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

[Vivek Goyal <vgoyal@redhat.com>]

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.

In practice does C2 really help or we can get rid of it entirely?

Thanks
Vivek