Re: [PATCH v3 1/2] xfs: set a mount flag when perag reservation is active

[Dave Chinner <david@fromorbit.com>]

On Thu, Mar 18, 2021 at 12:17:06PM -0400, Brian Foster wrote:
> perag reservation is enabled at mount time on a per AG basis. The
> upcoming in-core allocation btree accounting mechanism needs to know
> when reservation is enabled and that all perag AGF contexts are
> initialized. As a preparation step, set a flag in the mount
> structure and unconditionally initialize the pagf on all mounts
> where at least one reservation is active.

I'm not sure this is a good idea. AFAICT, this means just about any
filesystem with finobt, reflink and/or rmap will now typically read
every AGF header in the filesystem at mount time. That means pretty
much every v5 filesystem in production...

We've always tried to avoid needing to reading all AG headers at
mount time because that does not scale when we have really large
filesystems (I'm talking petabytes here). We should only read AG
headers if there is something not fully recovered during the mount
(i.e. slow path) and not on every mount.

Needing to do a few thousand synchonous read IOs during mount makes
mount very slow, and as such we always try to do dynamic
instantiation of AG headers...  Testing I've done with exabyte scale
filesystems (>10^6 AGs) show that it can take minutes for mount to
run when each AG header needs to be read, and that's on SSDs where
the individual read latency is only a couple of hundred
microseconds. On spinning disks that can do 200 IOPS, we're
potentially talking hours just to mount really large filesystems...

Hence I don't think that any algorithm that requires reading every
AGF header in the filesystem at mount time on every v5 filesystem
already out there in production (because finobt triggers this) is a
particularly good idea...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com