linux-ext4 - Re: Severe slowdown caused by jbd2 process

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Message-id: <4D3A2EC6.3020700@shiftmail.org>
Date:	Sat, 22 Jan 2011 02:11:34 +0100
From:	torn5 <torn5@...ftmail.org>
To:	Ted Ts'o <tytso@....edu>
Cc:	Josef Bacik <josef@...hat.com>,
	Jon Leighton <j@...athanleighton.com>,
	linux-ext4@...r.kernel.org
Subject: Re: Severe slowdown caused by jbd2 process

On 01/22/2011 12:56 AM, Ted Ts'o wrote:
> On Fri, Jan 21, 2011 at 09:31:45AM -0500, Josef Bacik wrote:
>    
>> Yup, whatever you are doing in your webapp is making your database do lots of
>> fsyncs, which is going to suck.  If you are on a battery backed system or just
>> don't care if you lose your database and rather it be faster you can mount your
>> ext4 fs with -o nobarrier.  Thanks,
>>      
> Note that if you don't use -o barrier on ext3, or use -o nobarrier on
> ext4, the chance of significant file system damage if you have a power
> failure, since without the barrier, the file system doesn't wait for
> disk to acknowledge that the data has hit the barrier.  The problem is
> that if you are using a barrier operation, you're not going to be able
> to get more than about 30-50 non-trivial[1] fsync's per second on a
> standard HDD; barriers are inherently slow.
>    

I think that currently the fsyncs have a double meaning: they are used 
to make a filesystem operation happen before another filesystem 
operation, and to make a filesystem operation happen before a network 
operation. I don't think the second case can be speeded up (there can be 
a distributed transaction involved) but the first could probably be 
speeded up, but I'm thinking how...

Do you think nobarrier + data=journal would provide the same guarantees 
of barrier and almost the same performances of nobarrier (for random I/O)?

Hmm maybe you need the barriers enabled to make even data=journal work 
reliably?
But then there should be a mount option (barriersonlyjournal?) so that 
barriers are only generated every so many seconds and only for 
committing a big transaction to the journal, while applications' fsyncs 
would be made with nobarriers.
This should provide the benefits I mentioned, for disk-to-disk 
sequentiality (not disk-to-network), shouldn't it?

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