Re: [RFC] block integrity: Fix write after checksum calculationproblem

From: Jan Kara
Date: Thu Feb 24 2011 - 11:43:46 EST

On Wed 23-02-11 11:24:50, Martin K. Petersen wrote:
> >>>>> "Dave" == Dave Chinner <david@xxxxxxxxxxxxx> writes:
> >> Agreed. I too am curious to study which circumstances favor copying
> >> vs blocking.
> Dave> IMO blocking is generally preferable in high throughput threaded
> Dave> workloads as there is always another thread that can do useful
> Dave> work while we wait for IO to complete. Most use cases for DIF
> Dave> center around high throughput environments....
> Yeah.
> A while back I did a bunch of tests with a liberal amount of
> wait_on_page_writeback() calls added to (I think) ext2, ext3, and
> XFS. For my regular workloads there was no measurable change (kernel
> builds, random database and I/O tests). I'm sure we'll unearth some apps
> that will suffer when DI is on but so far I'm not too worried about
> blocking in the data path.
> My main concern is wrt. metadata because that's where extN really
> hurts. Simple test: Unpack a kernel tarball and watch the directory
> block fireworks. Given how frequently those buffers get hit I'm sure
> blocking would cause performance to tank completely. I looked into
> fixing this in ext2 but I had to stop because my eyes were bleeding.
Ext2 is problematic yes, but ext[34] should be OK because we do
metadata copy anyway because of journalling. So for ext[34] you shouldn't
need any additional metadata protection since JBD does it for you (apart
from nojournal mode of ext4 of course).

Jan Kara <jack@xxxxxxx>
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