Messages in this thread | | | Date | Tue, 24 Mar 2015 19:42:21 -0600 | From | Jens Axboe <> | Subject | Re: [PATCH 1/6] block: add support for carrying a stream ID in a bio |
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On 03/24/2015 04:07 PM, Ming Lin-SSI wrote: >> -----Original Message----- >> From: Jens Axboe [mailto:axboe@kernel.dk] >> Sent: Tuesday, March 24, 2015 10:27 AM >> To: Matias Bjørling; Jens Axboe; linux-kernel@vger.kernel.org; linux- >> fsdevel@vger.kernel.org >> 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
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