Re: [PATCH v4 6/6] io_uring: add support for zone-append

[Kanchan Joshi]

On Tue, Sep 8, 2020 at 8:48 PM hch@xxxxxxxxxxxxx <hch@xxxxxxxxxxxxx> wrote:
>
> On Mon, Sep 07, 2020 at 12:31:42PM +0530, Kanchan Joshi wrote:
> > But there are use-cases which benefit from supporting zone-append on
> > raw block-dev path.
> > Certain user-space log-structured/cow FS/DB will use the device that
> > way. Aerospike is one example.
> > Pass-through is synchronous, and we lose the ability to use io-uring.
>
> So use zonefs, which is designed exactly for that use case.

Not specific to zone-append, but in general it may not be good to lock
new features/interfaces to ZoneFS alone, given that direct-block
interface has its own merits.
Mapping one file to a one zone is good for some use-cases, but
limiting for others.
Some user-space FS/DBs would be more efficient (less meta, indirection)
with the freedom to decide file-to-zone mapping/placement.
- Rocksdb and those LSM style DBs would map SSTable to zone, but
SSTable file may be two small (initially) and may become too large
(after compaction) for a zone.
- The internal parallelism of a single zone is a design-choice, and
depends on the drive. Writing multiple zones parallely (striped/raid
way) can give better performance than writing on one. In that case one
would want to file that seamlessly combines multiple-zones in a
striped fashion.

Also it seems difficult (compared to block dev) to fit simple-copy TP
in ZoneFS. The new
command needs: one NVMe drive, list of source LBAs and one destination
LBA. In ZoneFS, we would deal with N+1 file-descriptors (N source zone
file, and one destination zone file) for that. While with block
interface, we do not need  more than one file-descriptor representing
the entire device. With more zone-files, we face open/close overhead too.

-- 
Joshi