Re: rados read ordering

[Josh Durgin <josh.durgin@inktank.com>]

On 12/08/2014 09:03 AM, Sage Weil wrote:
> The current RADOS behavior is that reads (on any given object) are always
> processed in the order they are submitted by the client.  This causes a
> few headaches for the cache tiering that it would be nice to avoid.  It
> also occurs to me that there are likely cases where we could go a lot
> faster by not strictly ordering things.  For example, a stat can respond
> more quickly than a large read, and some reads may hit cache while others
> go to disk.  This doesn't happen currently because of the (lame) way we do
> reads synchronously, but hope that can change too.
>
> I propose we drop this semantic.  If a client wants reads to have a strict
> ordering, they can set the existing RWORDERED flag (which also orders them
> with respect to writes).  That's not the most general thing ever, but I'm
> not sure we care about callers who want reads ordered with respect to each
> other but not writes.
>
> The real question is whether there are any users that want/need this
> currently.  I can't think of any offhand.  In several places we submit
> multiple *writes* and expect them to be strictly ordered (e.g., we
> set a completion on teh last write only).  I don't think we do this
> anywhere for reads though...
>
> Josh, Yehuda, Jason--can you think of any in RBD or RGW that would depend
> on this?

Nope, I've thought we should fix this since I found out about it.