Re: [PATCH v2 00/16] Parallelizing filesystem writeback

Darrick J. Wong
2025-10-29T06:09:33Z

On Wed, Oct 29, 2025 at 11:35:21AM +0530, Kundan Kumar wrote:
> On 10/22/2025 10:09 AM, Christoph Hellwig wrote:
> > On Tue, Oct 21, 2025 at 09:46:30AM +1100, Dave Chinner wrote:
> >> Not necessarily. The allocator can (and will) select different AGs
> >> for an inode as the file grows and the AGs run low on space. Once
> >> they select a different AG for an inode, they don't tend to return
> >> to the original AG because allocation targets are based on
> >> contiguous allocation w.r.t. existing adjacent extents, not the AG
> >> the inode is located in.
> >
> > Also, as pointed out in the last discussion of this for the RT
> > subvolume there is zero relation between the AG the inode is in
> > and the data placement.
> >
> >
> I evaluated the effect of parallel writeback on realtime inodes and
> observed no improvement in IOPS. We can limit writes for realtime
> inodes to utilize a single default (0) writeback context. Do you
> see it differently?

Was that with or without rtgroups? metadir/rtgroups aren't enabled by
default yet so you'd have to select that manually with mkfs.xfs -m
metadir=1.

(and you might still not see much change because of what hch said)

--D