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@xxxxxxxxxxxxx
