Re: bitmap chunk size

[Doug Ledford <dledford@redhat.com>]

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On 11/10/2009 11:39 AM, Darius S. Naqvi wrote:
> Is there any possibility of having a bitmap chunk size of 512 bytes?
> I know that mdadm rejects anything under 4k.  I fear that the
> assumption of the 4k minimum is embedded fairly strongly in the code.
> Can my fear be alleviated?
> 

If you're putting any normal filesystem (with a block size of 4k) on
this, then it makes absolutely no sense to have a bitmap size less than
4k as any given filesystem block is either dirty or clean, sub-block
semantics make no sense in this scenario.  That said, unless you have a
specific need for this level of granularity, it is a really bad idea
(performance wise and space wise) to go with anything even close to
resembling the granularity you are requesting.  I usually go with
--bitmap-chunk=32768 (which since that's expressed in k means
32Megabytes).  I would actually suspect that if you have a truly
pressing need for a 512byte bitmap chunk, then you probably don't need a
bare raid, you need some sort of database underlying your data or
something else.

-- 
Doug Ledford <dledford@redhat.com>
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Infiniband specific RPMs available at
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