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

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Date:	Sat, 30 Jul 2011 15:25:32 +0800
From:	Coly Li <colyli@...il.com>
To:	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 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...


>> Yes, it makes sense to just put a "fake" dirent at the end of the leaf block,
>> or similar.  I don't think it is necessary to modify existing directories or
>> extent blocks to add these structures in, if there is no room, but for new
>> blocks, or blocks with space it is enough.
> 
> 	We have tunefs.ocfs2 code to adjust existing directories to add
> the trailer.  It's not too bad, really.

Agree, enable/disable the trailer isn't that difficult. We need more eyes to take care the user space tools.

Coly
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