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

[Jens Axboe]

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.

--
Jens Axboe

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