Hi,

This patch gives userspace ability to prevent underlying file of loop
device to be sparse. Currently, if loop device is 'trimmed'
(BLKDISCARD), then underlying file will always become sparse and disk
space freed. This behaviour is good for thin provisioning but not good
for preallocated disk images. This patch introduces LO_FLAGS_NO_DEALLOC
flag, which will write zeroes to underlying file instead of punching
holes when BLKDISCARD is requested. Thus reducing file fragmentation of
preallocated disk images and improve performance.

I will also submit patches to util-linux to provide userspace support if
this patch is merged.

Changes in V1->V2:
Renamed NODEALLOC to NO_DEALLOC, to avoid confusion between NO_DEALLOC
and NODE_ALLOC. Suggested by Jens Axboe.

Best Regards,
Zhang Boyang

On 2022/8/6 23:30, Zhang Boyang wrote:
> I will also submit patches to util-linux to provide userspace support if
> this patch is merged.

For anyone who want to test the LO_FLAGS_NO_DEALLOC feature, the 
userspace support patch for util-linux is done at 
https://github.com/zhangboyang/util-linux/tree/losetup-nodealloc-202207

Below is an example typescript. In the example, a 10G test file is 
created and used as backing file of /dev/loop0. Then the mkfs.ext4 will 
discard most blocks in /dev/loop0. As you can see in the output of `du' 
and `filefrag', the extents is not freed nor changed location, thus file 
fragmentation is avoided. (The filesystem of backing file is ext4. 
Things are different in XFS or Btrfs, they tend to allocate new extents 
elsewhere when doing FALLOC_FL_ZERO_RANGE.)

Best Regards,
Zhang Boyang


root@debian:~# fallocate -l 10G /data/test
root@debian:~# du -sh /data/test
11G	/data/test
root@debian:~# filefrag -v /data/test
Filesystem type is: ef53
File size of /data/test is 10737418240 (2621440 blocks of 4096 bytes)
  ext:     logical_offset:        physical_offset: length:   expected: 
flags:
    0:        0..   32767:   37715968..  37748735:  32768: 
unwritten
    1:    32768..  524287:   37781504..  38273023: 491520:   37748736: 
unwritten
    2:   524288.. 1015807:   38305792..  38797311: 491520:   38273024: 
unwritten
    3:  1015808.. 1507327:   38830080..  39321599: 491520:   38797312: 
unwritten
    4:  1507328.. 1998847:   39354368..  39845887: 491520:   39321600: 
unwritten
    5:  1998848.. 2490367:   39878656..  40370175: 491520:   39845888: 
unwritten
    6:  2490368.. 2621439:   40402944..  40534015: 131072:   40370176: 
last,unwritten,eof
/data/test: 7 extents found
root@debian:~# ./losetup -f --no-dealloc /data/test
root@debian:~# ./losetup
NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE  DIO NO-DEALLOC LOG-SEC
/dev/loop0         0      0         0  0 /data/test   0          1     512
root@debian:~# mkfs.ext4 /dev/loop0
mke2fs 1.46.2 (28-Feb-2021)
Discarding device blocks: done
Creating filesystem with 2621440 4k blocks and 655360 inodes
Filesystem UUID: 4abd9157-6e40-4842-bb33-e6807fc5c231
Superblock backups stored on blocks:
	32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632

Allocating group tables: done
Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done

root@debian:~# ./losetup -d /dev/loop0
root@debian:~# du -sh /data/test
11G	/data/test
root@debian:~# filefrag -v /data/test
Filesystem type is: ef53
File size of /data/test is 10737418240 (2621440 blocks of 4096 bytes)
  ext:     logical_offset:        physical_offset: length:   expected: 
flags:
    0:        0..    1027:   37715968..  37716995:   1028:
    1:     1028..    1042:   37716996..  37717010:     15: 
unwritten
    2:     1043..    1043:   37717011..  37717011:      1:
    3:     1044..    1058:   37717012..  37717026:     15: 
unwritten
    4:     1059..    1059:   37717027..  37717027:      1:
    5:     1060..    9250:   37717028..  37725218:   8191: 
unwritten
    6:     9251..    9256:   37725219..  37725224:      6:
    7:     9257..   32767:   37725225..  37748735:  23511: 
unwritten
    8:    32768..   32770:   37781504..  37781506:      3:   37748736:
    9:    32771..   98303:   37781507..  37847039:  65533: 
unwritten
   10:    98304..   98306:   37847040..  37847042:      3:
   11:    98307..  163839:   37847043..  37912575:  65533: 
unwritten
   12:   163840..  163842:   37912576..  37912578:      3:
   13:   163843..  229375:   37912579..  37978111:  65533: 
unwritten
   14:   229376..  229378:   37978112..  37978114:      3:
   15:   229379..  294911:   37978115..  38043647:  65533: 
unwritten
   16:   294912..  294914:   38043648..  38043650:      3:
   17:   294915..  524287:   38043651..  38273023: 229373: 
unwritten
   18:   524288..  524288:   38305792..  38305792:      1:   38273024:
   19:   524289..  819199:   38305793..  38600703: 294911: 
unwritten
   20:   819200..  819202:   38600704..  38600706:      3:
   21:   819203..  884735:   38600707..  38666239:  65533: 
unwritten
   22:   884736..  884738:   38666240..  38666242:      3:
   23:   884739.. 1015807:   38666243..  38797311: 131069: 
unwritten
   24:  1015808.. 1048575:   38830080..  38862847:  32768:   38797312: 
unwritten
   25:  1048576.. 1048577:   38862848..  38862849:      2:
   26:  1048578.. 1081343:   38862850..  38895615:  32766: 
unwritten
   27:  1081344.. 1081344:   38895616..  38895616:      1:
   28:  1081345.. 1507327:   38895617..  39321599: 425983: 
unwritten
   29:  1507328.. 1572863:   39354368..  39419903:  65536:   39321600: 
unwritten
   30:  1572864.. 1572864:   39419904..  39419904:      1:
   31:  1572865.. 1605631:   39419905..  39452671:  32767: 
unwritten
   32:  1605632.. 1605634:   39452672..  39452674:      3:
   33:  1605635.. 1998847:   39452675..  39845887: 393213: 
unwritten
   34:  1998848.. 2097151:   39878656..  39976959:  98304:   39845888: 
unwritten
   35:  2097152.. 2097152:   39976960..  39976960:      1:
   36:  2097153.. 2097166:   39976961..  39976974:     14: 
unwritten
   37:  2097167.. 2097167:   39976975..  39976975:      1:
   38:  2097168.. 2490367:   39976976..  40370175: 393200: 
unwritten
   39:  2490368.. 2621439:   40402944..  40534015: 131072:   40370176: 
last,unwritten,eof
/data/test: 7 extents found