Re: ext4 writeback performance issue in 6.12

["Theodore Ts'o" <tytso@mit.edu>]

On Thu, Oct 09, 2025 at 06:52:54PM +0100, Matt Fleming wrote:
> On Thu, Oct 09, 2025 at 11:22:59AM +0100, Matt Fleming wrote:
> > 
> > Thanks Ted. I'm going to try disabling the stripe parameter now. I'll report
> > back shortly.
> 
> Initial results look very good. No blocked tasks so far and the mb
> allocator latency is much improved.

OK, so that definitely confirms the theory of what's going on.  There
have been some changes in the latest kernel that *might* address what
you're seeing.  The challenge is that we don't have a easy reproducer
that doesn't involve using a large file system running a production
workload.  If you can only run this on a production server, it's
probably not fair to ask you to try running 6.17.1 and see if it shows
up there.

I do think in the long term, we need to augment thy buddy bitmap in
fs/ext4/mballoc.c with some data structure which tracks free space in
units of stripe blocks, so we can do block allocation in a much more
efficient way for RAID systems.  The simplest way would be to add a
counter of the number of aligned free stripes in the group info
structure, plus a bit array which indicates which aligned stripes are
free.  This is not just to improve stripe allocation, but also when
doing sub-stripe allocation, we preferentially try allocating out of
stripes which are already partially in use.

Out of curiosity, are you using the stride parameter because you're
using a SSD-based RAID array, or a HDD-based RAID array?

      		       	      	   	     	  - Ted