On Fri, Aug 08 2008, David Woodhouse wrote: > On Fri, 2008-08-08 at 12:30 +0200, Jens Axboe wrote: > > Not at all, please re-read that part. I objected to adding a strategy > > hook just for setting cmd type and flags for discard. > > Ah, now I understand what you were saying. And I vacillated about that > too -- I actually suggested to Gilad that we didn't want it, but then > went ahead and did it anyway. > > Adding the ->discard_fn hook provides two features: > > Firstly, it allows an early 'abort' of discard requests where the device > doesn't support them -- there's no point in allocating the request and > passing it down to the device to be discarded there, when we can just > notice that there's no ->discard_fn and return immediately from > blkdev_issue_discard(). > > Secondly, it allows drivers to insert the appropriate command onto their > queue -- I expect that SCSI and ATA drives won't use REQ_TYPE_DISCARD, > just as they don't use REQ_TYPE_FLUSH. They'll use REQ_TYPE_BLOCK_PC > with whatever opcode is allocated for trim/punch. > > We _could_ address those two differently -- maybe we could add a > QUEUE_FLAG_DISCARDS in q->queue_flags to indicate that discard requests > are supported, and we could make the driver 'translate' from > REQ_TYPE_DISCARD to whatever it actually wants to do. But that seems > strangely different to how we handle flushes, which is what I was basing > my implementation on. > Given that discard requests may become quite often issued by drivers, it does make sense to optimize for the case where we don't support them as well. So perhaps the ->discard_fn() can be tolerated, though I'd rather get rid of the ->issue_flush_fn instead :-) For now, lets just go with the discard_fn, I'll update the patch again. -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
