Tomasz Chmielewski
2011-May-21 12:58 UTC
curious writes on mounted, not used btrfs filesystem
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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Stephane Chazelas
2011-May-22 09:34 UTC
Re: curious writes on mounted, not used btrfs filesystem
2011-05-21 14:58:21 +0200, Tomasz Chmielewski:> I have a btrfs filesystem (2.6.39) which is mounted, but otherwise, not used: > > # lsof -n|grep /mnt/btrfsprocesses with open fds are one thing. You could also have loop devices setup on it for instance.> # > > > I noticed that whenever I do "sync", btrfs will write for around 6.5s and write 13 MB (see below).[...] You could try and play with /proc/sys/vm/block_dump to see what is being written (remember to disable logging of kernel messages by syslog). -- Stephane -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Tomasz Chmielewski
2011-May-22 09:52 UTC
Re: curious writes on mounted, not used btrfs filesystem
On 22.05.2011 11:34, Stephane Chazelas wrote:> 2011-05-21 14:58:21 +0200, Tomasz Chmielewski: >> I have a btrfs filesystem (2.6.39) which is mounted, but otherwise, not used: >> >> # lsof -n|grep /mnt/btrfs > > processes with open fds are one thing. You could also have loop > devices setup on it for instance.I know. It''s _not_ used by anything.>> # >> >> >> I noticed that whenever I do "sync", btrfs will write for around 6.5s and write 13 MB (see below). > [...] > > You could try and play with /proc/sys/vm/block_dump to see what > is being written (remember to disable logging of kernel messages > by syslog).It doesn''t give me any hints, really. Can you try running these commands yourself: iostat -k 1 </your/btrfs/device> And in a second terminal: while true; do sync ; done To see if your btrfs makes writes on sync each time? -- Tomasz Chmielewski http://wpkg.org -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Swâmi Petaramesh
2011-May-22 15:21 UTC
Re: curious writes on mounted, not used btrfs filesystem
Le dimanche 22 mai 2011 à 10:34 +0100, Stephane Chazelas a écrit :> > > > I noticed that whenever I do "sync", btrfs will write for around 6.5s and write 13 MB (see below). > [...]I had also noticed here, without really paying further attention, that external USB HDs with a BTRFS mounted, but without any open files (and no possibly open files, these disks being only used for files backups and no direct work, no libraries, no spools, no progs, etc...), show some activity (LED flashing) when executing a "sync" when I would expect there''s nothing to be synced on them... -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Stephane Chazelas
2011-May-22 18:26 UTC
Re: curious writes on mounted, not used btrfs filesystem
2011-05-22 11:52:37 +0200, Tomasz Chmielewski: [...]> Can you try running these commands yourself: > > iostat -k 1 </your/btrfs/device> > > > And in a second terminal: > > while true; do sync ; done > > > To see if your btrfs makes writes on sync each time?[...] Yes it does. And I see: <7>[38554.244219] sync(3354): dirtied inode 1 (?) on dm-11 <7>[38554.244740] btrfs-submit-0(29134): WRITE block 301128 on dm-11 (56 sectors) <7>[38554.244774] btrfs-submit-0(29134): WRITE block 741832 on dm-11 (56 sectors) <7>[38554.249963] sync(3354): WRITE block 128 on dm-11 (8 sectors) <7>[38554.250010] sync(3354): WRITE block 131072 on dm-11 (8 sectors) <7>[38567.312908] sync(3356): dirtied inode 1 (?) on dm-11 <7>[38567.313330] btrfs-submit-0(29134): WRITE block 301256 on dm-11 (24 sectors) <7>[38567.313350] btrfs-submit-0(29134): WRITE block 741960 on dm-11 (24 sectors) <7>[38567.313358] btrfs-submit-0(29134): WRITE block 301288 on dm-11 (24 sectors) <7>[38567.313366] btrfs-submit-0(29134): WRITE block 741992 on dm-11 (24 sectors) <7>[38567.313393] btrfs-submit-0(29134): WRITE block 301312 on dm-11 (8 sectors) <7>[38567.313403] btrfs-submit-0(29134): WRITE block 742016 on dm-11 (8 sectors) <7>[38567.325194] sync(3356): WRITE block 128 on dm-11 (8 sectors) <7>[38567.325244] sync(3356): WRITE block 131072 on dm-11 (8 sectors) <7>[38570.374449] sync(3358): dirtied inode 1 (?) on dm-11 <7>[38570.374976] btrfs-submit-0(29134): WRITE block 301128 on dm-11 (56 sectors) <7>[38570.375011] btrfs-submit-0(29134): WRITE block 741832 on dm-11 (56 sectors) <7>[38570.379221] sync(3358): WRITE block 128 on dm-11 (8 sectors) <7>[38570.379272] sync(3358): WRITE block 131072 on dm-11 (8 sectors) <7>[38572.170816] sync(3359): dirtied inode 1 (?) on dm-11 <7>[38572.171289] btrfs-submit-0(29134): WRITE block 301256 on dm-11 (24 sectors) <7>[38572.171300] btrfs-submit-0(29134): WRITE block 741960 on dm-11 (24 sectors) <7>[38572.171304] btrfs-submit-0(29134): WRITE block 301288 on dm-11 (24 sectors) <7>[38572.171308] btrfs-submit-0(29134): WRITE block 741992 on dm-11 (24 sectors) <7>[38572.171320] btrfs-submit-0(29134): WRITE block 301312 on dm-11 (8 sectors) <7>[38572.171325] btrfs-submit-0(29134): WRITE block 742016 on dm-11 (8 sectors) <7>[38572.180338] sync(3359): WRITE block 128 on dm-11 (8 sectors) <7>[38572.180386] sync(3359): WRITE block 131072 on dm-11 (8 sectors) <7>[38574.186559] sync(3360): dirtied inode 1 (?) on dm-11 <7>[38574.187090] btrfs-submit-0(29134): WRITE block 301128 on dm-11 (56 sectors) <7>[38574.187125] btrfs-submit-0(29134): WRITE block 741832 on dm-11 (56 sectors) <7>[38574.191602] sync(3360): WRITE block 128 on dm-11 (8 sectors) <7>[38574.191654] sync(3360): WRITE block 131072 on dm-11 (8 sectors) <7>[38576.370003] sync(3361): dirtied inode 1 (?) on dm-11 <7>[38576.370452] btrfs-submit-0(29134): WRITE block 301256 on dm-11 (24 sectors) <7>[38576.370470] btrfs-submit-0(29134): WRITE block 741960 on dm-11 (24 sectors) <7>[38576.370478] btrfs-submit-0(29134): WRITE block 301288 on dm-11 (24 sectors) <7>[38576.370485] btrfs-submit-0(29134): WRITE block 741992 on dm-11 (24 sectors) <7>[38576.370513] btrfs-submit-0(29134): WRITE block 301312 on dm-11 (8 sectors) <7>[38576.370523] btrfs-submit-0(29134): WRITE block 742016 on dm-11 (8 sectors) <7>[38576.379718] sync(3361): WRITE block 128 on dm-11 (8 sectors) <7>[38576.379766] sync(3361): WRITE block 131072 on dm-11 (8 sectors) Every other sync the same. -- Stephane -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html