On Sat, Nov 23, 2013 at 12:41:06AM -0800, Christoph Hellwig wrote:

On Sat, Nov 23, 2013 at 09:40:38AM +1100, Dave Chinner wrote:

quoted

quoted

geometry, and we already have it wired to to large sector size
testing in xfstests.

We don't need to screw around with the sector size - that is
irrelevant to the problem, and we have an allocation alignment
test that is supposed to catch these issues: generic/223.

It didn't imply we need large sector sizes, but the same mechanism
to expodse a large sector size can also be used to present large
stripe units/width.

quoted

As I said, I have seen occasional failures of that test (once a
month, on average) as a result of this bug. It was simply not often
enough - running in a hard loop didn't increase the frequency of
failures - to be able debug it or to reach my "there's a regression
I need to look at" threshold. Perhaps we need to revisit that test
and see if we can make it more likely to trigger failures...

Seems like 233 should have cought it regularly with the explicit
alignment options on mkfs time.  Maybe we also need a test mirroring
the plain dd more closely?

Preallocation showed the problem, too, so we probably don't even
need dd to check whether allocation alignment is working properly.
We should probably write a test that spefically checks all the
different anlignment/extent size combinations we can use.

Preallocation should behave very similarly to direct IO, but I'm
pretty sure that it won't do things like round up allocations to
stripe unit/widths like direct IO does. The fact that we do
allocation sunit/swidth size alignment for direct Io outside the
allocator and sunit/swidth offset alignment inside the allocation is
kinda funky....

I've not seen 233 fail for a long time..

Not surprising, it is a one in several hundred test runs occurrence
here...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com