Messages in this thread | | | Subject | Re: [RFC PATCH 00/17] btrfs zoned block device support | From | Hannes Reinecke <> | Date | Mon, 13 Aug 2018 21:20:35 +0200 |
| |
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
| |