Re: Query regarding stripe management in linux md-raid

[Kunal Kushwaha <kunal.kushwaha@gmail.com>]

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) ?




>>
>> 1. If one page is allocated for one chunk instead of actual buffer
>> size, isn't memory lot of memory is wasted?
>
> It depends on what you mean by "wasted".  The memory is used to
> provide adequate performance with manageable code complexity.
> Possibly the same performance could be achieved using less memory, but
> I suspect the code would be much more complex and so probably more
> buggy.  It is a trade off.
>
> If the 5MB eats in to you 256MB too much, you can reduce it be writing
> a number to /sys/block/mdX/md/stripe_cache_size.
> You could probably get away with as little as '4', but I suspect that
> would really kill performance.  With a 64K chunk size you want at
> least 16 entries, so 32 or 64 might be a suitable compromise for you.
>
>> 2. It will restrict the no. of stripes in memory (in case of
>> non-swappable memory, no will be very less) will effect performance of
>> IO badly.
>
> The number of (page-wide) stripes in memory is fixed by the cache
> size.  It defaults to 256, but you can change it.  A smaller size
> would be likely to negatively affect performance.  A larger cache can
> improve performance depending on workload.
>
>>
>> Please correct me, if I am wrong, also any suggestion to overcome this
>> problem is most welcomed.
>
> Do you have an actual observed problem, or are you just trying to
> discover what problems you might eventually run in to?
>
> NeilBrown
>



-- 
Thanks in Advance,
Kunal Kushwaha
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