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SubjectRe: bio too big - in nested raid setup
On 01/28/2010 12:50 PM, Neil Brown wrote:
> Both raid0 and linear register a 'bvec_mergeable' function (or whatever it is
> called today).
> This allows for the fact that these devices have restrictions that cannot be
> expressed simply with request sizes. In particular they only handle requests
> that don't cross a chunk boundary.
> As raid1 never calls the bvec_mergeable function of it's components (it would
> be very hard to get that to work reliably, maybe impossible), it treats any
> device with a bvec_mergeable function as though the max_sectors were one page.
> This is because the interface guarantees that a one page request will always
> be handled.

I'm also guilty of doing some mirror work, in exofs, over osd objects.

I was thinking about that reliability problem with mirrors, also related
to that infamous problem of coping the mirrored buffers so they do not
change while writing at the page cache level.

So what if we don't fight it? what if we just keep a journal of the mirror
unbalanced state and do not page_uptodate until the mirror is finally balanced.
Only then pages can be dropped from the cache, and journal cleared.

(Balanced-mirror-page is when a page has participated in an IO to all devices
without being marked dirty from the get-go to the completion of IO)

I think Trond's last work with adding that un_updated-but-committed state to
pages can facilitate in doing that, though I do understand that it is a major
conceptual change to the the VFS-BLOCKS relationship in letting the block devices
participate in the pages state machine (And md keeping a journal). Sigh


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