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
