Re: [PATCH 5/5] nvme: support for zoned namespaces

[Heiner Litz <hlitz@ucsc.edu>]

Thanks Damien,
the striping explanation makes sense. In this case will rephase to: It
is sufficient to support large enough un-splittable writes to achieve
full per-zone bandwidth with a single writer/single QD.

My main point is: There is no fundamental reason for splitting up
requests intermittently just to re-assemble them in the same form
later.

On Wed, Jun 17, 2020 at 10:15 PM Damien Le Moal <Damien.LeMoal@wdc.com> wrote:
>
> On 2020/06/18 13:24, Heiner Litz wrote:
> > What is the purpose of making zones larger than the erase block size
> > of flash? And why are large writes fundamentally unreasonable?
>
> It is up to the drive vendor to decide how zones are mapped onto flash media.
> Different mapping give different properties for different use cases. Zones, in
> many cases, will be much larger than an erase block due to stripping across many
> dies for example. And erase block size also has a tendency to grow over time
> with new media generations.
> The block layer management of zoned block devices also applies to SMR HDDs,
> which can have any zone size they want. This is not all about flash.
>
> As for large writes, they may not be possible due to memory fragmentation and/or
> limited SGL size of the drive interface. E.g. AHCI max out at 168 segments, most
> HBAs are at best 256, etc.
>
> > I don't see why it should be a fundamental problem for e.g. RocksDB to
> > issue single zone-sized writes (whatever the zone size is because
> > RocksDB needs to cope with it). The write buffer exists as a level in
> > DRAM anyways and increasing write latency will not matter either.
>
> Rocksdb is an application, so of course it is free to issue a single write()
> call with a buffer size equal to the zone size. But due to the buffer mapping
> limitations stated above, there is a very high probability that this single
> zone-sized large write operation will end-up being split into multiple write
> commands in the kernel.
>
> >
> > On Wed, Jun 17, 2020 at 6:55 PM Keith Busch <kbusch@kernel.org> wrote:
> >>
> >> On Wed, Jun 17, 2020 at 04:44:23PM -0700, Heiner Litz wrote:
> >>> Mandating zone-sized writes would address all problems with ease and
> >>> reduce request rate and overheads in the kernel.
> >>
> >> Yikes, no. Typical zone sizes are much to large for that to be
> >> reasonable.
> >
>
>
> --
> Damien Le Moal
> Western Digital Research