Re: raid0 vs. raid5 read performance

[Ingo Molnar]

On Thu, 29 Jul 1999, Lance Robinson wrote:

> AFAIK: RAID-5 accesses are always in stripes. All disks are read (or
> written) no matter how small the original read/write request. Whereas,
> RAID0 can read just one disk for smaller requests. RAID5 does a lot more
> work for smaller requests. 

this is true - but the 'bonnie' workload measures sequential access, so
stripe-granularity is not an issue here. My experience is that RAID5
read-performance is almost as high as RAID0 performance when using 4k
stripe size. In this case both Linux and the disk itself has more chances
to optimize. (disks will most likely read sequentially due to readahead
caching, and they will skip over parity blocks without skipping
physically). Nevertheless this means RAID5 performance will never be
better than (N-1)/N*RAID0_bandwith.

-- mingo