linux-ext4 - mke2fs and -E no discard

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Date:   Tue, 17 Apr 2018 11:58:16 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: mke2fs and -E no discard

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@...kie 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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