linux-ext4 - Re: changing stride and stripe

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Message-ID: <4ADE28BF.6000605@redhat.com>
Date:	Tue, 20 Oct 2009 16:16:47 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Andreas Dilger <adilger@....com>
CC:	Doug Hunley <doug@...ley.homeip.net>, linux-ext4@...r.kernel.org
Subject: Re: changing stride and stripe_width post-fs-creation?

Andreas Dilger wrote:
> On 20-Oct-09, at 11:32, Doug Hunley wrote:
>> Is it safe to use tune2fs to alter stride and stripe_width on an ext4
>> fs once it has been created? Any caveats I should know about? Thanks
> 
> 
> The stride is mostly used at fs creation time, but there is no problem
> with changing it.  The stripe_width is used by the allocator to align
> file allocations with the RAID layout.
> 
> One question for Eric is whether the new libdisk patches he made will set
> the stripe_width to something ridiculous like 512 or 4096 bytes, or if it
> just leaves that field unset in that case.  I suspect it would be bad for
> mballoc to see the stripe_width be such a small value.

well... yes, it does set it to whatever is reported:

+       min_io = blkid_topology_get_minimum_io_size(tp);
+       opt_io = blkid_topology_get_optimal_io_size(tp);
+       blocksize = EXT2_BLOCK_SIZE(fs_param);
+
+       fs_param->s_raid_stride = min_io / blocksize;
+       fs_param->s_raid_stripe_width = opt_io / blocksize;


if mballoc can't handle certain values then maybe the kernel code should 
be changed to ignore it?  Small values could just as easily come from a 
user too ...

-Eric
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