I have a btrfs filesystem (2.6.39) which is mounted, but otherwise, not used:
# lsof -n|grep /mnt/btrfs
#
I noticed that whenever I do "sync", btrfs will write for around 6.5s and write 13 MB (see below).
If I do a "while true; do sync; done", there will be constant writes to btrfs filesystem (again - with no userspace program accessing btrfs).
When it happens, "btrfs-submit-0" is in "D" state.
What are these writes? I didn't notice anything like this with other filesystems.
$ iostat -t -k 1 sdb4|grep sdb4
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb4 320.00 0.00 1660.00 0 1660
sdb4 419.00 0.00 1720.00 0 1720
sdb4 291.00 0.00 1224.00 0 1224
sdb4 417.00 0.00 1736.00 0 1736
sdb4 609.00 0.00 2628.00 0 2628
sdb4 350.00 0.00 1452.00 0 1452
sdb4 606.00 0.00 2620.00 0 2620
sdb4 281.00 0.00 1516.00 0 1516
sdb4 352.00 0.00 1472.00 0 1472
sdb4 384.00 0.00 1584.00 0 1584
sdb4 533.00 0.00 2232.00 0 2232
sdb4 640.00 0.00 2660.00 0 2660
When btrfs is mounted with "compress" flag, there will be also reads when doing a similar operation.
See MB_wrtn before/after the sync:
$ iostat -t -m sdb4
Linux 2.6.39-020639-generic (dom) 05/21/2011 _x86_64_ (8 CPU)
05/21/2011 02:45:46 PM
avg-cpu: %user %nice %system %iowait %steal %idle
2.47 0.01 1.25 8.01 0.00 88.26
Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn
sdb4 127.39 0.92 1.94 94207 199343
$ time sync
real 0m6.808s
user 0m0.000s
sys 0m0.110s
$ iostat -t -m sdb4
Linux 2.6.39-020639-generic (dom) 05/21/2011 _x86_64_ (8 CPU)
05/21/2011 02:46:02 PM
avg-cpu: %user %nice %system %iowait %steal %idle
2.47 0.01 1.25 8.01 0.00 88.26
Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn
sdb4 127.40 0.92 1.94 94207 199356
$ time sync
real 0m6.628s
user 0m0.000s
sys 0m0.110s
$ iostat -t -m sdb4
Linux 2.6.39-020639-generic (dom) 05/21/2011 _x86_64_ (8 CPU)
05/21/2011 02:46:55 PM
avg-cpu: %user %nice %system %iowait %steal %idle
2.47 0.01 1.25 8.01 0.00 88.27
Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn
sdb4 127.37 0.92 1.94 94207 199369
--
Tomasz Chmielewski
http://wpkg.org
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