Re: [PATCH 1/6] block: add support for carrying a stream ID in a bio

From: Jens Axboe
Date: Tue Mar 24 2015 - 21:42:31 EST


On 03/24/2015 04:07 PM, Ming Lin-SSI wrote:
-----Original Message-----
From: Jens Axboe [mailto:axboe@xxxxxxxxx]
Sent: Tuesday, March 24, 2015 10:27 AM
To: Matias Bjørling; Jens Axboe; linux-kernel@xxxxxxxxxxxxxxx; linux-
fsdevel@xxxxxxxxxxxxxxx
Cc: Ming Lin-SSI
Subject: Re: [PATCH 1/6] block: add support for carrying a stream ID in a bio

On 03/24/2015 11:11 AM, Matias Bjørling wrote:
On 03/24/2015 04:26 PM, Jens Axboe wrote:
The top bits of bio->bi_flags are reserved for keeping the allocation
pool, set aside the next four bits for carrying a stream ID. That
leaves us with support for 15 streams,
0 is reserved as a "stream not set" value.

15 streams seem very limited. Can this be extended? e.g. 16 bits.

15 streams is enough for 1-4 applications. More, and applications
starts to fight over the same stream id's, leading them to place
different age data in same flash blocks and push us back to square one.

I understand that Samsung multi-stream SSD supports a limited amount
of streams, more advance implementations should provide higher limits.

Pushing it higher is not a big deal as far as the implementation goes, though
16 bits might be stealing a bit too much space for this. On 32-bit archs, we
have 18 bits currently free that we can abuse. The Samsung device supports
16 streams. That's honestly a lot more than I would expect most devices to
support in hardware, 16 is a lot of open erase blocks and write append points.
Obviously the open channel effort would make that more feasible, though.

Can we use 8 bits at least? I'll test performance with 16 streams.

We could, but I still question whether that's really useful. I'd rather start smaller and go bigger if there's a real use case for it. It wont change the user space ABI if we later make it larger.

--
Jens Axboe

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/