Re: RFC: detection of silent corruption via ATA long sector reads

["NeilBrown" <neilb@suse.de>]

On Mon, January 5, 2009 1:45 pm, Martin K. Petersen wrote:
>>>>>> "John" == John Robinson <john.robinson@anonymous.org.uk> writes:
>
> John> Essentially DIF on 32k blocks instead of 512 byte ones. But
> John> perhaps this is a bad assumption and MD/DM already optimises out
> John> whole-chunk reads and writes where they're not required (for very
> John> short, less-than-one-chunk transactions), and I've no idea whether
> John> this happens a lot.
>
> I haven't looked at the RAID4/5/6 code for a long time so I'm not sure
> whether they only write dirty pages or the whole chunk + parity ditto.
> Neil?

md/RAID456 writes whole pages (aligned to the array) but not whole chunks.

If a filesystem request to write one page which is at a sector address
which is not a multiple of the page size, we will pre-read the read of
the two array-aligned pages, and when write them (and parity) back out.

Otherwise, it will just write the requested pages plus parity updates.

NeilBrown