vinum performance

[Jens Rehsack]

Jason Andresen wrote:
> Mattias Pantzare wrote:
> 
>> Lukas Ertl wrote:
>>
>>> Ok. But I still don't understand why RAID 5 write performance is _so_ 
>>> bad.
>>> The CPU is not the bottle neck, it's rather bored. And I don't 
>>> understand
>>> why RAID 0 doesn't give a big boost at all. Is the ahc driver known 
>>> to be
>>> slow?
>>
>>
>>
>> To do a RAID 5 write you do this:
>> 1. Read the old data on the blocks that you will write to.
>> 2. Read the coresponding parity data.
>> 3. Write the new data.
>> 4. Write the new parity.
> 
> 
> Hmm, how about the case where you're writing new data?  You shouldn't 
> have to do steps 1 & 2, and yet the RAID5 write performance is still 
> abysmial.

Remember for that case that a block covered by the raid-system may be 
larger than 512 bytes. I use 32K for my fileserver, so to skip reading 
old data I had to write 32K blocks at once.

Of course, the system software (either vinum or the controller software) 
caches a little bit, so if you write enough small data you may get a 32K 
block (or whatever you use), full.

> I get 4565 K/sec on modern ATA/133 HDDs.
> 
> Reading is much better at 91908 K/sec at least.
> 

So long,
Jens