rehsack at liwing.de
Mon Mar 31 13:11:07 PST 2003
Jason Andresen wrote:
> Mattias Pantzare wrote:
>> Lukas Ertl wrote:
>>> Ok. But I still don't understand why RAID 5 write performance is _so_
>>> The CPU is not the bottle neck, it's rather bored. And I don't
>>> why RAID 0 doesn't give a big boost at all. Is the ahc driver known
>>> to be
>> 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
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.
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