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

[Jeff Garzik]

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?

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.

	Jeff



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