[dm-devel] [PATCH v2 3/3] zonefs: fix synchronous write to sequential zone files

[Damien Le Moal]

On 2021/04/19 15:45, Christoph Hellwig wrote:
> On Sat, Apr 17, 2021 at 11:33:23AM +0900, Damien Le Moal wrote:
>> Synchronous writes to sequential zone files cannot use zone append
>> operations if the underlying zoned device queue limit
>> max_zone_append_sectors is 0, indicating that the device does not
>> support this operation. In this case, fall back to using regular write
>> operations.
> 
> Zone append is a mandatory feature of the zoned device API.

Yes, I am well aware of that. All physical zoned devices and null blk do support
zone append, but the logical device created by dm-crypt is out. And we cannot
simply disable zone support in dm-crypt as there are use cases out there in the
field that I am aware of, in SMR space.

So this series is a compromise: preserve dm-crypt zone support for SMR (no one
uses the zone append emulation yet, as far as I know) by disabling zone append.

For zonefs, we can:
1) refuse to mount if ZA is disabled, same as btrfs
2) Do as I did in the patch, fallback to regular writes since that is easy to do
(zonefs file size tracks the WP position already).

I chose option (2) to allow for SMR+dm-crypt to still work with zonefs.


-- 
Damien Le Moal
Western Digital Research