Josef Bacik wrote on 2015/10/08 21:36 -0700:
On 10/08/2015 07:11 PM, Qu Wenruo wrote:
In previous rework of qgroup, we succeeded in fixing qgroup accounting
part, making the rfer/excl numbers accurate.
But that's just part of qgroup work, another part of qgroup still has
quite a lot problem, that's qgroup reserve space part which will lead to
EQUOT even we are far from the limit.
[[BUG]]
The easiest way to trigger the bug is,
1) Enable quota
2) Limit excl of qgroup 5 to 16M
3) Write [0,2M) of a file inside subvol 5 10 times without sync
EQUOT will be triggered at about the 8th write.
But after remount, we can still write until about 15M.
[[CAUSE]]
The problem is caused by the fact that qgroup will reserve space even
the data space is already reserved.
In above reproducer, each time we buffered write [0,2M) qgroup will
reserve 2M space, but in fact, at the 1st time, we have already reserved
2M and from then on, we don't need to reserved any data space as we are
only writing [0,2M).
Also, the reserved space will only be freed *ONCE* when its backref is
run at commit_transaction() time.
That's causing the reserved space leaking.
[[FIX]]
The fix is not a simple one, as currently btrfs_qgroup_reserve() will
allocate whatever caller asked for.
So for accurate qgroup reserve, we introduce a completely new framework
for data and metadata.
1) Per-inode data reserve map
Now, each inode will have a data reserve map, recording which range
of data is already reserved.
If we are writing a range which is already reserved, we won't need to
reserve space again.
Also, for the fact that qgroup is only accounted at commit_trans(),
for data commit into disc and its metadata is also inserted into
current tree, we should free the data reserved range, but still keep
the reserved space until commit_trans().
So delayed_ref_head will have new members to record how much space is
reserved and free them at commit_trans() time.
This is already handled by setting DELALLOC in the io_tree, we do
similar sort of stuff for the normal enospc accounting, why not use
that? Thanks,
Josef
Thanks for pointing this out.
I was also searching for a existing facility, but didn't find one as I'm
not familiar with io_tree.
After a quick glance, it seems quite fit the need, but not completely sure.
I'll keep investigating on it and try to use it.
BTW, from what I understand, __btrfs_buffered_write() should cause the
range to be DEALLOC, but I didn't find any call to set_extent_delalloc(),
it that done in other place?
Thanks,
Qu
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