Re: Interesting problem with write data.

[Chris Mason <chris.mason@oracle.com>]

Excerpts from Tomasz Chmielewski's message of 2010-11-18 11:00:16 -0500:
> On 18.11.2010 16:54, Chris Mason wrote:
> > Excerpts from Tomasz Chmielewski's message of 2010-11-18 10:39:05 -0500:
> >> On 18.11.2010 16:07, Chris Mason wrote:
> >>
> >> (...)
> >>
> >>>> [27821.906513] btrfs-cache-8 D ffff88050c5fde98     0  8089      2 0x00000000
> >>>> [27821.906517]  ffff88051c3a9b60 0000000000000046 ffff88051c3a9b00 ffff88051c3a9fd8
> >>>> [27821.906522]  00000000000139c0 00000000000139c0 ffff88051c3a9fd8 ffff88051c3a9fd8
> >>>> [27821.906526]  00000000000139c0 ffff88050c5fde98 ffff88050c5fdea0 ffff88050c5fdb00
> >>>> [27821.906530] Call Trace:
> >>>> [27821.906534]  [<ffffffff8159fc4e>] io_schedule+0x5e/0xa0
> >>>> [27821.906538]  [<ffffffff81109f15>] sync_page+0x45/0x60
> >>>
> >>> So, you're caching block groups.  What you want to do is use Josef's new
> >>> block group caching code.
> >>>
> >>> mount -o space_cache /dev/xxx
> >>>
> >>> Do the test and let the caching threads finish, then unmount and then
> >>> your next run should be fast.
> >>
> >> # mount -o space_cache /dev/sdb4 /mnt/btrfs/
> >> [29720.305741] btrfs: enabling disk space caching
> >> [29720.305743] btrfs: force clearing of disk cache
> >>
> >>
> >> I don't see any difference in behaviour with this mount option; it still
> >> "hangs" for quite a bit at around ~1.8 GB (and reading with ~500 kB/s
> >> when the hangs happens) on a subsequent dd run (several runs,
> >> unmounts/mounts).
> >
> > Right, you have to wait for all the caching threads to finish before you
> > unmount.
> 
> How do I find out? Is non changing reads/writes enough?

Look for whose name starts with btrfs-cache

-chris