On Mon, 2009-03-30 at 15:05 -0400, Jeff Garzik wrote:James Bottomley wrote:On Wed, 2009-03-25 at 16:25 -0400, Ric Wheeler wrote:Quite true, though wondering aloud...Jeff Garzik wrote:What do you mean by well supported? The way the SCSI standard isRic Wheeler wrote:> And, as I am sure that you do know, to add insult to injury, FLUSH_CACHEHow well supported is this in SCSI? Can we try it out with a commodity SAS drive?is per device (not file system).SCSI'S SYNCHRONIZE CACHE command already accepts an (LBA, length) pair. We could make use of that.
When you issue an fsync() on a disk with multiple partitions, you will flush the data for all of its partitions from the write cache....
And I bet we could convince T13 to add FLUSH CACHE RANGE, if we could demonstrate clear benefit.
written, a device can do a complete cache flush when a range flush is
requested and still be fully standards compliant. There's no easy way
to tell if it does a complete cache flush every time other than by
taking the firmware apart (or asking the manufacturer).
How difficult would it be to pass the "lower-bound" LBA to SYNCHRONIZE CACHE, where "lower bound" is defined as the lowest sector in the range of sectors to be flushed?
Actually, the implementation is designed to allow this. The standard
says if the number of blocks is zero that means flush from the specified
LBA to the end of the device. The sync cache we currently use has LBA 0
and number of blocks zero (which means flush everything).
That seems like a reasonable optimization -- it gives the drive an easy way to skip sync'ing sectors lower than the lower-bound LBA, if it is capable. Otherwise, a standards-compliant firmware will behave as you describe, and do what our code currently expects today -- a full cache flush.
This seems like a good way to speed up cache flush [on SCSI], while also perhaps experimenting with a more fine-grained way to pass down write barriers to the device.
Not a high priority thing overall, but OTOH, consider the case of placing your journal at the end of the disk. You could then issue a cache flush with a non-zero starting offset:
SYNCHRONIZE CACHE (max sectors - JOURNAL_SIZE, ~0)
That should be trivial even for dumb disk firmwares to optimize.
We could try it ... I'm still not sure how we'd tell the device is
actually implementing it and not flushing the entire device.