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.
And statfs() can also grab that pre-calculated value for instance usage.
For metadata over-commit, statfs() falls back to factor based educated
guess method.
Since over-commit can only happen when we have unallocated space, the
problem caused by over-commit should only be a first world problem.
Changelog:
v1:
- Fix a bug where we forgot to update per-profile array after allocating
a chunk.
To avoid ABBA deadlock, this introduce a small windows at the end
__btrfs_alloc_chunk(), it's not elegant but should be good enough
before we rework chunk and device list mutex.
- Make statfs() to use virtual chunk allocator to do better estimation
Now statfs() can report not only more accurate result, but can also
handle RAID5/6 better.
v2:
- Fix a deadlock caused by acquiring device_list_mutex under
__btrfs_alloc_chunk()
There is no need to acquire device_list_mutex when holding
chunk_mutex.
Fix it and remove the lockdep assert.
v3:
- Use proper chunk_mutex instead of device_list_mutex
Since they are protecting two different things, and we only care about
alloc_list, we should only use chunk_mutex.
With improved lock situation, it's easier to fold
calc_per_profile_available() calls into the first patch.
- Add performance benchmark for statfs() modification
As Facebook seems to run into some problems with statfs() calls, add
some basic ftrace results.
v4:
- Keep the lock-free design for statfs()
As extra sleeping in statfs() may not be a good idea, keep the old
lock-free design, and use factor based calculation as fall back.
Qu Wenruo (3):
btrfs: Introduce per-profile available space facility
btrfs: space-info: Use per-profile available space in can_overcommit()
btrfs: statfs: Use virtual chunk allocation to calculation available
data space
fs/btrfs/space-info.c | 15 ++-
fs/btrfs/super.c | 174 +++++++++-------------------------
fs/btrfs/volumes.c | 212 ++++++++++++++++++++++++++++++++++++++----
fs/btrfs/volumes.h | 15 +++
4 files changed, 257 insertions(+), 159 deletions(-)
--
2.24.1
