vinum performance

[Greg 'groggy' Lehey]

[Format recovered--see http://www.lemis.com/email/email-format.html]

Alternate long/short.

On Sunday, 30 March 2003 at 18:50:20 +0200, 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.

These two go in parallel.

> 3. Write the new data.
> 4. Write the new parity.

So do these.

> For RAID0 to be faster than a singel disk you have to do reads that
> match or are larger than a stripe. Or have several processes
> generate the IO.

Several processes is the obvious one.  The random seeks test does 3
processes (and you'll note that in this one, RAID-5 exceeds the
performance of a single disk), but that's still not enough.  rawio
uses 8 processes by default, but you can change it.

Greg
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