linux-kernel - Re: [dm-devel] Barriers still not passing on simple dm devices...

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Date:	Wed, 8 Apr 2009 08:54:12 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Ric Wheeler <rwheeler@...hat.com>
cc:	Jens Axboe <jens.axboe@...cle.com>,
	device-mapper development <dm-devel@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	ak@...ux.intel.com, "MASON, CHRISTOPHER" <CHRIS.MASON@...cle.com>
Subject: Re: [dm-devel] Barriers still not passing on simple dm devices...

> And I will restate that back at EMC we tested the original barriers (with
> reiserfs mostly, a bit on ext3 and ext2) and saw significant reduction in file
> system integrity issues after power loss.

You saw that barrier-enabled filesystem was worse than the same filesystem 
without barriers? And what kind of issues were that? Disks writing damaged 
sectors if powered-off in the middle of the writes? Or data corruptions 
due to bugs in ReiserFS?

> The vantage point I had at EMC while testing and deploying the original
> barrier work done by Jens and Chris was pretty unique - full ability to do
> root cause failures of any component when really needed, a huge installed base
> which could send information home on a regular basis about crashes/fsck
> instances/etc and the ability (with customer permission) to dial into any box
> and diagnose issues remotely. Not to mention access to drive vendors to
> pressure them to make the flushes more robust. The application was also able
> to validate that all acknowledged writes were consistent.
> 
> Barriers do work as we have them, but as others have mentioned, it is not a
> "free" win - fsync will actually move your data safely out to persistent
> storage for a huge percentage of real users (including every ATA/S-ATA and SAS
> drive I was able to test).  The file systems I monitored in production use
> without barriers were much less reliable.

With write cache or without write cache?

With cache and without barriers the system is violating the specification. 
There just could be data corruption ... and it will eventually happen.

If you got corruption without cache and without barriers, there's a bug 
and it needs to be investigated.

> As others have noted, some storage does not need barriers or flushed (high end
> arrays, drives with no volatile write cache) and some need it but stink (low
> cost USB flash sticks?) so warning is a good thing to do...
> 
> ric

Mikulas
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