Re: [PATCHSET 3/3] xfsprogs: scale shards on ssds

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Jun 03, 2024 at 10:09:10PM -0700, Christoph Hellwig wrote:
> On Mon, Jun 03, 2024 at 01:12:05PM -0700, Darrick J. Wong wrote:
> > This patchset adds a different computation for AG count and log size
> > that is based entirely on a desired level of concurrency.  If we detect
> > storage that is non-rotational (or the sysadmin provides a CLI option),
> > then we will try to match the AG count to the CPU count to minimize AGF
> > contention and make the log large enough to minimize grant head
> > contention.
> 
> Do you have any performance numbers for this?
> 
> Because SSDs still have a limited number of write streams, and doing
> more parallel writes just increases the work that the 'blender' has
> to do for them.  Typical number of internal write streams for not
> crazy expensive SSDs would be at most 8.

Not much other than the AG[IF] and log grant heads becoming less hot.
That pushes the bottlenecks to the storage device, which indeed is about
8 per device.  More if you can raid0 them.

*Fortunately* for metadata workloads the logging code is decent about
deduplicating repeated updates, so unless you're doing something truly
nasty like synchronous direct writes to a directory tree with parent
pointers that is being modified heavily, it takes some effort to
overload the ssd.

(Or a crappy ssd, I guess.  Maybe I'll pull out the 860 QVO and see how
it does.)

--D