Messages in this thread | | | Date | Mon, 30 Mar 2009 15:05:42 -0400 | From | Jeff Garzik <> | Subject | range-based cache flushing (was Re: Linux 2.6.29) |
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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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