Re: [PATCH v2 0/3] dm zoned: fix two idle reclaim problems

[Damien Le Moal]

On 2026/10/10 18:16, illofspeed wrote:
> v2: no code changes. The only change is my author address: v1 went out on
> 2026-10-10 from my previous address. Please apply this version instead of v1.

OK. Missed this. So this answer my question about the changes.

> 
> These patches fix two problems in dm-zoned's reclaim worker, found while
> running md RAID5 on three dm-zoned targets on 27 TB host-managed SMR drives
> (WD Ultrastar DC HC680), each target with a regular cache device.
> 
> 1. On a target with cache zones and no free sequential zone, an idle
>    target keeps copying random zones into other random zones, forever
>    (patch 2; patch 1 makes the resulting -ENOSPC back off instead of
>    spinning). After a full md rebuild every chunk is mapped, so this
>    happens right after a rebuild: the idle member read and wrote about
>    72 MB/s nonstop for hours.
> 
> 2. After a reclaim pass that ends while the target is busy, the idle poll
>    is never re-armed, so zones filled under load are not reclaimed when the
>    target goes idle (patch 3). Users notice it as "the buffer never drains";
>    the workaround has been a timer sending "dmsetup message <dev> 0 reclaim".
> 
> Testing: the three patches were built as an out-of-tree dm-zoned module for
> 7.2.6 (the changed code is identical in current master) and tested
> - on emulated host-managed drives (tcmu-runner ZBC handler) in a VM: the
>   idle copy loop reproduced with the stock module (340 MB/s on an idle
>   target, zone counters unchanged), 0 MB/s with the patches;
> - on the three real drives since 2026-10-05: crash test, three 1.5 TiB
>   write benchmarks, and eight conversions between single-device and
>   cache-device layouts; emptied zones were reclaimed within minutes of the
>   targets going idle without the reclaim timer.
> - with a reproducer that needs no special hardware (scsi_debug zbc=managed,
>   64 MiB zones, plus a 1 GiB loop cache device): stock module ~600 MB/s of
>   read+write on the idle target with unchanged zone counters, patched
>   module 0 MB/s:
>   https://github.com/illofspeed/synology-host-managed-smr/tree/main/repro
> Not done: a test on current master itself (only 7.2.6), and a rebuild onto a
> cached member with the patches (the first problem was seen after a rebuild
> with the stock module).
> 
> Related, not addressed here: the double-free in dmz_load_sb() reported on
> dm-devel on 2026-05-30.
> 
> The problems were found and the patches written with the help of an AI
> coding assistant (hence Assisted-by); the analysis, the tests on real
> hardware and this description were reviewed by me.
> 
> illofspeed (3):
>   dm zoned: back off when reclaim finds no destination zone
>   dm zoned: do not reclaim a random zone into another random zone
>   dm zoned: keep polling for idle reclaim after a reclaim pass
> 
>  drivers/md/dm-zoned-reclaim.c | 28 ++++++++++++++++++++++++++--
>  1 file changed, 26 insertions(+), 2 deletions(-)
> 


-- 
Damien Le Moal
Western Digital Research