[PATCH 3/6] block: add support for zone offline transition

[Damien Le Moal]

On 2020/06/26 18:11, hch at lst.de wrote:
> On Fri, Jun 26, 2020 at 01:14:30AM +0000, Damien Le Moal wrote:
>> As long as you keep ZNS namespace report itself as being "host-managed" like
>> ZBC/ZAC disks, we need the consistency and common interface. If you break that,
>> the meaning of the zoned model queue attribute disappears and an application or
>> in-kernel user cannot rely on this model anymore to know how the drive will behave.
> 
> We just need a way to expose to applications that new feature are
> supported.  Just like we did with zone capacity support.  That is why
> we added the feature flags to uapi zone structure.
> 
>> Think of a file system, or any other in-kernel user. If they have to change
>> their code based on the device type (NVMe vs SCSI), then the zoned block device
>> interface is broken. Right now, that is not the case, everything works equally
>> well on ZNS and SCSI, modulo the need by a user for conventional zones that ZNS
>> do not define. But that is still consistent with the host-managed model since
>> conventional zones are optional.
> 
> That is why we have the feature flag.  That user should not know the
> underlying transport or spec.  But it can reliably discover "this thing
> support zone capacity" or "this thing supports offline zones" or even
> nasty thing like "this zone can time out when open" which are much
> harder to deal with.
> 
>> For this particular patch, there is currently no in-kernel user, and it is not
>> clear how this will be useful to applications. At least please clarify this. And
> 
> The main user is the ioctl.  And if you think about how offline zones are
> (suppose to) be used driving this from management tools in userspace
> actually totally make sense.  Unlike for example open/close all which
> just don't make sense as primitives to start with.

OK. Adding a new BLKZONEOFFLINE ioctl is easy though and fits into the current
zone management plumbing well, I think. So the patch can be significantly
simplified (no need for the new zone management op function with flags).

> 


-- 
Damien Le Moal
Western Digital Research