On 12/27/19 8:09 PM, Qu Wenruo wrote:
On 2019/12/28 上午2:32, Josef Bacik wrote:
On 12/25/19 8:39 AM, Qu Wenruo wrote:
There are several bug reports of ENOSPC error in
btrfs_run_delalloc_range().
With some extra info from one reporter, it turns out that
can_overcommit() is using a wrong way to calculate allocatable metadata
space.
The most typical case would look like:
devid 1 unallocated: 1G
devid 2 unallocated: 10G
metadata profile: RAID1
In above case, we can at most allocate 1G chunk for metadata, due to
unbalanced disk free space.
But current can_overcommit() uses factor based calculation, which never
consider the disk free space balance.
To address this problem, here comes the per-profile available space
array, which gets updated every time a chunk get allocated/removed or a
device get grown or shrunk.
This provides a quick way for hotter place like can_overcommit() to grab
an estimation on how many bytes it can over-commit.
The per-profile available space calculation tries to keep the behavior
of chunk allocator, thus it can handle uneven disks pretty well.
The RFC tag is here because I'm not yet confident enough about the
implementation.
I'm not sure this is the proper to go, or just a over-engineered mess.
In general I like the approach, however re-doing the whole calculation
once we add or remove a chunk seems like overkill. Can we get away with
just doing the big expensive calculation on mount, and then adjust
available up and down as we add and remove chunks?
That looks good on a quick glance, but in practice it may not work as
expected, mostly due to the small difference in sort.
Current chunk allocator works by sorting the max hole size as primary
sort index, thus it may cause difference on some corner case.
Without proper re-calculation, the difference may drift larger and larger.
Thus I prefer to be a little safer to do extra calculation each time
chunk get allocated/remove.
And that calculation is not that heavy, it just iterate the device lists
several times, and all access are in-memory without sleep, it should be
pretty fast.
Ahh I hadn't thought of different hole sizes. You're right that it shouldn't
matter in practice, it's not like chunk allocation is a fast path. This seems
reasonable to me then, I'll go through the patches properly. Thanks,
Josef
