vinum performance

[Jason Andresen]

Michael C. Brenner wrote:

> Writing to a RAID5 stripe set requires that all disks in the array 
> successfully report completion before the RAID5 controller's buffer can 
> be released back to the cache. (Applies to either software or hardware 
> raid.) If you are doing a large block write (like dd) you can easily 
> fill the cache on most controllers. Once the cache is full, the 
> controller slows each write to the LONGEST completion time of each 
> spindle in the array. ECC calculation becomes part of the latency also. 
> In a 5 drive system (other than one where the cache is larger than the 
> largest file being written as in a large EMC array) the writes are 
> always about 4-5 times longer than the reads. Tuning stripes and 
> blocking factors can speed up a specific transfer but RAID5 has always 
> been slow to write large data and best for read mostly data.

Ahh, that makes sesnse.

> Read operations benefit from RAID5 or mirrors. Now the shortest 
> completion time of the minimal drive set is the gating event. The first 
> set of drives to deliver the data block ends the operation. This makes a 
> 2 to 1 difference into a 4 to one difference.

Thanks for clearing this up.

-- 
   \  |_ _|__ __|_ \ __| Jason Andresen        jandrese at mitre.org
  |\/ |  |    |    / _|  Network and Distributed Systems Engineer
_|  _|___|  _| _|_\___| Office: 703-883-7755