Documentation/yaffs2.html

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  13 <H1>YAFFS2</H1>
  14 <H4>Summary</H4>
  15 <P>The original motivation for YAFFS 2 was to add support for the new
  16 NAND with 2kB pages instead of 512-byte pages and strictly sequential
  17 page writing order.
  18 </P>
  19 <P>To achieve this, a new design is used which also realises the
  20 following benefits:</P>
  21 <UL>
  22         <LI><P>zero page rewrites, which means faster operation. (YAFFS1
  23         uses a single rewrite in the spare area to delete a page).</P>
  24         <LI><P>ability to exploit simultaneous page programming on some
  25         chips.</P>
  26         <LI><P>improves performance relative to YAFFS1 speed(write:1.5x to
  27         5x, delete: 4x, garbage collection: 2x)</P>
  28         <LI><P>lower RAM footprint (approx. 25% to 50% of YAFFS1, depending
  29         on chunk size used).</P>
  30         <LI><P>Can support Toshiba/Sandisk MLC parts.</P>
  31         <LI><P>Runtime selection between various chunk sizes.</P>
  32 </UL>
  33 <P>Most of YAFFS and YAFFS2 code is common, therefore the code will
  34 likely be kept common with YAFFS vs YAFFS2 being run-time selected.</P>
  35 <H4>Method</H4>
  36 <P>The main philosophical difference between YAFFS and YAFFS2 is how
  37 discarded status is tracked. Since we can't do any re-writing, we
  38 can't use the &quot;discarded&quot; flags in YAFFS2.</P>
  39 <P>Instead YAFFS2 uses two mechanisms to resolve data state.</P>
  40 <UL>
  41         <LI><P>YAFFS2 chunks have more tag information, including a block
  42         sequence Id. From that we can determine the chunk sequence Id since
  43         the chunks are allocated sequentially in a block. Thus we always
  44         know the patch order for all chunks in the system.</P>
  45         <LI><P>The above helps us track stale -vs- fresh data, but does not
  46         help determine when a file/object is deleted. Deletion is achieved
  47         by moving the object to the &quot;unlinked&quot; directory. We also
  48         keep track of the number of chunks (both stale and current) in the
  49         system for each object. While this number indicates that there are
  50         still chunks associated with this object we keep the deletion
  51         record. When the last trace of the object has been really erased
  52         from NAND, we can forget about the deletion record too.
  53         </P>
  54         <LI><P>Since there is no deletion, a resize (shrinking) of a file
  55         will still have valid data chunks past the end of file on the NAND.
  56         However, we write a new ObjectHeader at the time of the resize,
  57         therefore this shows the shrunken file size. The ObjectHeader also
  58         carries information to say that this is a shrink, for some special
  59         handling in the garbage collector.</P>
  60 </UL>
  61 <P><BR><BR>
  62 </P>
  63 <P>This changes erasure slightly:</P>
  64 <UL>
  65         <LI><P>During garbage collection we can't just look at chunk state
  66         flags, instead we must read the tags of each chunk to determine
  67         which object's chunk count we must decrement. This step must also be
  68         performed when a block is erased (as part of deletion). This is
  69         already done in YAFFS by<I> soft deletion.</I></P>
  70 </UL>
  71 <P>This makes erasure &amp; garbage collection more expensive (by
  72 adding reads), but remember that ion YAFFS2 we don't need to do page
  73 deletions which are much more expensive operations. Thus, all-up
  74 YAFFS2 wins.</P>
  75 <H4>Resize Handling</H4>
  76 <P>In YAFFS, soft deletion is ued for everything but resizing
  77 (shrinking) a file which has some particularly ugly cases that can
  78 complicate garbage collection.</P>
  79 <P>As mentioned before, we write a new ObjectHeader to indicate the
  80 shrinking.  However, it is important that this ObjectHeader does not
  81 get destroyed (erased) before the data chunks that were discarded
  82 during the shrink are destroyed (erased). If this precaution is not
  83 taken then it is possible that the deleted chunks might be brought
  84 back to life.</P>
  85 <P>The modification to the garbage collector is as follows:</P>
  86 <UL>
  87         <LI><P>The block info flags are expanded as follows:</P>
  88         <LI><P>containsShrinkObjectHeader: Indicates that one or more of the
  89         chunks in the block are object headers to indicate a file shrink.</P>
  90         <LI><P>containsShrinkDataChunks: Indicates that one or more of the
  91         chunks in the block are data chunks discarded by a file shrink.</P>
  92 </UL>
  93 <UL>
  94         <LI><P>Before we allow a block with containsShrinkObjectHeader set
  95         to be erased (garbage collected), we must first ensure that all the
  96         earlier blocks (ie according to the block sequence number) that have
  97         containsShrinkDataChunks set are erased (garbage collected) which
  98         ensures that the shrunk data chunks are deleted. The mechanisms to
  99         do this are as follows:</P>
 100         <LI><P>If the garbage collector attempts to select a block with
 101         containsShrinkObjectHeader set, we check that the above criterion in
 102         met before selecting the block.</P>
 103         <LI><P>Periodically select the oldest block with
 104         containsShrinkDataChunks for garbage collection.</P>
 105 </UL>
 106 <P><BR><BR>
 107 </P>
 108 <P><BR><BR>
 109 </P>
 110 <H4>Tag structure</H4>
 111 <P>Each chunk in YAFFS2 has the following information:</P>
 112 <TABLE WIDTH=100% BORDER=1 CELLPADDING=4 CELLSPACING=3>
 113         <COL WIDTH=45*>
 114         <COL WIDTH=83*>
 115         <COL WIDTH=64*>
 116         <COL WIDTH=64*>
 117         <THEAD>
 118                 <TR VALIGN=TOP>
 119                         <TH WIDTH=18%>
 120                                 <P>Field</P>
 121                         </TH>
 122                         <TH WIDTH=32%>
 123                                 <P>Comment</P>
 124                         </TH>
 125                         <TH WIDTH=25%>
 126                                 <P>Size for 1kb chunks</P>
 127                         </TH>
 128                         <TH WIDTH=25%>
 129                                 <P>Size for 2kB chunks</P>
 130                         </TH>
 131                 </TR>
 132         </THEAD>
 133         <TBODY>
 134                 <TR VALIGN=TOP>
 135                         <TD WIDTH=18%>
 136                                 <P>blockState</P>
 137                         </TD>
 138                         <TD WIDTH=32%>
 139                                 <P>Block state. non-0xFF for bad block</P>
 140                         </TD>
 141                         <TD WIDTH=25%>
 142                                 <P>1 byte</P>
 143                         </TD>
 144                         <TD WIDTH=25%>
 145                                 <P>1 byte</P>
 146                         </TD>
 147                 </TR>
 148                 <TR VALIGN=TOP>
 149                         <TD WIDTH=18%>
 150                                 <P>chunkId</P>
 151                         </TD>
 152                         <TD WIDTH=32%>
 153                                 <P>32-bit chunk Id</P>
 154                         </TD>
 155                         <TD WIDTH=25%>
 156                                 <P>4 bytes</P>
 157                         </TD>
 158                         <TD WIDTH=25%>
 159                                 <P>4 bytes</P>
 160                         </TD>
 161                 </TR>
 162                 <TR VALIGN=TOP>
 163                         <TD WIDTH=18%>
 164                                 <P>objectId</P>
 165                         </TD>
 166                         <TD WIDTH=32%>
 167                                 <P>32-bit object Id</P>
 168                         </TD>
 169                         <TD WIDTH=25%>
 170                                 <P>4 bytes</P>
 171                         </TD>
 172                         <TD WIDTH=25%>
 173                                 <P>4 bytes</P>
 174                         </TD>
 175                 </TR>
 176                 <TR VALIGN=TOP>
 177                         <TD WIDTH=18%>
 178                                 <P>nBytes</P>
 179                         </TD>
 180                         <TD WIDTH=32%>
 181                                 <P>Number of data bytes in this chunk</P>
 182                         </TD>
 183                         <TD WIDTH=25%>
 184                                 <P>2 bytes</P>
 185                         </TD>
 186                         <TD WIDTH=25%>
 187                                 <P>2 bytes</P>
 188                         </TD>
 189                 </TR>
 190                 <TR VALIGN=TOP>
 191                         <TD WIDTH=18%>
 192                                 <P>blockSequence</P>
 193                         </TD>
 194                         <TD WIDTH=32%>
 195                                 <P>sequence number for this block</P>
 196                         </TD>
 197                         <TD WIDTH=25%>
 198                                 <P>4 bytes</P>
 199                         </TD>
 200                         <TD WIDTH=25%>
 201                                 <P>4 bytes</P>
 202                         </TD>
 203                 </TR>
 204                 <TR VALIGN=TOP>
 205                         <TD WIDTH=18%>
 206                                 <P>tagsEcc</P>
 207                         </TD>
 208                         <TD WIDTH=32%>
 209                                 <P>ECC on tags area</P>
 210                         </TD>
 211                         <TD WIDTH=25%>
 212                                 <P>3 bytes</P>
 213                         </TD>
 214                         <TD WIDTH=25%>
 215                                 <P>3 bytes</P>
 216                         </TD>
 217                 </TR>
 218                 <TR VALIGN=TOP>
 219                         <TD WIDTH=18%>
 220                                 <P>ecc</P>
 221                         </TD>
 222                         <TD WIDTH=32%>
 223                                 <P>ECC, 3 bytes/256 bytes of data</P>
 224                         </TD>
 225                         <TD WIDTH=25%>
 226                                 <P>12 bytes</P>
 227                         </TD>
 228                         <TD WIDTH=25%>
 229                                 <P>24 bytes</P>
 230                         </TD>
 231                 </TR>
 232                 <TR VALIGN=TOP>
 233                         <TD WIDTH=18%>
 234                                 <P><B>Total</B></P>
 235                         </TD>
 236                         <TD WIDTH=32%>
 237                                 <P><BR>
 238                                 </P>
 239                         </TD>
 240                         <TD WIDTH=25%>
 241                                 <P ALIGN=LEFT><B>30 bytes</B></P>
 242                         </TD>
 243                         <TD WIDTH=25%>
 244                                 <P><B>42 bytes</B></P>
 245                         </TD>
 246                 </TR>
 247         </TBODY>
 248 </TABLE>
 249 <P><BR><BR>
 250 </P>
 251 <P>To get enough spare bytes for this tagging structure requires a
 252 chunk-size of at least 1kB. YAFFS1 is still used for 512-byte chunk
 253 sizes.</P>
 254 <P>The blockSequence increments each time a block is allocated. (ie.
 255 the first block allocated is block 1, and so on).</P>
 256 <H4>Scanning</H4>
 257 <P>The only reason we need to keep track of data status on NAND is to
 258 be able to recreate the file system state during scanning. Since we
 259 no longer have chunk deletion status flags we use a slightly
 260 different process for scanning a YAFFS2 system.</P>
 261 <P>In effect, YAFFS2 recreates its state by &quot;replaying the
 262 tape&quot;. ie. it scans the chunks in their allocation order (block
 263 sequence Id order) rather than in their order on the media. This
 264 implies that at start up, the blocks must be read and their block
 265 sequence determined.</P>
 266 <H4>Performance</H4>
 267 <P>These numbers are indicative of relative performance. These only
 268 apply to the NAND data transfer and do not include other overheads.</P>
 269 <P>As an example, read/write cycle times of 100nS are used (though
 270 NAND can typically do 50nS), &quot;seek time&quot; of 10uS and
 271 program time of 200uS. Mileage will vary.</P>
 272 <P>NB x16 means using a 16-bit bus. Clearly this cuts down on data
 273 transfer time relative to an 8-bit bus.</P>
 274 <P>Times for 2kB read(units of 1uS).</P>
 275 <TABLE WIDTH=937 BORDER=1 CELLPADDING=4 CELLSPACING=3>
 276         <COL WIDTH=175>
 277         <COL WIDTH=176>
 278         <COL WIDTH=177>
 279         <COL WIDTH=176>
 280         <COL WIDTH=173>
 281         <THEAD>
 282                 <TR VALIGN=TOP>
 283                         <TH WIDTH=175>
 284                                 <P>Operation</P>
 285                         </TH>
 286                         <TH WIDTH=176>
 287                                 <P>YAFFS1</P>
 288                         </TH>
 289                         <TH WIDTH=177>
 290                                 <P>YAFFS2 (512b pages)</P>
 291                         </TH>
 292                         <TH WIDTH=176>
 293                                 <P>YAFFS2 (2kB pages)</P>
 294                         </TH>
 295                         <TH WIDTH=173>
 296                                 <P>YAFFS2(2kB pages, x16)</P>
 297                         </TH>
 298                 </TR>
 299         </THEAD>
 300         <TBODY>
 301                 <TR>
 302                         <TD WIDTH=175 VALIGN=TOP>
 303                                 <P>Seek
 304                                 </P>
 305                         </TD>
 306                         <TD WIDTH=176 VALIGN=BOTTOM SDVAL="40" SDNUM="5129;">
 307                                 <P ALIGN=RIGHT>40</P>
 308                         </TD>
 309                         <TD WIDTH=177 VALIGN=BOTTOM SDVAL="40" SDNUM="5129;">
 310                                 <P ALIGN=RIGHT>40</P>
 311                         </TD>
 312                         <TD WIDTH=176 VALIGN=BOTTOM SDVAL="10" SDNUM="5129;">
 313                                 <P ALIGN=RIGHT>10</P>
 314                         </TD>
 315                         <TD WIDTH=173 VALIGN=BOTTOM SDVAL="10" SDNUM="5129;">
 316                                 <P ALIGN=RIGHT>10</P>
 317                         </TD>
 318                 </TR>
 319                 <TR>
 320                         <TD WIDTH=175 VALIGN=TOP>
 321                                 <P>Read</P>
 322                         </TD>
 323                         <TD WIDTH=176 VALIGN=BOTTOM SDVAL="220" SDNUM="5129;">
 324                                 <P ALIGN=RIGHT>220</P>
 325                         </TD>
 326                         <TD WIDTH=177 VALIGN=BOTTOM SDVAL="220" SDNUM="5129;">
 327                                 <P ALIGN=RIGHT>220</P>
 328                         </TD>
 329                         <TD WIDTH=176 VALIGN=BOTTOM SDVAL="220" SDNUM="5129;">
 330                                 <P ALIGN=RIGHT>220</P>
 331                         </TD>
 332                         <TD WIDTH=173 VALIGN=BOTTOM SDVAL="110" SDNUM="5129;">
 333                                 <P ALIGN=RIGHT>110</P>
 334                         </TD>
 335                 </TR>
 336                 <TR>
 337                         <TD WIDTH=175 VALIGN=TOP>
 338                                 <P>Total</P>
 339                         </TD>
 340                         <TD WIDTH=176 VALIGN=BOTTOM SDVAL="260" SDNUM="5129;">
 341                                 <P ALIGN=RIGHT>260</P>
 342                         </TD>
 343                         <TD WIDTH=177 VALIGN=BOTTOM SDVAL="260" SDNUM="5129;">
 344                                 <P ALIGN=RIGHT>260</P>
 345                         </TD>
 346                         <TD WIDTH=176 VALIGN=BOTTOM SDVAL="230" SDNUM="5129;">
 347                                 <P ALIGN=RIGHT>230</P>
 348                         </TD>
 349                         <TD WIDTH=173 VALIGN=BOTTOM SDVAL="120" SDNUM="5129;">
 350                                 <P ALIGN=RIGHT>120</P>
 351                         </TD>
 352                 </TR>
 353                 <TR>
 354                         <TD WIDTH=175 VALIGN=TOP>
 355                                 <P>MB/s</P>
 356                         </TD>
 357                         <TD WIDTH=176 VALIGN=BOTTOM SDVAL="7.6" SDNUM="5129;">
 358                                 <P ALIGN=RIGHT>7.6</P>
 359                         </TD>
 360                         <TD WIDTH=177 VALIGN=BOTTOM SDVAL="7.6" SDNUM="5129;">
 361                                 <P ALIGN=RIGHT>7.6</P>
 362                         </TD>
 363                         <TD WIDTH=176 VALIGN=BOTTOM SDVAL="8.7" SDNUM="5129;">
 364                                 <P ALIGN=RIGHT>8.7</P>
 365                         </TD>
 366                         <TD WIDTH=173 VALIGN=BOTTOM SDVAL="16.7" SDNUM="5129;">
 367                                 <P ALIGN=RIGHT>16.7</P>
 368                         </TD>
 369                 </TR>
 370                 <TR>
 371                         <TD WIDTH=175 VALIGN=TOP>
 372                                 <P>Relative speed</P>
 373                         </TD>
 374                         <TD WIDTH=176 VALIGN=BOTTOM SDVAL="1" SDNUM="5129;">
 375                                 <P ALIGN=RIGHT>1</P>
 376                         </TD>
 377                         <TD WIDTH=177 VALIGN=BOTTOM SDVAL="1" SDNUM="5129;">
 378                                 <P ALIGN=RIGHT>1</P>
 379                         </TD>
 380                         <TD WIDTH=176 VALIGN=BOTTOM SDVAL="1.1" SDNUM="5129;">
 381                                 <P ALIGN=RIGHT>1.1</P>
 382                         </TD>
 383                         <TD WIDTH=173 VALIGN=BOTTOM SDVAL="2.2" SDNUM="5129;">
 384                                 <P ALIGN=RIGHT>2.2</P>
 385                         </TD>
 386                 </TR>
 387         </TBODY>
 388 </TABLE>
 389 <P><BR><BR>
 390 </P>
 391 <P>Times for 2kB writes(units of 1uS).</P>
 392 <TABLE WIDTH=100% BORDER=1 CELLPADDING=4 CELLSPACING=3>
 393         <COL WIDTH=51*>
 394         <COL WIDTH=51*>
 395         <COL WIDTH=51*>
 396         <COL WIDTH=51*>
 397         <COL WIDTH=51*>
 398         <THEAD>
 399                 <TR VALIGN=TOP>
 400                         <TH WIDTH=20%>
 401                                 <P>Operation</P>
 402                         </TH>
 403                         <TH WIDTH=20%>
 404                                 <P>YAFFS1</P>
 405                         </TH>
 406                         <TH WIDTH=20%>
 407                                 <P>YAFFS2 (512b pages)</P>
 408                         </TH>
 409                         <TH WIDTH=20%>
 410                                 <P>YAFFS2 (2kB pages)</P>
 411                         </TH>
 412                         <TH WIDTH=20%>
 413                                 <P>YAFFS2(2kB pages, x16)</P>
 414                         </TH>
 415                 </TR>
 416         </THEAD>
 417         <TBODY>
 418                 <TR>
 419                         <TD WIDTH=20% VALIGN=TOP>
 420                                 <P>Seek</P>
 421                         </TD>
 422                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="40" SDNUM="5129;">
 423                                 <P ALIGN=RIGHT>40</P>
 424                         </TD>
 425                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="40" SDNUM="5129;">
 426                                 <P ALIGN=RIGHT>40</P>
 427                         </TD>
 428                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="10" SDNUM="5129;">
 429                                 <P ALIGN=RIGHT>10</P>
 430                         </TD>
 431                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="10" SDNUM="5129;">
 432                                 <P ALIGN=RIGHT>10</P>
 433                         </TD>
 434                 </TR>
 435                 <TR>
 436                         <TD WIDTH=20% VALIGN=TOP>
 437                                 <P>Program</P>
 438                         </TD>
 439                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="800" SDNUM="5129;">
 440                                 <P ALIGN=RIGHT>800</P>
 441                         </TD>
 442                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="800" SDNUM="5129;">
 443                                 <P ALIGN=RIGHT>800</P>
 444                         </TD>
 445                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="200" SDNUM="5129;">
 446                                 <P ALIGN=RIGHT>200</P>
 447                         </TD>
 448                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="200" SDNUM="5129;">
 449                                 <P ALIGN=RIGHT>200</P>
 450                         </TD>
 451                 </TR>
 452                 <TR>
 453                         <TD WIDTH=20% VALIGN=TOP>
 454                                 <P>Seek</P>
 455                         </TD>
 456                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="40" SDNUM="5129;">
 457                                 <P ALIGN=RIGHT>40</P>
 458                         </TD>
 459                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="40" SDNUM="5129;">
 460                                 <P ALIGN=RIGHT>40</P>
 461                         </TD>
 462                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="10" SDNUM="5129;">
 463                                 <P ALIGN=RIGHT>10</P>
 464                         </TD>
 465                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="10" SDNUM="5129;">
 466                                 <P ALIGN=RIGHT>10</P>
 467                         </TD>
 468                 </TR>
 469                 <TR>
 470                         <TD WIDTH=20% VALIGN=TOP>
 471                                 <P>Read</P>
 472                         </TD>
 473                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="220" SDNUM="5129;">
 474                                 <P ALIGN=RIGHT>220</P>
 475                         </TD>
 476                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="220" SDNUM="5129;">
 477                                 <P ALIGN=RIGHT>220</P>
 478                         </TD>
 479                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="220" SDNUM="5129;">
 480                                 <P ALIGN=RIGHT>220</P>
 481                         </TD>
 482                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="110" SDNUM="5129;">
 483                                 <P ALIGN=RIGHT>110</P>
 484                         </TD>
 485                 </TR>
 486                 <TR>
 487                         <TD WIDTH=20% VALIGN=TOP>
 488                                 <P>Total</P>
 489                         </TD>
 490                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="1320" SDNUM="5129;">
 491                                 <P ALIGN=RIGHT>1320</P>
 492                         </TD>
 493                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="1320" SDNUM="5129;">
 494                                 <P ALIGN=RIGHT>1320</P>
 495                         </TD>
 496                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="660" SDNUM="5129;">
 497                                 <P ALIGN=RIGHT>660</P>
 498                         </TD>
 499                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="440" SDNUM="5129;">
 500                                 <P ALIGN=RIGHT>440</P>
 501                         </TD>
 502                 </TR>
 503                 <TR>
 504                         <TD WIDTH=20% VALIGN=TOP>
 505                                 <P>MB/s</P>
 506                         </TD>
 507                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="1.5" SDNUM="5129;">
 508                                 <P ALIGN=RIGHT>1.5</P>
 509                         </TD>
 510                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="1.5" SDNUM="5129;">
 511                                 <P ALIGN=RIGHT>1.5</P>
 512                         </TD>
 513                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="3" SDNUM="5129;">
 514                                 <P ALIGN=RIGHT>3</P>
 515                         </TD>
 516                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="4.5" SDNUM="5129;">
 517                                 <P ALIGN=RIGHT>4.5</P>
 518                         </TD>
 519                 </TR>
 520                 <TR>
 521                         <TD WIDTH=20% VALIGN=TOP>
 522                                 <P>Relative speed</P>
 523                         </TD>
 524                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="1" SDNUM="5129;">
 525                                 <P ALIGN=RIGHT>1</P>
 526                         </TD>
 527                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="1" SDNUM="5129;">
 528                                 <P ALIGN=RIGHT>1</P>
 529                         </TD>
 530                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="2" SDNUM="5129;">
 531                                 <P ALIGN=RIGHT>2</P>
 532                         </TD>
 533                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="3" SDNUM="5129;">
 534                                 <P ALIGN=RIGHT>3</P>
 535                         </TD>
 536                 </TR>
 537         </TBODY>
 538 </TABLE>
 539 <P><BR><BR>
 540 </P>
 541 <P>Times for 1MB delete (units of 1uS).</P>
 542 <TABLE WIDTH=100% BORDER=1 CELLPADDING=4 CELLSPACING=3>
 543         <COL WIDTH=51*>
 544         <COL WIDTH=51*>
 545         <COL WIDTH=51*>
 546         <COL WIDTH=51*>
 547         <COL WIDTH=51*>
 548         <THEAD>
 549                 <TR VALIGN=TOP>
 550                         <TH WIDTH=20%>
 551                                 <P>Operation</P>
 552                         </TH>
 553                         <TH WIDTH=20%>
 554                                 <P>YAFFS1</P>
 555                         </TH>
 556                         <TH WIDTH=20%>
 557                                 <P>YAFFS2 (512b pages)</P>
 558                         </TH>
 559                         <TH WIDTH=20%>
 560                                 <P>YAFFS2 (2kB pages)</P>
 561                         </TH>
 562                         <TH WIDTH=20%>
 563                                 <P>YAFFS2(2kB pages, x16)</P>
 564                         </TH>
 565                 </TR>
 566         </THEAD>
 567         <TBODY>
 568                 <TR>
 569                         <TD WIDTH=20% VALIGN=TOP>
 570                                 <P>Seek</P>
 571                         </TD>
 572                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="20480" SDNUM="5129;">
 573                                 <P ALIGN=RIGHT>20480</P>
 574                         </TD>
 575                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="0" SDNUM="5129;">
 576                                 <P ALIGN=RIGHT>0</P>
 577                         </TD>
 578                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="0" SDNUM="5129;">
 579                                 <P ALIGN=RIGHT>0</P>
 580                         </TD>
 581                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="0" SDNUM="5129;">
 582                                 <P ALIGN=RIGHT>0</P>
 583                         </TD>
 584                 </TR>
 585                 <TR>
 586                         <TD WIDTH=20% VALIGN=TOP>
 587                                 <P>Program</P>
 588                         </TD>
 589                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="409600" SDNUM="5129;">
 590                                 <P ALIGN=RIGHT>409600</P>
 591                         </TD>
 592                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="0" SDNUM="5129;">
 593                                 <P ALIGN=RIGHT>0</P>
 594                         </TD>
 595                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="0" SDNUM="5129;">
 596                                 <P ALIGN=RIGHT>0</P>
 597                         </TD>
 598                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="0" SDNUM="5129;">
 599                                 <P ALIGN=RIGHT>0</P>
 600                         </TD>
 601                 </TR>
 602                 <TR>
 603                         <TD WIDTH=20% VALIGN=TOP>
 604                                 <P>Erase</P>
 605                         </TD>
 606                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="128000" SDNUM="5129;">
 607                                 <P ALIGN=RIGHT>128000</P>
 608                         </TD>
 609                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="128000" SDNUM="5129;">
 610                                 <P ALIGN=RIGHT>128000</P>
 611                         </TD>
 612                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="16000" SDNUM="5129;">
 613                                 <P ALIGN=RIGHT>16000</P>
 614                         </TD>
 615                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="16000" SDNUM="5129;">
 616                                 <P ALIGN=RIGHT>16000</P>
 617                         </TD>
 618                 </TR>
 619                 <TR>
 620                         <TD WIDTH=20% VALIGN=TOP>
 621                                 <P>Total</P>
 622                         </TD>
 623                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="558080" SDNUM="5129;">
 624                                 <P ALIGN=RIGHT>558080</P>
 625                         </TD>
 626                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="128000" SDNUM="5129;">
 627                                 <P ALIGN=RIGHT>128000</P>
 628                         </TD>
 629                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="16000" SDNUM="5129;">
 630                                 <P ALIGN=RIGHT>16000</P>
 631                         </TD>
 632                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="16000" SDNUM="5129;">
 633                                 <P ALIGN=RIGHT>16000</P>
 634                         </TD>
 635                 </TR>
 636                 <TR>
 637                         <TD WIDTH=20% VALIGN=TOP>
 638                                 <P>MB/s</P>
 639                         </TD>
 640                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="1.8" SDNUM="5129;">
 641                                 <P ALIGN=RIGHT>1.8</P>
 642                         </TD>
 643                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="7.8" SDNUM="5129;">
 644                                 <P ALIGN=RIGHT>7.8</P>
 645                         </TD>
 646                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="62.5" SDNUM="5129;">
 647                                 <P ALIGN=RIGHT>62.5</P>
 648                         </TD>
 649                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="62.5" SDNUM="5129;">
 650                                 <P ALIGN=RIGHT>62.5</P>
 651                         </TD>
 652                 </TR>
 653                 <TR>
 654                         <TD WIDTH=20% VALIGN=TOP>
 655                                 <P>Relative speed</P>
 656                         </TD>
 657                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="1" SDNUM="5129;">
 658                                 <P ALIGN=RIGHT>1</P>
 659                         </TD>
 660                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="4" SDNUM="5129;">
 661                                 <P ALIGN=RIGHT>4</P>
 662                         </TD>
 663                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="34" SDNUM="5129;">
 664                                 <P ALIGN=RIGHT>34</P>
 665                         </TD>
 666                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="34" SDNUM="5129;">
 667                                 <P ALIGN=RIGHT>34</P>
 668                         </TD>
 669                 </TR>
 670         </TBODY>
 671 </TABLE>
 672 <P><BR><BR>
 673 </P>
 674 <P>Times for 1MB of garbage collection at 50% dirty (units of 1uS).</P>
 675 <TABLE WIDTH=100% BORDER=1 CELLPADDING=4 CELLSPACING=3>
 676         <COL WIDTH=51*>
 677         <COL WIDTH=51*>
 678         <COL WIDTH=51*>
 679         <COL WIDTH=51*>
 680         <COL WIDTH=51*>
 681         <THEAD>
 682                 <TR VALIGN=TOP>
 683                         <TH WIDTH=20%>
 684                                 <P>Operation</P>
 685                         </TH>
 686                         <TH WIDTH=20%>
 687                                 <P>YAFFS1</P>
 688                         </TH>
 689                         <TH WIDTH=20%>
 690                                 <P>YAFFS2 (512b pages)</P>
 691                         </TH>
 692                         <TH WIDTH=20%>
 693                                 <P>YAFFS2 (2kB pages)</P>
 694                         </TH>
 695                         <TH WIDTH=20%>
 696                                 <P>YAFFS2(2kB pages, x16)</P>
 697                         </TH>
 698                 </TR>
 699         </THEAD>
 700         <TBODY>
 701                 <TR>
 702                         <TD WIDTH=20% VALIGN=TOP>
 703                                 <P>Delete 1MB</P>
 704                         </TD>
 705                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="558080" SDNUM="5129;">
 706                                 <P ALIGN=RIGHT>558080</P>
 707                         </TD>
 708                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="128000" SDNUM="5129;">
 709                                 <P ALIGN=RIGHT>128000</P>
 710                         </TD>
 711                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="16000" SDNUM="5129;">
 712                                 <P ALIGN=RIGHT>16000</P>
 713                         </TD>
 714                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="16000" SDNUM="5129;">
 715                                 <P ALIGN=RIGHT>16000</P>
 716                         </TD>
 717                 </TR>
 718                 <TR>
 719                         <TD WIDTH=20% VALIGN=TOP>
 720                                 <P>Write 0.5MB</P>
 721                         </TD>
 722                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="337920" SDNUM="5129;">
 723                                 <P ALIGN=RIGHT>337920</P>
 724                         </TD>
 725                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="337920" SDNUM="5129;">
 726                                 <P ALIGN=RIGHT>337920</P>
 727                         </TD>
 728                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="168960" SDNUM="5129;">
 729                                 <P ALIGN=RIGHT>168960</P>
 730                         </TD>
 731                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="112640" SDNUM="5129;">
 732                                 <P ALIGN=RIGHT>112640</P>
 733                         </TD>
 734                 </TR>
 735                 <TR>
 736                         <TD WIDTH=20% VALIGN=TOP>
 737                                 <P>Total</P>
 738                         </TD>
 739                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="896000" SDNUM="5129;">
 740                                 <P ALIGN=RIGHT>896000</P>
 741                         </TD>
 742                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="465920" SDNUM="5129;">
 743                                 <P ALIGN=RIGHT>465920</P>
 744                         </TD>
 745                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="184960" SDNUM="5129;">
 746                                 <P ALIGN=RIGHT>184960</P>
 747                         </TD>
 748                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="128640" SDNUM="5129;">
 749                                 <P ALIGN=RIGHT>128640</P>
 750                         </TD>
 751                 </TR>
 752                 <TR>
 753                         <TD WIDTH=20% VALIGN=TOP>
 754                                 <P>MB/s</P>
 755                         </TD>
 756                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="1.1" SDNUM="5129;">
 757                                 <P ALIGN=RIGHT>1.1</P>
 758                         </TD>
 759                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="2.1" SDNUM="5129;">
 760                                 <P ALIGN=RIGHT>2.1</P>
 761                         </TD>
 762                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="5.4" SDNUM="5129;">
 763                                 <P ALIGN=RIGHT>5.4</P>
 764                         </TD>
 765                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="7.7" SDNUM="5129;">
 766                                 <P ALIGN=RIGHT>7.7</P>
 767                         </TD>
 768                 </TR>
 769                 <TR>
 770                         <TD WIDTH=20% VALIGN=TOP>
 771                                 <P>Relative speed</P>
 772                         </TD>
 773                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="1" SDNUM="5129;">
 774                                 <P ALIGN=RIGHT>1</P>
 775                         </TD>
 776                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="1.9" SDNUM="5129;">
 777                                 <P ALIGN=RIGHT>1.9</P>
 778                         </TD>
 779                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="4.9" SDNUM="5129;">
 780                                 <P ALIGN=RIGHT>4.9</P>
 781                         </TD>
 782                         <TD WIDTH=20% VALIGN=BOTTOM SDVAL="7" SDNUM="5129;">
 783                                 <P ALIGN=RIGHT>7</P>
 784                         </TD>
 785                 </TR>
 786         </TBODY>
 787 </TABLE>
 788 <P><BR><BR>
 789 </P>
 790 <H2>MTD Interface</H2>
 791 <P>As mentioned before, YAFFS2 requires a chunk-size of at least 1kB
 792 to get a large enough spare area to support the increased size of the
 793 tags. This is not really a disadvantage, but should rather be viewed
 794 as an opportunity to exploit the NAND hardware more effectively. In
 795 particular:</P>
 796 <UL>
 797         <LI><P>Newer, larger, NAND with 2kB pages can be used in chunks of
 798         2kB. Keeping the relationship of one chunk per page improves
 799         robustness and performance (rather than say trying to &quot;fake&quot;
 800         512byte pages). A block comprises 64x2kB pages.</P>
 801         <LI><P>Some devices have 512byte pages, but are arranged as multiple
 802         &quot;bit planes&quot; which can be programmed and erased in
 803         parallel. For example, the Samsung K9K1G08U0M can support 4
 804         simultaneous operations. YAFFS2 can exploit this by using 2kB chunks
 805         by using groups of 4 pages - one on each bitplane. Virtual blocks
 806         would be built which comprise 32x2kB chunks.</P>
 807 </UL>
 808 <P>To this end, yaffs_guts is being re-crafted to support arbitrary
 809 chunk size (power of 2, &gt;= 1024), with arbitrary block size.</P>
 810 <P>Currently, YAFFS also makes some other assumptions which will need
 811 to be changed:</P>
 812 <UL>
 813         <LI><P>Spare layout. Might need to be changed. This is relatively
 814         simple to shuffle around.</P>
 815         <LI><P>Bad block detection. Currently YAFFS uses the SmartMedia
 816         detection (checks first two pages for bad block markers). Needs to
 817         be more flexible. eg. Sandisk/Toshiba MLC parts mark the last two
 818         pages in a block.</P>
 819         <LI><P>ECC was on this list, but now seems flexible enough. Thanx
 820         Thomas.</P>
 821 </UL>
 822 <P>Some of these differences can be absorbed in the yaffs_mtd layer.
 823 Some will need to be handles inside the mtd itself.</P>
 824 <P>$Id: yaffs2.html,v 1.3 2003-09-16 06:48:38 charles Exp $</P>
 825 <P><BR><BR>
 826 </P>
 827 <P><BR><BR>
 828 </P>
 829 </BODY>
 830 </HTML>