Re: Query regarding stripe management in linux md-raid

[Goswin von Brederlow <goswin-v-b@web.de>]

Kunal Kushwaha <kunal.kushwaha@gmail.com> writes:

> Hi Neil,
>
> Thanks for your reply. I am trying to find out  problems that I could
> face later on.
>
> On Wed, Apr 29, 2009 at 7:15 AM, Neil Brown <neilb@suse.de> wrote:
>> On Tuesday April 28, kunal.kushwaha@gmail.com wrote:
>>> Hi,
>>>
>>> I am trying to put Linux raid in Box with 256 MB of RAM. The kernel is
>>> compiled with non-swappable memory management option. I looked into
>>> raid5.c and found, it allocates one page for each chunk.  I am using 5
>>> disks for 64k chunk size. considering my kernel is within 30 MB.
>>
>> That isn't quite right.   It is not 1 page per chunk.
>> raid5 maintains a stripe cache.  Each entry in the cache has one page
>> per device, and there are 256 entries by default.
>> So for a 5-disk array, that is 5*256 == 1280 pages or 5MB (plus
>> overhead).
>>
>
> Sorry I missed to mention about stripe cache. Yes we will be using
> only 5MB if we
> consider page size is of 4k.
>
> I have one more doubt regarding this. How a page of 4k will be able to
> store data of
> 64k( my chunk size if of 64k) ?

A chunk size of 64k does in no way mean all transactions are done in
64k chunks.

What effect does chunk size actually do have? Was it just the amount
of sequential data before the parity is rotated in a raid5/6.

MfG
        Goswin
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