linux-ext4 - Re: Ext4 MAX journal size ?

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Date:	Tue, 08 Dec 2009 11:26:40 -0700
From:	Andreas Dilger <adilger@....com>
To:	Iavor Stoev <iavor@...soft.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: Ext4 MAX journal size ?

On 2009-12-08, at 09:43, Iavor Stoev wrote:
> We use this setup for our backup servers.
> We use rsync via SSH using hard links as backup technology; the  
> backup server is pulling the data from several servers.
>
> The setup is 12x1TB disks in RAID6 128k stripe, using ext3 4k block  
> + lvm2 with the journal on Gigabyte I-RAM drive 1GB DDR400.
> The server has 8GB RAM.
>
> The journal mode is data.
> The journal size is 400MB.
>
> When we moved the journal on the external device we have gained like  
> 20%
> performance improvement with our backup.
> I'm converting several servers to ext4 to see what will be the  
> performance improvement for our workload.
>
> Do you have any suggestions regarding the journal size and the  
> overall file system setup?

You should definitely make sure you create enough inodes (use -i or - 
N), and use the flex_bg option (enabled by default for ext4) to  
improve metadata performance.

> Andreas Dilger wrote:
>> On 2009-12-07, at 14:46, Iavor Stoev wrote:
>>> I wonder if the Ext3's MAX journal size of 102,400 file system  
>>> blocks
>>> has been increased in Ext4.
>>>
>>> I'm using 10TB 4k block Ext3 file system with external journal on  
>>> Gigabyte I-Ram drive and I'm planning a migration to Ext4 system.
>>> And I wonder if I can increase the journal size over 400MB.
>> Well, even with ext3 the maximum journal size was only for internal  
>> journals.  It was always possible to have larger external journal  
>> devices.
>> With ext4, the maximum journal size WAS increased, though this is  
>> in fact a mke2fs/tune2fs limit so it is also increased for new ext3  
>> filesystems.
>> Note that with large journals you are also consuming an equal  
>> amount of RAM as the size of the journal, so don't make it crazy  
>> big.  Having a journal on SSD is only really noticable for sync- 
>> happy workloads.  It isn't noticably better than using a regular  
>> disk for the external journal if you aren't doing a lot of syncs  
>> (e.g. NFS or email).
>> I've thought in the past that it might be an interesting hack to  
>> use a huge journal device (say 32GB) with data journaling, and then  
>> have the JBD layer get the data blocks from the journal for  
>> checkpointing to the filesystem instead of keeping the buffers  
>> pinned in RAM.  That would would allow blazing metadata workloads,  
>> zero seeking, and then checkpointing in bulk to the  
>> filesystem.  ... but unfortunately not something I have time to  
>> test out.
>> Cheers, Andreas
>> -- 
>> Andreas Dilger
>> Sr. Staff Engineer, Lustre Group
>> Sun Microsystems of Canada, Inc.
>
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Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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