linux-kernel - Re: [RFC] block integrity: Fix write after checksum calculation problem

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Date:	Thu, 24 Feb 2011 17:43:35 +0100
From:	Jan Kara <jack@...e.cz>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
Cc:	Dave Chinner <david@...morbit.com>,
	"Darrick J. Wong" <djwong@...ibm.com>,
	Andreas Dilger <adilger@...ger.ca>,
	Jens Axboe <axboe@...nel.dk>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Mingming Cao <mcao@...ibm.com>,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: [RFC] block integrity: Fix write after checksum calculation
 problem

On Wed 23-02-11 11:24:50, Martin K. Petersen wrote:
> >>>>> "Dave" == Dave Chinner <david@...morbit.com> 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).

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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