[PATCH 0/3] btrfs: rework background block group relocation

[Josef Bacik <josef@toxicpanda.com>]

Hello,

Currently the background block group relocation code only works for zoned
devices, as it prevents the file system from becoming unusable because of block
group fragmentation.

However inside Facebook our common workload is to download tens of gigabytes
worth of send files or package files, and it does this by fallocate()'ing the
entire package, writing into it, and then free'ing it up afterwards.
Unfortunately this leads to a similar problem as zoned, we get fragmented data
block groups, and this trends towards filling the entire disk up with partly
used data block groups, which then leads to ENOSPC because of the lack of
metadata space.

Because of this we have been running balance internally forever, but this was
triggered based on different size usage hueristics and stil gave us a high
enough failure rate that it was annoying (figure 10-20 machines needing to be
reprovisioned per week).

So I modified the existing bg_reclaim_threshold code to also apply in the !zoned
case, and I also made it only apply to DATA block groups.  This has completely
eliminated these random failure cases, and we're no longer reprovisioning
machines that get stuck with 0 metadata space.

However my internal patch is kind of janky as it hard codes the DATA check.
What I've done here is made the bg_reclaim_threshold per-space_info, this way
a user can target all block group types or just the ones they care about.  This
won't break any current users because this only applied in the zoned case
before.

Additionally I've added the code to allow this to work in the !zoned case, and
loosened the restriction on the threshold from 50-100 to 0-100.

I tested this on my vm by writing 500m files and then removing half of them and
validating that the block groups were automatically reclaimed.  Thanks,

Josef

Josef Bacik (3):
  btrfs: make the bg_reclaim_threshold per-space info
  btrfs: allow block group background reclaim for !zoned fs'es
  btrfs: change the bg_reclaim_threshold valid region from 0 to 100

 fs/btrfs/block-group.c      | 31 ++++++++++++++++
 fs/btrfs/ctree.h            |  1 -
 fs/btrfs/disk-io.c          |  1 -
 fs/btrfs/free-space-cache.c |  4 +--
 fs/btrfs/space-info.c       |  9 +++++
 fs/btrfs/space-info.h       |  6 ++++
 fs/btrfs/sysfs.c            | 71 +++++++++++++++++++------------------
 fs/btrfs/zoned.h            |  6 ----
 8 files changed, 84 insertions(+), 45 deletions(-)

-- 
2.26.3