Example VDI Defragmentation Utility

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Example VDI Defragmentation Utility

Postby TerryE » 15. Jul 2008, 02:25

I am just finishing off a tutorial on VDIs; how they work; and how you work with them to get the most out of your host and VMs. I reference a Q&D example in the tutorial and I thought that I might post it up for comment.

My real point is to emphasise that there's no reason why VBox couldn't separated out its VDI and VMDK manipulation utilities, to make them easier to use as a FLOSS baseline.

It's a little utility which creates a copy (not a clone -- the UUIDs are the same) of a dynamic VDI, but shuffles the 1Mbyte allocation pages into ascending sequence, thereby defragmenting the VDI. OK, its in Perl, but why write 1,000 lines of C++ when you can do it in <150 lines of Perl. And I reckon that more readers will be familiar with Perl. I nearly did it in Python, but I thought that would be cruel. :lol: It still drives my HDD flat out. Anyway here it is. Please treat it as Alpha code. It works fine for me, but I won't warrant it.
DO NOT USE —— I'VE FOUND A CASE WHERE THIS CORRUPTS THE VDI
(I will post an update when I've debugged it)
Code: Select all   Expand viewCollapse view
#
#  A liile demo to show that it is quite easy to manipulate VDIs
#  ~ 140 lines of Perl to do a defrag
#  TerryE   15 Jul 2008
#  This is an original work by me and placed unwarranted into the public domain.
#
use strict;

sub printUUID($);
sub printMap($@);
sub dumpVDIheader($);

my( $vdiName, $headerSize, $nextBlock ) =( shift(@ARGV), 512, 0 );
my( $header, $allocMap, @currentMap, @marshalOrder, @allocatedBlock, $newAllocMap );

open INVDI, '<:raw:bytes', $vdiName or die "cannot open VDI\n";

die "cannot read header\n"
    unless read( INVDI, $header, $headerSize ) == $headerSize;

my( $blockSize, $blocksInHDD, $blocksAllocated ) = dumpVDIheader $header;
my $mapSize  = $blocksInHDD * 4;
   
die "cannot read allocation map\n"
    unless read( INVDI, $allocMap, $mapSize ) == $mapSize;

@currentMap     = map  { unpack "l<", substr( $allocMap, 4*$_, 4 ) ; }
                      ( 0..($blocksInHDD-1) ) ;
@marshalOrder   = grep { $_ > -1 } @currentMap;
@allocatedBlock = map  { $_ == -1 ? -1 : $nextBlock++ } @currentMap;
$newAllocMap    = join '', map( { pack( "l<", $_ ); } @allocatedBlock ) ;

die "problem with setting up Allocation Map\n"
    if length( $newAllocMap ) != $mapSize;

printMap "Allocation map",                 @currentMap;
printMap "Marshalling Block sequence map", @marshalOrder;
printMap "New Allocation map",             @allocatedBlock;

my $vdiTmp = $vdiName . "${$}tmp";
open OUTVDI, '>:raw:bytes', $vdiTmp or die "cannot open VDI\n";
seek OUTVDI, 0, 0;
 
die "cannot write header\n"
    unless syswrite( OUTVDI, $header, $headerSize ) == $headerSize;
die "cannot write index mao\n"
    unless syswrite( OUTVDI, $newAllocMap, $mapSize ) == $mapSize;     
my $block = ' ' x $blockSize;
my $cnt = 0;
foreach my $i(@marshalOrder) {
   my($chunk, $offset) =( '', 0 );
   
   print( ($cnt++ % 10 ) == 9 ? "+":"." );
   
    die "seek error on input file\n"
       unless seek INVDI, $headerSize + $mapSize + $i*$blockSize , 0;
      
   while( $offset < $blockSize ) {
      my $n = read( INVDI, $chunk, $blockSize - $offset );
      substr( $block, $offset, $n) = $chunk;
      $offset += $n;
       die "cannot read block\n" if $n  == 0;
   }
   die "die invalid block length \n" unless $offset == $blockSize;
   
   $offset = 0;
   while( $offset < $blockSize ) {
      my $n = syswrite( OUTVDI, substr( $block, $offset), $blockSize - $offset );
      $offset += $n;
       die "cannot write block\n" if $n  == 0;
   }
    die "cannot write block\n" unless $offset == $blockSize;
}

close INVDI; close OUTVDI;
exit;

#===========================================================================
sub dumpVDIheader($) {
    my( $fileInfo,    $imageSignture,   $version,      $headerSize,     $imageType,
         $imageFlags,  $imageDescription,$offsetBlocks, $offsetData,
         $nCylinders,  $nHeads,          $nSectors,     $sectorSize,     $unused,
         $diskSizeLSL, $diskSizeMSL,     $blockSize,    $blockExtraData, $blocksInHDD,
         $blocksAllocated
         ) = unpack "a[64]VVVVVa[256]VVVVVVVVVVVVVVV", $header;
    my $thisUUID   = substr $header, 0x188, 16;
    my $lastUUID   = substr $header, 0x198, 16;
    my $linkUUID   = substr $header, 0x1a8, 16;
    my $parentUUID = substr $header, 0x1b8, 16;
   
    die "Unable to process this VDI format\n"
       unless $imageSignture == 0xBEDA107F && $version == 0x00010001;
   
    my $diskSize = $diskSizeMSL * 256 * 256 * 256 * 256 + $diskSizeLSL;
    $fileInfo =~ s/[\0x00\n]*//g;
    $imageDescription =~ s/[\0x00\n]*//g;
   
    print "fileInfo = $fileInfo\n";
    print "imageSignture = $imageSignture\n";
    print "version = $version\n";
    print "headerSize = $headerSize\n";
    print "imageType = $imageType\n";
    print "imageFlags = $imageFlags\n";
    print "imageDescription = $imageDescription\n";
    print "offsetBlocks = $offsetBlocks\n";
    print "offsetData = $offsetData\n";
    print "nCylinders = $nCylinders\n";
    print "nHeads = $nHeads\n";
    print "nSectors = $nSectors\n";
    print "sectorSize = $sectorSize\n";
    print "diskSize = $diskSize\n";
    print "blockSize = $blockSize\n";
    print "blockExtraData = $blockExtraData\n";
    print "blocksInHDD = $blocksInHDD\n";
    print "blocksAllocated = $blocksAllocated\n";
    print "This UUID   = ", printUUID($thisUUID), "\n";
    print "Last UUID   = ", printUUID($lastUUID), "\n";
    print "Link UUID   = ", printUUID($linkUUID), "\n";
    print "Parent UUID = ", printUUID($parentUUID), "\n";

    return( $blockSize, $blocksInHDD, $blocksAllocated );
}

sub printUUID($) {
    return sprintf "%08x-%04x-%04x-%04x-%08x%04x", unpack("VvvnNn", shift);
}

sub printMap($@) {
    my $mapTitle = shift;
    my @map = @_;
   
    print "\n$mapTitle = \n    |", map {sprintf "%4d", $_}(0..19) ;
    print  "\n----+",("-" x 80);
    foreach my $i( 0..$#map ) {
       printf "\n%4d|", $i if($i % 20) == 0;
        printf "%4d", $map[$i];
    }
    print "\n\n";
}
. Out of interest here was the allocation map of a 1920 Mbyte Ext3 system disk prior to the shuffle:
Code: Select all   Expand viewCollapse view
fileInfo = <<< Sun VM VirtualBo Disk Image >>>
imageSignture = 3201962111
version = 65537
headerSize = 400
imageType = 1
imageFlags = 0
imageDescription =
offsetBlocks = 512
offsetData = 8192
nCylinders = 0
nHeads = 0
nSectors = 0
sectorSize = 512
diskSize = 2013265920
blockSize = 1048576
blockExtraData = 0
blocksInHDD = 1920
blocksAllocated = 523
This UUID   = 2762085a-b6a8-4469-a157-e2b243a58fcb
Last UUID   = e3b15c0c-73c5-40ed-aef7-06d620690c96
Link UUID   = 00000000-0000-0000-0000-000000000000
Parent UUID = 00000000-0000-0000-0000-000000000000

Allocation map =
    |   0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19
----+--------------------------------------------------------------------------------
   0|   0  -1   1  54  77  98   2 159 182 217   3 259 272 285   4 372 397 398   5 399
  20| 412  -1   6  -1  -1  -1   7  -1  -1  -1   8  -1  -1  -1 519  -1  -1  -1  -1  -1
  40|  -1  -1  -1  -1  -1  -1  -1  -1 204 205 206 207 208 209 210 211 212 213 214 215
  60| 216 218 220 221 222 223 224 225 226 227 234 235 236 237 238 240 241 242 243 244
  80| 245 246 247 289 288 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333
 100| 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353
 120| 354 355 356 364 357 358 359 360   9 392 361 362 363 365 366 367 368 369 370 371
 140| 373 374 375 376 377 378 379 380 386 385 384 381 382 383 387 388 389 390 391 393
 160| 394  -1  -1  -1  -1  -1  -1  -1 400  -1  -1  -1  -1  -1  -1  -1  28  40  39  43
 180|  44  45  46  47 248 249 250 251 252 253 254 255 256 257 258 260 262 263 264 265
 200| 266 267 268 269 270 271 273 274 275 279 278 277 276 280 281 282 283 284 286 287
 220| 290 291 292 293 294 295  -1  -1  -1  -1  -1  -1 522  -1  -1  -1  -1  -1  -1  -1
 240|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  10  -1  -1  -1
 260|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 280|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 300|  -1  -1  -1  -1  23  24  25  26 296 297  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 320|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 340|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 360|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 380|  -1  -1  -1  -1  11  -1 408  -1  -1  -1  -1  -1 409  -1  -1  -1  -1  -1  -1  -1
 400| 410  -1  -1  -1  -1  -1  -1  -1 411  -1  -1  -1  -1  -1  -1  -1 407  -1  -1  -1
 420|  -1  -1  -1  -1 406  -1  -1  -1  -1  -1  -1  -1  27  38  29  30  31  32  33  34
 440|  35  36  37  41  96  97  95  94  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 460|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1 401  -1  -1  -1  -1  -1  -1  -1
 480| 402  -1  -1  -1  -1  -1  -1  -1 403  -1  -1  -1  -1  -1  -1  -1 404  -1  -1  -1
 500|  -1  -1  -1  -1 405  -1  -1  -1  -1  -1  -1  -1  12  -1  -1  -1  -1  -1  -1  -1
 520|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 540|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 560| 261  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 580|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 600|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 620|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 640|  13  -1  18  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 660|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 680|  -1  -1  -1  -1  -1  -1  -1  -1  42  48  49  50  51  52  53  55  56  57  58  59
 700|  60  61  62  63  64  65  66  67  68  69  70  71  72  73  74  75  76  78  79  80
 720|  81  82  83  84  85  86  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 740|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 760|  -1  -1  -1  -1  -1  -1  -1  -1  14  19  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 780|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 800|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1 308  -1  -1  -1
 820|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 840|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 860|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 880|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  15  -1  20  -1
 900|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 920|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
 940|  -1  -1  -1  -1  87  88  89  90  91  92  93  99 100 101 102 103 104 105 106 107
 960| 108 109 110 115 111 112 113 114  22 116 117 118 119 120 121 122 123 124 125 126
 980| 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146
1000| 147 148 171 170 299 300 301 298 302 303 304 305 306 307 309 310 311 312 313 314
1020| 315 316 317 318  16  21  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
1040|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
1060|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1 149 150 151 152 153 154 155 156
1080| 157 158 160 161 162 163 164 165 166 167 168 169 172 173 174 175 176 177 178 179
1100| 180 181 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200
1120| 201 202 203 219 228 229 230 231 232 233 239  -1  -1  -1  -1  -1  -1  -1  -1  -1
1140|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  17  -1 520  -1  -1  -1  -1  -1
1160|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
1180|  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1
1200| 395 396  -1  -1  -1  -1 413 419 418 417 416 415 420 421 414 462  -1  -1  -1  -1
1220|  -1  -1 422  -1  -1  -1  -1  -1  -1  -1 423  -1  -1  -1  -1  -1  -1  -1 424  -1
1240|  -1  -1  -1  -1  -1  -1 425  -1  -1  -1  -1  -1  -1  -1 426  -1  -1  -1  -1  -1
1260|  -1  -1 427  -1  -1  -1  -1  -1  -1  -1 428  -1  -1  -1  -1  -1  -1  -1 429  -1
1280|  -1  -1  -1  -1  -1  -1 430  -1  -1  -1  -1  -1  -1  -1 431  -1  -1  -1  -1  -1
1300|  -1  -1 432  -1  -1  -1  -1  -1  -1  -1 433  -1  -1  -1  -1  -1  -1  -1 434  -1
1320|  -1  -1  -1  -1  -1  -1 435  -1  -1  -1  -1  -1  -1  -1 436  -1  -1  -1  -1  -1
1340|  -1  -1 437  -1  -1  -1  -1  -1  -1  -1 438  -1  -1  -1  -1  -1  -1  -1 439  -1
1360|  -1  -1  -1  -1  -1  -1 440  -1  -1  -1  -1  -1  -1  -1 441  -1  -1  -1  -1  -1
1380|  -1  -1 442  -1  -1  -1  -1  -1  -1  -1 443  -1  -1  -1  -1  -1  -1  -1 444  -1
1400|  -1  -1  -1  -1  -1  -1 445  -1  -1  -1  -1  -1  -1  -1 446  -1  -1  -1  -1  -1
1420|  -1  -1 447  -1  -1  -1  -1  -1  -1  -1 448  -1  -1  -1  -1  -1  -1  -1 449  -1
1440|  -1  -1  -1  -1  -1  -1 450  -1  -1  -1  -1  -1  -1  -1 451  -1  -1  -1  -1  -1
1460|  -1  -1 452  -1  -1  -1  -1  -1  -1  -1 453  -1  -1  -1  -1  -1  -1  -1 454  -1
1480|  -1  -1  -1  -1  -1  -1 455  -1  -1  -1  -1  -1  -1  -1 456  -1  -1  -1  -1  -1
1500|  -1  -1 457  -1  -1  -1  -1  -1  -1  -1 458  -1  -1  -1  -1  -1  -1  -1 459  -1
1520|  -1  -1  -1  -1  -1  -1 460  -1  -1  -1  -1  -1  -1  -1 461  -1  -1  -1  -1  -1
1540|  -1  -1 463  -1  -1  -1  -1  -1  -1  -1 464  -1  -1  -1  -1  -1  -1  -1 465  -1
1560|  -1  -1  -1  -1  -1  -1 466  -1  -1  -1  -1  -1  -1  -1 467  -1  -1  -1  -1  -1
1580|  -1  -1 468  -1  -1  -1  -1  -1  -1  -1 469  -1  -1  -1  -1  -1  -1  -1 470  -1
1600|  -1  -1  -1  -1  -1  -1 471  -1  -1  -1  -1  -1  -1  -1 472  -1  -1  -1  -1  -1
1620|  -1  -1 473  -1  -1  -1  -1  -1  -1  -1 474  -1  -1  -1 475 476 477 478 479 485
1640| 486 487 488 489  -1  -1  -1  -1  -1  -1 490  -1  -1  -1  -1  -1  -1  -1 491  -1
1660|  -1  -1  -1  -1  -1  -1 492  -1  -1  -1  -1  -1  -1  -1 493  -1  -1  -1  -1  -1
1680|  -1  -1 494  -1  -1  -1  -1  -1  -1  -1 495  -1  -1  -1  -1  -1  -1  -1 496  -1
1700|  -1  -1  -1  -1  -1  -1 497  -1  -1  -1  -1  -1  -1  -1 498  -1  -1  -1  -1  -1
1720|  -1  -1 499  -1  -1  -1  -1  -1  -1  -1 500  -1  -1  -1  -1  -1  -1  -1 501  -1
1740|  -1  -1  -1  -1  -1  -1 502  -1  -1  -1 521  -1  -1  -1 480  -1  -1  -1  -1  -1
1760|  -1  -1 481  -1  -1  -1  -1  -1  -1  -1 482  -1  -1  -1  -1  -1  -1  -1 483  -1
1780|  -1  -1  -1  -1  -1  -1 484  -1  -1  -1  -1  -1  -1  -1 503  -1  -1  -1  -1  -1
1800|  -1  -1 504  -1  -1  -1  -1  -1  -1  -1 505  -1  -1  -1  -1  -1  -1  -1 506  -1
1820|  -1  -1  -1  -1  -1  -1 507  -1  -1  -1  -1  -1  -1  -1 508  -1  -1  -1  -1  -1
1840|  -1  -1 509  -1  -1  -1  -1  -1  -1  -1 510  -1  -1  -1  -1  -1  -1  -1 511  -1
1860|  -1  -1  -1  -1  -1  -1 512  -1  -1  -1  -1  -1  -1  -1 513  -1  -1  -1  -1  -1
1880|  -1  -1 514  -1  -1  -1  -1  -1  -1  -1 515  -1  -1  -1  -1  -1  -1  -1 516  -1
1900|  -1  -1  -1  -1  -1  -1 517  -1  -1  -1  -1  -1  -1  -1 518  -1  -1  -1  -1  -1


Marshalling Block sequence map =
    |   0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19
----+--------------------------------------------------------------------------------
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Re: Example VDI Defragmentation Utility

Postby techtonik » 7. Mar 2009, 12:23

Have you found the case where VDI become corrupted? Is because some information is missing from VDI description (garbage fields) or because of script logic?
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Re: Example VDI Defragmentation Utility

Postby TerryE » 7. Mar 2009, 16:29

Yes, I have. IIRC, it was due to rounding error in that VBox always aligns the start of data to the next sector. However, with Vbox 2.1, the "write to VDI" process now optimises away writes of zeros to an unallocate 1Mb page and the clonehd basically just does a dd from the old image to the new, so doing a clonehd now does compact and defrag on the fly, and this script is redundant.
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Re: Example VDI Defragmentation Utility

Postby techtonik » 8. Mar 2009, 10:12

I've heard that clonehd doesn't preserve UUID of cloned image. In addition it still requires a lot of free space (and a lot of time to find out). I would still prefer a script that does the same in place, because when you need to compact the image - you've probably run out of space completely.

Looking at VDI tutorial, can you clarify why there is a second copy of block mapping table? I.e. what is the second 4N in the header size calculator expression
Hsize = 512 + 4N + (508 + 4N) mod 512
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Re: Example VDI Defragmentation Utility

Postby TerryE » 8. Mar 2009, 17:49

techtonik wrote:I've heard that clonehd doesn't preserve UUID of cloned image.

Correct. It creates a new copy of the disk image with its own UUID identity. In essence it does the same as if you attached the old disk + a new one of the same size to a LiveCD VM and did a dd from the old to new. Any zero 1Mb pages would be optimised out in the copy.

Yes, an in place defrag is entirely possible, but this has two drawbacks: (i) the shuffle down process to move used blocks from the end of the VDI to empty blocks in the compacted VDI increases VDI fragmentation (though this isn't a huge performance issue); (ii) we would need to do some transactional analysis and think about storing intermediate copies of the the block mapping table so that we could recover from system failure during the compact. I feel that this second issue is important since IMHO the probability of loosing your FS if your system craters during compaction is unacceptable.

I have also come to think that having a policy of regular compaction is wrong. The designers of file systems assume that the have full and equal access to the entire partition and use assumption in their optimisation of FS performance. So you should assume that over time (and in fact quite rapidly) every 1Mb block in every partition will become dirty. The VDI will grow to the size of its active partitions. If you have allocated a larger disk for growth contingency then this is a separate issue. My approach is to allocate adequate space for each partition but interleave the partitions with unallocated space (perhaps 100-200%). This unallocated space will not be used and therefore never need to be compacted. However, if it becomes clear that you need to increase the allocation to a partition, there are a number of Windows and Linux LiveCD tools that can do this safely in a matter of minutes. Since you have dead space between partitions to grow into, this is a really quick operation as no data blocks need to be relocated.

The only time I want to compact a file system is prior to distribution especially if I intend to use it as a snapshot baseline,

What you do need to do in the case of NTFS is to defrag the partition regularly as NTFS is appalling at controlling file fragmentation. Also if you use a compressing backup, then run a tool such as sDelete or on Linux do the dd if=/dev/zero trick to zero all unallocated blocks before backing up the VDI.

techtonik wrote:Looking at VDI tutorial, can you clarify why there is a second copy of block mapping table? I.e. what is the second 4N in the header size calculator expression: Hsize = 512 + 4N + (508 + 4N) mod 512

If you look at the code, I don't actually use this. There is also a typo in it. Sorry! It should read Hsize = 512 + 4N - (508 + 4N) mod 512. This is just another way of saying that 4N is rounded up to a 512 byte boundary.
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Re: Example VDI Defragmentation Utility

Postby jimklimov » 12. Jul 2009, 21:04

> So you should assume that over time (and in fact quite rapidly) every 1Mb block in every partition will become dirty
Concerning this line: I believe the "dirtiness" will progress as you defragment the VM's "guest filesystem" (i.e. NTFS in the virtual WinXP).

However I think there are some tools out there which can be used to zero out unallocated blocks, such as wipeinfo/wipedisk for windows.
If this can be used to minimize "golden image VIDs" (i.e. set up a system or new patches, defragment, wipe unallocated blocks; VDI is ready for cloning) - it is a benefit.

My questions are:
1) If a previously allocated large block of a VM's filesystem is overwritten with zero's (by the guest OS), will the sparse VDI become smaller?
2) Can the readers of this recommend known-safe wiping tools which can write zero's, for all the popular filesystems out there (FAT, NTFS, ext*, reiserfs, UFS, ZFS and so on)? By "safe" I mean that they don't delete the whole disk on occasion; and by "writing zeros" - that they have this mode, not the secure random DOD codes for anti-undeletion protection ;)
3) Would this actually be useful, beside staring off into the graybarland? ^)

//Jim

PS: I've seen the relevant post in All about VDIs tutorial (http://forums.virtualbox.org/viewtopic.php?p=29272#p29272).
It does explain much, but does not offer tools for other filesystems than NTFS and ext3. So parts of my question still hold :)
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Re: Example VDI Defragmentation Utility

Postby baf » 12. Jul 2009, 21:28

1) No it won't shrink. But a clone will now be smaller.

2) sdelete works on windows. zerofree on ext*. No more known to me.

3) Yes before cloning.
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Re: Example VDI Defragmentation Utility

Postby TerryE » 13. Jul 2009, 00:51

+1. sDelete is a Sysinternals (now part of MS) utility. Defrag before you use this. sdelete is now on debian distros so for ubuntu and debian at least you can just do a sudo apt-get install zerofree to get this one. zerofree need to be run with the target fs either unmounted or mounted ro.

Clonehd optimises out writes of zeros to an as yet unallocated 1Mb page, so running defrag+sdelete+clonehd will free up space in the target FS.
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Re: Example VDI Defragmentation Utility

Postby kernelOfTruth » 10. Aug 2010, 16:40

I don't know if it still works like in December 2008 but Will Kemp seems to have a nice solution (also) based on sdelete and VBoxManage:

http://novemberechoromeodelta.com/20081220-shrinking-virtualbox-vdi-files

cheers ! :)
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Re: Example VDI Defragmentation Utility

Postby stefan.becker » 10. Aug 2010, 19:15

This is the approach posted by Fred Flintstone 1000 of years before. Part of the Howto for Zuse Linux.

Search forum for CloneVDI.
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