Re: [RFC PATCH 00/17] btrfs zoned block device support

[Hannes Reinecke]

On 08/13/2018 08:42 PM, David Sterba wrote:
> On Fri, Aug 10, 2018 at 03:04:33AM +0900, Naohiro Aota wrote:
> > This series adds zoned block device support to btrfs.
>
> Yay, thanks!
>
> As this a RFC, I'll give you some. The code looks ok for what it claims
> to do, I'll skip style and unimportant implementation details for now as
> there are bigger questions.
>
> The zoned devices bring some constraints so not all filesystem features
> cannot be expected to work, so this rules out any form of in-place
> updates like NODATACOW.
>
> Then there's list of 'how will zoned device work with feature X'?
>
> You disable fallocate and DIO. I haven't looked closer at the fallocate
> case, but DIO could work in the sense that open() will open the file but
> any write will fallback to buffered writes. This is implemented so it
> would need to be wired together.
>
> Mixed device types are not allowed, and I tend to agree with that,
> though this could work in principle.  Just that the chunk allocator
> would have to be aware of the device types and tweaked to allocate from
> the same group. The btrfs code is not ready for that in terms of the
> allocator capabilities and configuration options.
>
> Device replace is disabled, but the changlog suggests there's a way to
> make it work, so it's a matter of implementation. And this should be
> implemented at the time of merge.
How would a device replace work in general?
While I do understand that device replace is possible with RAID 
thingies, I somewhat fail to see how could do a device replacement 
without RAID functionality.
Is it even possible?
If so, how would it be different from a simple umount?

> RAID5/6 + zoned support is highly desired and lack of it could be
> considered a NAK for the whole series. The drive sizes are expected to
> be several terabytes, that sounds be too risky to lack the redundancy
> options (RAID1 is not sufficient here).
That really depends on the allocator.
If we can make the RAID code to work with zone-sized stripes it should 
be pretty trivial. I can have a look at that; RAID support was on my 
agenda anyway (albeit for MD, not for btrfs).

> The changelog does not explain why this does not or cannot work, so I
> cannot reason about that or possibly suggest workarounds or solutions.
> But I think it should work in principle.
As mentioned, it really should work for zone-sized stripes. I'm not sure 
we can make it to work with stripes less than zone sizes.

> As this is first post and RFC I don't expect that everything is
> implemented, but at least the known missing points should be documented.
> You've implemented lots of the low-level zoned support and extent
> allocation, so even if the raid56 might be difficult, it should be the
> smaller part.
FYI, I've run a simple stress-test on a zoned device (git clone linus && 
make) and haven't found any issue with those; compilation ran without a 
problem, and with quite decent speed.
Good job!

Cheers,

Hannes