Re: [PATCH 3/3] block: reimplement FLUSH/FUA to support merge

[Vivek Goyal]

On Fri, Jan 21, 2011 at 01:56:17PM -0500, Vivek Goyal wrote:
> On Fri, Jan 21, 2011 at 04:59:58PM +0100, Tejun Heo wrote:
> 
> [..]
> > + * The actual execution of flush is double buffered.  Whenever a request
> > + * needs to execute PRE or POSTFLUSH, it queues at
> > + * q->flush_queue[q->flush_pending_idx].  Once certain criteria are met, a
> > + * flush is issued and the pending_idx is toggled.  When the flush
> > + * completes, all the requests which were pending are proceeded to the next
> > + * step.  This allows arbitrary merging of different types of FLUSH/FUA
> > + * requests.
> > + *
> > + * Currently, the following conditions are used to determine when to issue
> > + * flush.
> > + *
> > + * C1. At any given time, only one flush shall be in progress.  This makes
> > + *     double buffering sufficient.
> > + *
> > + * C2. Flush is not deferred if any request is executing DATA of its
> > + *     sequence.  This avoids issuing separate POSTFLUSHes for requests
> > + *     which shared PREFLUSH.
> 
> Tejun, did you mean "Flush is deferred" instead of "Flush is not deferred"
> above?
> 
> IIUC, C2 might help only if requests which contain data are also going to 
> issue postflush. Couple of cases come to mind.
> 
> - If queue supports FUA, I think we will not issue POSTFLUSH. In that
>   case issuing next PREFLUSH which data is in flight might make sense.
> 
> - Even if queue does not support FUA and we are only getting requests
>   with REQ_FLUSH then also waiting for data requests to finish before
>   issuing next FLUSH might not help.
> 
> - Even if queue does not support FUA and say we have a mix of REQ_FUA
>   and REQ_FLUSH, then this will help only if in a batch we have more
>   than 1 request which is going to issue POSTFLUSH and those postflush
>   will be merged.
> 
> - Ric Wheeler was once mentioning that there are boxes which advertise
>   writeback cache but are battery backed so they ignore flush internally and
>   signal completion immediately. I am not sure how prevalent those
>   cases are but I think waiting for data to finish will delay processing
>   of new REQ_FLUSH requests in pending queue for such array. There
>   we will not anyway benefit from merging of FLUSH.
> 
> Given that C2 is going to benefit primarily only if queue does not support
> FUA and we have many requets with REQ_FUA set, will it make sense to 
> put additional checks for C2. Atleast a simple queue support FUA
> check might help.

Reading through the blk_insert_flush() bit more, looks like pure REQ_FUA
requests will not even show up in data list if queue supports FUA. But
IIUC requests with both REQ_FLUSH and REQ_FUA set will still show up even if
queue supports FUA.

Thanks
Vivek
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