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

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Date:	Mon, 01 Aug 2011 07:52:41 +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31日 15:08, Joel Becker Wrote:
> 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.
> 

Yes, my idea was similar to you.
One thing not clear to me is, in non-journal mode, how to make the page of bitmap block being stable. Because bits
setting in Ext4 bitmap is non-locking, it might be possible that new bit setting after check sum is calculated.

Coly



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