Re: RAID5 created by 8 disks works with xfs

[John Robinson <john.robinson@anonymous.org.uk>]

On 02/04/2012 11:01, Jack Wang wrote:
> 2012/4/2 John Robinson<john.robinson@anonymous.org.uk>
>> On 02/04/2012 04:15, Jack Wang wrote:
>>> 2012/4/1 John Robinson<john.robinson@anonymous.org.uk
>>> <mailto:john.robinson@anonymous.org.uk>>
>> [...]
>>>     This might also be a good application for bcache, FlashCache or
>>>     whatever.
>>>
>>> Using bcache/flashcache is what I'm considering, does above
>>> configuration option still needed?
[...]
> Thanks for reply , I mean using SSD as cache device, and raid5
> 16*7200rpm disks, is this works for streaming workload above?
>
> I know bcache is try to turn rand write to seqencial write, but most
> ssd is not good at RAND write as I know.

I don't know where you got that from; SSDs are absolutely brilliant at 
small random writes and reads, hundreds of times better than spinning 
hard discs. bcache and flashcache were written specifically to take 
advantage of SSDs' high I/Os per second for small (random) reads and writes.

I wouldn't touch RAID5 for anything other than 3 drives; I'd use RAID6 
or RAID60 for 4-plus where I didn't need the better small/random write 
performance of RAID10. I haven't actually used bcache or flashcache, 
which is why I introduced them with "might".

My fantasy configuration in your 16-drive chassis would be 2 6-drive 
RAID6s, striped together in RAID0, with an SSD cache over the top built 
from two SSDs in RAID10 (or if I was feeling really paranoid, 3 SSDs in 
RAID10,n3), with the remaining slot containing a hot spare for the RAID6s.

Cheers,

John.