On 03/24/2015 03:46 PM, Ming Lin-SSI wrote:
-----Original Message-----
From: Jens Axboe [mailto:ax...@kernel.dk]
Sent: Tuesday, March 24, 2015 10:08 AM
To: Jeff Moyer; Jens Axboe
Cc: linux-kernel@vger.kernel.org; linux-fsde...@vger.kernel.org; Ming Lin-
SSI
Subject: Re: [PATCH RFC] Support for write stream IDs
On 03/24/2015 11:03 AM, Jeff Moyer wrote:
Jens Axboe <ax...@fb.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.
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.
Can you share the nvme spec proposal with us?
--
Jens Axboe
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