RE: [PATCH RFC] Support for write stream IDs

[Ming Lin-SSI]

> -----Original Message-----
> From: Jens Axboe [mailto:axboe@xxxxxxxxx]
> Sent: Tuesday, March 24, 2015 10:08 AM
> To: Jeff Moyer; Jens Axboe
> Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-fsdevel@xxxxxxxxxxxxxxx; Ming Lin-
> SSI
> Subject: Re: [PATCH RFC] Support for write stream IDs
> 
> On 03/24/2015 11:03 AM, Jeff Moyer wrote:
> > Jens Axboe <axboe@xxxxxx> 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.
> 
> It doesn't have it, yet, completely. Ming Lin can expand on what it looks like
> for the Samsung nvme devices. Current nvme does have vague support for it,
> however. The write command does have bits for frequent vs infrequent vs
> one-time writes.

Samsung extended NVMe spec to add "stream control command" to open/close stream.
And with small modification to the "Write command" to write to open stream.

We are promoting the multi-stream spec to NVMe group.

Thanks,
Ming

> 
> --
> Jens Axboe

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