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

Jeff Garzik
2009-03-30T19:07:17Z

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