Re: [PATCH 0/11] Update version of write stream ID patchset

[Jeff Moyer <jmoyer@redhat.com>]

Jens Axboe <axboe@fb.com> writes:

> It's been a while since I last posted the write stream ID patchset, but
> here is an updated version.
>
> The original patchset was centered around the current NVMe streams
> proposal, but there was a number of issues with that. It's now in a
> much beter state, and hopefully will make it into 1.3 of the spec
> soon.

But the spec is still not public.  The only documentation I can find on
this stuff is from t10, dated May of last year.

> To quickly re-summarize the intent behind write stream IDs, it's to
> be able to provide a hint to the underlying storage device on what
> writes could feasibly be grouped together. If the device is able to
> group writes of similar life times on media, then we can greatly reduce
> the amount of data that needs to be copied around at garbage collection
> time. This gives us a better write amplification factor, which leads
> to better device life times and better (and more predictable)
> performance at steady staet.
>
> There's been a number of changes to this patchset since it was last
> posted. In summary:
>
> 1) The bio parts have been bumped to carry 16 bits of stream data, up
>    from 8 and 12 in the original series.
>
> 2) Since the interface grew some more options, I've moved away from
>    fadvise and instead added a new system call. I don't feel strongly
>    about what interface we use here, another option would be to have a
>    (big) set of fcntl() commands instead.
>
> 3) The kernel now manages the ID space, since we have moved to a host
>    assigned model. This is done on a backing_dev_info basis, and the
>    btrfs patch has been updated to show how this can be used for nested
>    devices on btrfs/md/dm/etc. This could be moved to the request queue
>    as well, again I don't feel too strongly aboout this specific part.
>
> Those are the big changes.

My main question is why expose this to userspace at all?  If we're
keeping track of write streams per file, then why not implement that in
the kernel, transparent to the application?  That would benefit all
applications instead of requiring application developers to opt in.

I'm sure your argument will have something to do with how stream id's
are allocated/freed (expensive/slow, limited resource, whatever), but
that really just gets back to Martin's original questions about what we
should expect from the hardware and what the programming model should
look like (questions that are, afaik, still open).

I'm not against write streams, I think it's a neat idea.  I just think
it will die on the vine if you require application developers to opt
in.  Not all storage is SSDs, and I don't like that SSDs now have to be
treated differently by the programmer.

Cheers,
Jeff