Re: O_DIRECT please; Sybase 12.5

[Andrew Morton]

Andrea Arcangeli wrote:
> 
> On Fri, Jul 06, 2001 at 12:28:15AM +1000, Andrew Morton wrote:
> > ext3 journals data.  That's unique and it breaks things (or rather,
> > things break it).   It'd be trivial to support O_DIRECT in ext3's
> > writeback mode (metadata-only), but nobody uses that.
> 
> I thought everybody uses metadata-only to avoid killing data-write
> performance.

ext3 has three modes:

data=journal

	Data is journalled.  Yes, this slows things down
	significantly.

data=ordered

	The default mode and the most popular.  All data is written
	to disk prior to a commit.  Write throughput is good, and
	you don't have uninitialised data in your files after a
	crash.

data=writeback

	Metadata-only.   Better write throughput (in dbench, anyway),
	but only metadata integrity is preserved after a crash. ie:
	fsck says the fs is fine, but files can (and almost always do)
	contain random stuff after a crash.

Ordered data mode is really nice.  It's not magical though - for example,
if you reset the machine during a kernel build, a subsequent `make' will
fail because you have a number of .o files which have zero length.
That's the length they happened to have when the machine went down.

For ordered-data mode we need to keep track of all the buffers which
are associated with a transaction's journalled metadata and ensure that
they are written out before the transaction commits.  That is done with
a little structure which hangs off ->b_private.

> So I thought it was ok to at first support O_DIRECT only
> for metadata journaling, doing that should be a three liner as you said
> and that is what I expected.

Yup.  metadata-only journalling is all-round much, much simpler.