[lustre-discuss] Constant small writes on ZFS backend even while idle

[Andreas Dilger]

It could be Multi-Mount Protection (MMP)?  

Cheers, Andreas

> On Oct 7, 2025, at 08:52, Alex Vodeyko via lustre-discuss <lustre-discuss at lists.lustre.org> wrote:
> 
> Hi,
> 
> I'm in the process of testing lustre-2.15.7_next on rocky-9.6, kernel
> 5.14.0-570.17.1.el9_6.x86_64, zfs-2.3.4.
> 84 disk shelf, multipath.
> 2x OSTs per OSS
> OST is on the zpool with 4x(8+2)raidz2=40 hdds configuration (btw -
> also tested on draid - the same problem).
> atime=off (also tested with relatime=on)
> recordsize=1M, compression=off
> 
> During benchmarks I've found that even on the completely idle system,
> zpool-iostat shows 40-160 4k (ashift=12) writes (1-4 per hdd) every
> second.
> # zpool iostat 1
>              capacity     operations     bandwidth
> pool        alloc   free   read  write   read  write
> ----------  -----  -----  -----  -----  -----  -----
> ..
> ost00       482G   145T      0    158      0   634K
> ost01       401G   145T      0     40      0   161K
> ----------  -----  -----  -----  -----  -----  -----
> ost00       482G   145T      0     40      0   161K
> ost01       401G   145T      0    157      0   629K
> ----------  -----  -----  -----  -----  -----  -----
> ost00       482G   145T      0     40      0   161K
> ost01       401G   145T      0     40      0   161K
> ----------  -----  -----  -----  -----  -----  -----
> ost00       482G   145T      0     38      0   153K
> ost01       401G   145T      0     39      0   157K
> 
> Could you please advise if I can turn off something (probably in
> lustre, because local zfs does not show this behaviour) to avoid these
> writes because they affect read performance (and cause huge cpu load
> average and iowait numbers especially during multiple concurrent reads
> from the single OST).
> 
> Many thanks,
> Alex
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