On Tue, 18 Jun 2013 16:52:09 +0400
Роман Алексеев  [off-list ref] wrote:

Hello,

I want to speed up rebuilding process of the raid to do that I'm going
to apply the command:

mdadm --grow --bitmap=internal --bitmap-chunk=NNNN /dev/mdX

But I don't know how to calculate optimal value of bitmap-chunk option.
How can I do that?

It's just the minimum granularity of pieces that get rebuilt after unclean
shutdowns. E.g. if your array decides it's "dirty" in 5 different areas, with
a 16MB chunk the rebuild will process 5*16MB, but with a 128MB it will rebuild
5*128MB. On modern systems the difference in time required to do the rebuild
between these two will be negligible. But having a larger size helps
noticeably in everyday operation (makes writes faster). So there is nothing to
"calculate" really, just use a reasonably large size like 128 or 256 MB.

https://raid.wiki.kernel.org/index.php/Write-intent_bitmap
http://techblog.tgharold.com/2013/01/mdadm-using-bitmaps-to-speed-up-rebuilds.html
http://blog.liw.fi/posts/write-intent-bitmaps/ (the 1st comment)

-- 
With respect,
Roman