I was just looking at a posting on stackexchange related to mke2fs
and filesystem image fragmentation:
https://unix.stackexchange.com/questions/287133/why-is-fragmentation-level-so-huge-in-files-that-contain-other-filesystems
It seems like "mke2fs" is causing issues for creation of filesystems in
preallocated files, as it is discarding the previously-allocated blocks,
even if the "-E nodiscard" option is used.
# dd if=/dev/zero of=/var/tmp/tt bs=1M count=100
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 0.272006 s, 385 MB/s
# sync
# filefrag -v /var/tmp/tt
Filesystem type is: ef53
File size of /var/tmp/tt is 104857600 (25600 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected: flags:
0: 0.. 24575: 1261568.. 1286143: 24576:
1: 24576.. 25599: 1292288.. 1293311: 1024: 1286144: last,eof
/var/tmp/tt: 2 extents found
# ./misc/mke2fs -E nodiscard /var/tmp/tt
mke2fs 1.43-WIP (15-Mar-2016)
Creating filesystem with 102400 1k blocks and 25688 inodes
Filesystem UUID: 3f8a0ca8-70b8-4801-b834-99dff0a6a642
Superblock backups stored on blocks:
8193, 24577, 40961, 57345, 73729
Allocating group tables: done
Writing inode tables: done
Writing superblocks and filesystem accounting information: done
[root@mookie e2fsprogs-git]# filefrag -v /var/tmp/tt
Filesystem type is: ef53
File size of /var/tmp/tt is 104857600 (25600 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected: flags:
0: 0.. 65: 1261568.. 1261633: 66:
1: 127.. 2113: 1261695.. 1263681: 1987: 1261634:
2: 2175.. 4096: 1263743.. 1265664: 1922: 1263682:
3: 4158.. 6209: 1265726.. 1267777: 2052: 1265665:
4: 6271.. 8192: 1267839.. 1269760: 1922: 1267778:
5: 8254.. 10305: 1269822.. 1271873: 2052: 1269761:
6: 10367.. 12288: 1271935.. 1273856: 1922: 1271874:
7: 12350.. 14401: 1273918.. 1275969: 2052: 1273857:
8: 14463.. 16384: 1276031.. 1277952: 1922: 1275970:
9: 16446.. 18497: 1278014.. 1280065: 2052: 1277953:
10: 18559.. 20480: 1280127.. 1282048: 1922: 1280066:
11: 20542.. 22528: 1282110.. 1284096: 1987: 1282049:
12: 22590.. 24575: 1284158.. 1286143: 1986: 1284097:
13: 24576.. 24576: 1292288.. 1292288: 1: 1286144:
14: 24638.. 25583: 1292350.. 1293295: 946: 1292289: last
/var/tmp/tt: 15 extents found
Without "-E nodiscard", mke2fs prints the message "Discarding device blocks:
done" and calls fallocate() (from strace):
fallocate(3, 03, 0, 1048576) = 0
fallocate(3, 03, 1048576, 103809024) = 0
which is NOT called in the "-E nodiscard" case, but it somehow is discarding
the allocated blocks anyway during inode table allocation:
write(1, "Writing inode tables: ", 22) = 22
write(1, " 0/13", 5) = 5
write(1, "\10\10\10\10\10", 5) = 5
fstat(3, {st_mode=S_IFREG|0644, st_size=104857600, ...}) = 0
fallocate(3, 03, 267264, 252928) = 0
fstat(3, {st_mode=S_IFREG|0644, st_size=104857600, ...}) = 0
fallocate(3, 03, 8655872, 252928) = 0
fstat(3, {st_mode=S_IFREG|0644, st_size=104857600, ...}) = 0
fallocate(3, 03, 16780288, 252928) = 0
:
:
Cheers, Andreas
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