Re: [RFC] [PATCH 5/8] inode-diet: Eliminate i_blksize and use a per-superblock default

["Nate Diller" <nate.diller@gmail.com>]

On 6/19/06, Theodore Tso <tytso@mit.edu> wrote:
> On Mon, Jun 19, 2006 at 09:16:51AM -0700, Joel Becker wrote:
> > On Mon, Jun 19, 2006 at 04:58:21PM +0100, Christoph Hellwig wrote:
> > > Blease don't add a field to the superblock for the optimal I/O size.
> > > Any filesystem that wants to override it can do so directly in ->getattr.
> >
> >       I don't disagree with you, but the idea of everyone implementing
> > ->getattr where they just let it work before scares me.  It's a ton of
> > cut-n-paste error waiting to happen.  Especially if we make something
> > stale.
> >       Perhaps add generic_fillattr_blksize()?
>
> Well, as far as I know the only filesystems today that would need to
> do something different are xfs, ocfs2, and reiserfs, and IMHO only the
> first two have any kind of justification for doing it.  Part of the
> problem is what st_blksize actually means was never well-defined; it
> was never in POSIX, and in SuSv3 all that is stated is, "A file
> system-specific preferred I/O block size for this object."  This is
> why Reiserfs got away with specifying 128 megs (I assume it helped on
> some benchmark), and why being ill-defined, using such a large value
> might cause some applications (like /bin/cp) to core dump.
>
> Given that most filesystems use the generic page cache read/write
> functions, using PAGE_CACHE_SIZE as the default seems to make a huge
> amount of sense.  I really wonder how useful setting st_blksize really
> is, actually, at least in the real-world, as opposed to just for
> benchmarks.

I assume "128 megs" is a typo, it's 128k, of course.  And certainly it
would have helped speed things up, not just benchmarks, because for
modern disks, doing 128k vs 4k takes like 25% more time.  Wu's
adaptive readahead patches might make this outdated, though, since
they now support "read 10 pages, seek, read 10 pages, seek, etc" type
workloads.

NATE
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