Re: [PATCH 00/16] support non power of 2 zoned devices

[Pankaj Raghav <p.raghav@samsung.com>]

Hi Johannes,
On 2022-05-03 00:07, Johannes Thumshirn wrote:
>> There was an effort previously [1] to add support to non po2 devices via
>> device level emulation but that was rejected with a final conclusion
>> to add support for non po2 zoned device in the complete stack[2].
> 
> Hey Pankaj,
> 
> One thing I'm concerned with this patches is, once we have npo2 zones (or to be precise 
> not fs_info->sectorsize aligned zones) we have to check on every allocation if we still 
> have at least have fs_info->sectorsize bytes left in a zone. If not we need to 
> explicitly finish the zone, otherwise we'll run out of max active zones. 
> 
This commit: `btrfs: zoned: relax the alignment constraint for zoned
devices` makes sure the zone size is BTRFS_STRIPE_LEN aligned (64K). So
even the npo2 zoned device should be aligned to `fs_info->sectorsize`,
which is typically 4k.

This was one of the comment that came from David Sterba:
https://lore.kernel.org/all/20220315142740.GU12643@twin.jikos.cz/
where he suggested to have some sane alignment for the zone sizes.

> This is a problem for zoned btrfs at the moment already but it'll be even worse
> with npo2, because we're never implicitly finishing zones.
> 
> See also 
> https://lore.kernel.org/linux-btrfs/42758829d8696a471a27f7aaeab5468f60b1565d.1651157034.git.naohiro.aota@wdc.com
> 
I did take a look at this few days back and the patch should work fine
also for npo2 zoned device as we allow only zone sizes that are
BTRFS_STRIPE_LEN aligned. So even the max nodesize for METADATA BGs is
only 64k and it should be aligned correctly to implicitly finish the zone.

Let me know your thoughts and if I am missing something.

Regards,
Pankaj