linux-kernel - Re: [PATCH RFC v2] Support for write stream IDs

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Message-ID: <x498uel12ri.fsf@segfault.boston.devel.redhat.com>
Date:	Wed, 25 Mar 2015 12:05:21 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <axboe@...com>
Cc:	<linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
	<ming.l@....samsung.com>, <david@...morbit.com>
Subject: Re: [PATCH RFC v2] Support for write stream IDs

Jens Axboe <axboe@...com> writes:

> One of the things that exacerbates write amplification on flash
> based devices is that fact that data with different lifetimes get
> grouped together on media. Currently we have no interface that
> applications can use to separate different types of writes. This
> patch set adds support for that.
>
> The kernel has no knowledge of what stream ID is what. The idea is
> that writes with identical stream IDs have similar life times, not
> that stream ID 'X' has a shorter lifetime than stream ID 'X+1'.

And presumably the device also has no knowledge of what stream ID is
what, right?

> There are basically two interfaces that could be used for this. One
> is fcntl, the other is fadvise. This patchset uses fadvise, with a
> new POSIX_FADV_STREAMID hint. The 'offset' field is used to pass
> the relevant stream ID. Switching to fcntl (with a SET/GET_STREAMID)
> would be trivial.
>
> The patchset wires up the block parts, adds buffered and O_DIRECT
> support, and modifies btrfs/xfs too. It should be trivial to extend
> this to all other file systems, I just used xfs and btrfs for testing.
>
> No block drivers are wired up yet. Patches are against current -git.

This proposal leaves lot to the reviewer's imagination.  Is there any
research in this area you can point to?

At a high level, are you sure you've got the right interface?  I would
think data lifetime would be tied to the file.  If that's the case, it
might be possible to not export this to userspace at all, and simply
make it work under the covers.  After all, what prevents multiple
applications from using the same stream id at the same time?

Cheers,
Jeff
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