Re: Filesystem benchmarking for pg 8.3.3 server

On Sat, 16 Aug 2008, Decibel! wrote:

> On Aug 13, 2008, at 2:54 PM, Henrik wrote:
>>> Additionally, you need to be careful of what size writes you're using. If 
>>> you're doing random writes that perfectly align with the raid stripe size, 
>>> you'll see virtually no RAID5 overhead, and you'll get the performance of 
>>> N-1 drives, as opposed to RAID10 giving you N/2.
>> But it still needs to do 2 reads and 2 writes for every write, correct?
>
>
> If you are completely over-writing an entire stripe, there's no reason to 
> read the existing data; you would just calculate the parity information from 
> the new data. Any good controller should take that approach.

in theory yes, in practice the OS writes usually aren't that large and 
aligned, and as a result most raid controllers (and software) don't have 
the special-case code to deal with it.

there's discussion of these issues, but not much more then that.

David Lang