Re: [PATCH v3 2/3] dm zoned: do not reclaim a random zone into another random zone

[Damien Le Moal <dlemoal@kernel.org>]

On 2026/10/11 19:54, Kyrylo Biryuk wrote:
> dmz_reclaim_rnd_data() falls back to a random destination zone when no
> free sequential zone is available and the target has cache zones. When
> the target is idle and the cache zone list is empty,
> dmz_get_rnd_zone_for_reclaim() selects random zones of the zoned device
> as well, so a random zone is then copied into another random zone. That
> frees nothing: the next pass picks the next random zone and copies it
> again, for as long as the target stays idle.
> 
> Seen on a two-device target (256 GiB regular cache device plus a 27 TB
> host-managed drive) used as an md RAID5 member: after a full rebuild
> every chunk was mapped and no sequential zone was free, and the idle
> drive read and wrote about 72 MB/s each without pause for hours while
> the free zone counters in "dmsetup status" did not change. Reproduced on
> emulated host-managed drives (tcmu-runner ZBC handler): 340 MB/s of
> copying on an idle target, 0 MB/s with this patch.
> 
> Only switch to a random destination zone when the zone being reclaimed
> is a cache zone: moving it to the zoned device still frees cache space.
> 
> Fixes: c5c788595292 ("dm zoned: start reclaim with sequential zones")
> Assisted-by: LLM
> Signed-off-by: Kyrylo Biryuk <illofspeed@gmail.com>

Looks good.

Reviewed-by: Damien Le Moal <dlemoal@kernel.org>

-- 
Damien Le Moal
Western Digital Research