Re: range-based cache flushing (was Re: Linux 2.6.29)

James Bottomley
2009-04-01T00:14:33Z

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:
> >> Jeff Garzik wrote:
> >>> Ric Wheeler wrote:> And, as I am sure that you do know, to add insult
> >>> to injury, FLUSH_CACHE
> >>>> is per device (not file system).
>
> >>>> 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....
> >>> SCSI'S SYNCHRONIZE CACHE command already accepts an (LBA, length)
> >>> pair. We could make use of that.
>
> >>> And I bet we could convince T13 to add FLUSH CACHE RANGE, if we could
> >>> demonstrate clear benefit.
>
> >> How well supported is this in SCSI? Can we try it out with a commodity
> >> SAS drive?
>
> > What do you mean by well supported? The way the SCSI standard is
> > 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).
>
> Quite true, though wondering aloud...
>
> 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.

James