/preallocate

preallocate

Primary LanguageCMIT LicenseMIT

preallocate

IO helper for suppress fragmentation of large files(backup, Blu-ray image, SDHC-card etc).

(or thing just I using daily basis)

what the heck is this for?

for example:

$ dd if=/dev/vg/lv_large_snapshot bs=4096 | \
    gzip > /mnt/backup/backup_2020_06_06.gz

depend on /mnt/backup contents however this is not good in term of fragmentation. because bash(or zsh or whatever) is no way to know > /mnt/backup/backup_2020_06_06.gz's size, before work done. it's mean, no way to use fallocate().

let's see actual 60GB file example.

$ dd if=/dev/vg/lv_large_snapshot bs=4096 | \
  gzip | dd of=/mnt/backup/backup_2020_06_06.gz status=progress
~~snip~~
$
$ filefrag -v /mnt/backup/backup_2020_06_06.gz
Filesystem type is: 58465342
File size of /mnt/backup/backup_2020_06_06.gz is 65143209044 (15904104 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..  262127:  142449069.. 142711196: 262128:
   1:   262128.. 1048559:  146483443.. 147269874: 786432:  142711197:
   2:  1048560.. 2097135:  148304107.. 149352682: 1048576:  147269875:
   3:  2097136.. 3145711:  149352683.. 150401258: 1048576:
   4:  3145712.. 5242862:   73590272..  75687422: 2097151:  150401259:
   5:  5242863.. 7145021:    2604764..   4506922: 1902159:   75687423:
   6:  7145022.. 7339998:    2080376..   2275352: 194977:    4506923:
   7:  7339999.. 8388559:   24583686..  25632246: 1048561:    2275353:
   8:  8388560.. 9437135:   66764570..  67813145: 1048576:   25632247:
   9:  9437136..10938887:  132545031.. 134046782: 1501752:   67813146:
  10: 10938888..11534286:  134060490.. 134655888: 595399:  134046783:
  11: 11534287..12582847:  150401259.. 151449819: 1048561:  134655889:
  12: 12582848..13631423:  156011326.. 157059901: 1048576:  151449820:
  13: 13631424..15068563:   71649882..  73087021: 1437140:  157059902:
  14: 15068564..15728574:   70674710..  71334720: 660011:   73087022:
  15: 15728575..15904103:   95949946..  96125474: 175529:   71334721: last,eof
/mnt/backup/backup_2020_06_06.gz: 15 extents found

filesystem is XFS, about 90% used. file is fragmented in this case, compared to

$ rm /mnt/backup/backup_2020_06_06.gz
$ sync
$ fallocate -l 65143209044 /mnt/backup/backup_2020_06_06.gz
$ filefrag -v /mnt/backup/backup_2020_06_06.gz
Filesystem type is: 58465342
File size of /mnt/backup/backup_2020_06_06.gz is 65143209044 (15904104 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0.. 2097150:  148304107.. 150401257: 2097151:             unwritten
   1:  2097151.. 4194301:   73590272..  75687422: 2097151:  150401258: unwritten
   2:  4194302.. 6096460:    2604764..   4506922: 1902159:   75687423: unwritten
   3:  6096461.. 7598212:  132545031.. 134046782: 1501752:    4506923: unwritten
   4:  7598213.. 9035352:   71649882..  73087021: 1437140:  134046783: unwritten
   5:  9035353..10451117:  150401258.. 151817022: 1415765:   73087022: unwritten
   6: 10451118..11756279:  146483443.. 147788604: 1305162:  151817023: unwritten
   7: 11756280..13031353:  156011326.. 157286399: 1275074:  147788605: unwritten
   8: 13031354..14221255:   24583686..  25773587: 1189902:  157286400: unwritten
   9: 14221256..15299485:   66764570..  67842799: 1078230:   25773588: unwritten
  10: 15299486..15904103:   70059668..  70664285: 604618:   67842800: last,unwritten,eof
/mnt/backup/backup_2020_06_06.gz: 11 extents found
$
$ sudo xfs_fsr -v /mnt/backup/backup_2020_06_06.gz
No improvement will be made (skipping): /mnt/backup/backup_2020_06_06.gz
$
$ dd if=/dev/vg/lv_large_snapshot bs=4096 | gzip | \
  dd of=/mnt/backup/backup_2020_06_06.gz conv=notrunc status=progress
...

notice length column became descending order. because allocate as large-freespace as possible first (at least with XFS). well done fallocate. (also conv=notrunc option was used with dd)

reason for do this is not fragmentation of large file, but keep small free space for small file.

effectivery preallocate is same thing but one line.

$ dd if=/dev/vg/lv_large_snapshot bs=4096 | \
  gzip | preallocate -l 60gb /mnt/backup/backup_2020_06_06.gz

-l 60gb is kind of hint to how many extents require. no need to exactly.

this is actual code without error handling.

static int preallocate_io(int fd, off_t len, int opt_sync)
{
    // allocate
    fallocate(fd, 0, 0L, len);

    // read/write stream
    off_t wrote = do_read_write(fd, opt_sync);
 
    // truncate
    if (wrote < len) {
        ftruncate(fd, wrote);
    }
    return 0;
}

fiemap script

fiemap.py spits out image of extents in device.

$ python fiemap/fiemap.py <mount-point or dir or file>

TLDR;

  • keep small free space for small files
  • file fragmentation is caused by free space fragmentation
  • free space fragmentation is not easily avoidable, because people delete file.
  • delayed allocation desen't much help when you dealing with single 100GiB file
  • (for SSD, not much difference I guess)

link