linux-kernel - Re: [PATCH 0/2] Add inode checksum support to ext4

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Message-ID: <20110731070832.GA2848@noexit.corp.google.com>
Date:	Sun, 31 Jul 2011 00:08:32 -0700
From:	Joel Becker <jlbec@...lplan.org>
To:	Coly Li <colyli@...il.com>
Cc:	Andreas Dilger <adilger.kernel@...ger.ca>,
	"Darrick J. Wong" <djwong@...ibm.com>,
	Andreas Dilger <aedilger@...il.com>,
	Mingming Cao <cmm@...ibm.com>, Theodore Ts'o <tytso@....edu>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] Add inode checksum support to ext4

On Sat, Jul 30, 2011 at 03:25:32PM +0800, Coly Li wrote:
> On 2011年07月29日 21:19, Joel Becker Wrote:
> > On Fri, Jul 29, 2011 at 03:48:45AM -0600, Andreas Dilger wrote:
> >> On 2011-07-28, at 4:07 PM, Joel Becker wrote:
> >>> 	We use ethernet crc32 in ocfs2.  btrfs uses crc32c.  Frankly, I
> >>> could have used crc32c if I'd really thought about the hardware
> >>> acceleration benefits.  I think it's a good idea for ext4.
> >>
> >> The problem with crc32[c] is that if you don't have hardware acceleration
> >> it is terribly slow.
> > 
> > 	We find ethernet crc32 just fine in ocfs2.  I use the kernel's
> > implementation, which survives everyone's network traffic, and of course
> > we added the triggers to jbd2 so we only have to do the calculations on
> > read and write.
> > 
> 
> Ext4 supports non-journal mode, and there are a few users (Google, Taobao, etc.).
> A trigger of jbd2 may not work well for non-journal Ext4 ...
> 
> And in non-journal mode, there is not copy of any meta data block in jbd2, we need to be
> more careful in check summing, e.g. inode/block bitmap blocks...

	Sure, but you could use a trigger in journaled mode and then do
the checksums directly in the __ext4_handle_journal_dirty_*() functions
in non-journaled mode.  Sure, it would be a little more CPU time, but
the user picked "checksums + no journal" at mkfs time.

Joel

-- 

 Joel's Second Law:

	If a code change requires additional user setup, it is wrong.

			http://www.jlbec.org/
			jlbec@...lplan.org
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