Re: [PATCH 3/6] direct-io: do not merge logically non-contiguousrequests

From: Josef Bacik
Date: Fri May 21 2010 - 11:30:21 EST

On Fri, May 21, 2010 at 11:21:11AM -0400, Christoph Hellwig wrote:
> On Wed, May 19, 2010 at 04:24:51PM -0400, Josef Bacik wrote:
> > Btrfs cannot handle having logically non-contiguous requests submitted. For
> > example if you have
> >
> > Logical: [0-4095][HOLE][8192-12287]
> > Physical: [0-4095] [4096-8191]
> >
> > Normally the DIO code would put these into the same BIO's. The problem is we
> > need to know exactly what offset is associated with what BIO so we can do our
> > checksumming and unlocking properly, so putting them in the same BIO doesn't
> > work. So add another check where we submit the current BIO if the physical
> > blocks are not contigous OR the logical blocks are not contiguous.
> This gets us slightly less optimal I/O patters for other filesystems in
> this case. But it's probably corner case enough to not care and make it
> the default.
> But please make the comment in the comment as verbose as the commit
> message so that people understand why we're doing this when reading the
> code in a few years.

So after I sent this I thought that maybe I could make that test _only_ if we
provide submit_bio, that way it only affects btrfs and not everybody else, would
you prefer I do something like that? I will make the commit log a bit more
verbose. Thanks,

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