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 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
