Re: [PATCH 1/7] block: add support for carrying a stream ID in a bio

[Jens Axboe <axboe@fb.com>]

On 04/09/2015 04:46 PM, Andreas Dilger wrote:
> On Mar 25, 2015, at 9:07 AM, Jens Axboe <axboe@fb.com> wrote:
>>
>> The top bits of bio->bi_flags are reserved for keeping the
>> allocation pool, set aside the next eight bits for carrying
>> a stream ID. That leaves us with support for 255 streams,
>> 0 is reserved as a "stream not set" value.
>
> I understand that the stream ID is not related to specific priority
> of an IO request.  However, I'm wondering how this patch series
> interacts with some of the other patch series that add cache priority
> hints?  Is there a danger of running out of space in the IO pipeline
> for the additional cache hints if this is using 8 bits?

That's always a risk, of course, but that goes for most features that 
need to carry more data in struct bio (or elsewhere). Otherwise we'll 
have to bite the bullet and add a new field.

>> +/*
>> + * after the pool bits, next 8 bits are for the stream id
>> + */
>> +#define BIO_STREAM_BITS		(8)
>> +#define BIO_STREAM_OFFSET	(BITS_PER_LONG - 12)
>
> Should this really be:
>
> #define BIO_STREAM_OFFSET	(BIO_POOL_OFFSET - BIO_STREAM_BITS)
>
> Otherwise there is a risk of conflict if someone changes BIO_POOL_BITS.

Good point, that would be cleaner. I'll make that change.

-- 
Jens Axboe