Re: changing stride and stripe_width post-fs-creation?

[Eric Sandeen <sandeen@redhat.com>]

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