Re: Possible to change chunk size on RAID-1 without re-init or destructive result?

[Roman Mamedov <rm@romanrm.ru>]

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On Wed, 27 Mar 2013 09:23:52 -0700
Jeff Johnson <jeff.johnson@aeoncomputing.com> wrote:

> And yet I have this output from /proc/mdstat:
> 
> md1 : active raid1 sdb3[1] sda3[0]
>       288567164 blocks super 1.1 [2/2] [UU]
>       bitmap: 3/3 pages [12KB], 65536KB chunk
> 
> It is very strange. the responsiveness on small file i/o tends to
> support the notion that this mirror really has a 64MB chunk size. This
> is practically an order of magnitude larger than what is prudent. The
> iowait on simple things like a sync or writing out small files seems
> to support what mdstat is reporting. Of course, I'd like to change
> this but how to do so without breaking the RAID or risking data is not
> obvious.

This is the array _bitmap_ chunk size. In simple terms, it determines
granularity of array resyncs on unclean shutdowns.

You can change it by

  mdadm --grow /dev/md1 --bitmap=none

  mdadm --grow /dev/md1 --bitmap=internal --bitmap-chunk=131072

But the size you already have is okay, there is no need to change it, and I'd
say certainly no need to lower it (this will decrease performance).

-- 
With respect,
Roman

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