hello,
On 09/09/2016 06:56 PM, Holger Hoffstätte wrote:
On 09/09/16 12:18, Holger Hoffstätte wrote:
On Fri, 09 Sep 2016 16:17:48 +0800, Wang Xiaoguang wrote:
cleaner_kthread() may run at any time, in which it'll call
btrfs_delete_unused_bgs()
to delete unused block groups. Because this work is asynchronous, it may also
result
in false ENOSPC error.
<snip>
With this v3 I can now no longer balance (tested only with metadata).
New chunks are allocated (as balance does) but nothing ever shrinks, until
after unmount/remount, when the cleaner eventually kicks in.
This might be related to the recent patch by Naohiro Aota:
"btrfs: let btrfs_delete_unused_bgs() to clean relocated bgs"
which by itself doesn't seem to do any harm (i.e. everything still seems
to work as expected).
Actually even that is not true; both patches seem to be wrong in subtle
ways. Naohiro's patch seems to prevent the deletion during balance, whereas
yours prevents the cleaner from kicking in.
Indeed in my patch, I just change "struct mutex delete_unused_bgs_mutex"
to "struct rw_semaphore bg_delete_sem", and try to get bg_delete_sem when
we allocate data space, so this patch should work as before :)
As a simple reproducer you can convert from -mdup to -msingle (to create
bloat) and then balance with -musage=10. Depending on which of the two
patches are applied, you end with bloat that only grows and never shrinks,
or bloat that ends up in mixed state (dup and single).
Can you give me a simple test script to reproduce your problem.
(I can write it myself, but I'm afraid I may misunderstand you :) )
Regards,
Xiaoguang Wang
Undoing both makes both balancing and cleaning work again.
-h
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