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

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Date:	Tue, 24 Mar 2015 13:03:39 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <axboe@...com>
Cc:	<axboe@...nel.dk>, <linux-kernel@...r.kernel.org>,
	<linux-fsdevel@...r.kernel.org>, <ming.l@....samsung.com>
Subject: Re: [PATCH RFC] Support for write stream IDs

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

> Hi,
>
> 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'.
>
> 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.

Can you give an idea of how the stream id would be communicated to the
device?  NVMe doesn't appear to have any notion of a data stream ID.

Cheers,
Jeff
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