Re: FFS write coalescing

[Chuck Silvers <chuq%chuq.com@localhost>]

On Mon, Dec 03, 2012 at 06:21:30PM +0100, Edgar Fu wrote:
> I could find out myself by digging through the source, but probabely someone
> here knows the answer off his head:
> When FFS does write coalescing, will it try to align the resulting 64k chunk?
> I.e., if I have 32k blocks and I write blocks 1, 2, 3, 4; will it write (1,2)
> and (3,4) or 1, (2,3) and 4?
> Of course, the background for my question is RAID stripe alignment.

currently no, the genfs_putpages() code that (almost?) all the file systems
use at this point doesn't align disk writes to anything larger than
a page boundary.

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.)

-Chuck