Re: FFS write coalescing

[Alan Barrett <apb%cequrux.com@localhost>]

On Mon, 03 Dec 2012, Chuck Silvers wrote:
> the genfs code also never writes clean pages to disk, even though for
> RAID5 storage it would likely be more efficient to write clean pages
> that are in the same stripe as dirty pages if that would avoid issuing
> partial-stripe writes.  (which is basically another way of saying
> what david said.)

Perhaps there should be a way for block devices to report at least three
block sizes:

a) smallest possible block size (512 for almost all disks)

b) smallest efficient block size and alignment (4k for modern disks,
stripe size for raid)

c) largest possible size (a device and bus-dependent variant of MAXPHYS)

Then the file system could use (b) to know when it's a good idea to
combine dirty and clean pages into the same write.

--apb (Alan Barrett)