> -----Original Message-----
> From: [EMAIL PROTECTED] <[EMAIL PROTECTED]>
> To: [EMAIL PROTECTED] <[EMAIL PROTECTED]>
> Date: Monday, May 24, 1999 1:02 PM
> Subject: Re: raid1 on ide decreases read performance
>
> raid-1 also increases read performance. It can do reads just like raid0,
> because both disks contain the same data. It doesn't read the same block
> from both disks and compare, instead it reads like from a stripe set
(raid0).
>
> At least that's the theory. Your points about several drives on the same
> bus etc. are ofcourse valid. If two IDE disks are put on _separate_
busses
> and the IDE chipset doesn't stink too much, you should see good read
performance
> both from raid0 and raid1.
The reason you don't get a speed up under raid1 is because
each drive is a physical device. As such there is no advantage
to asking 1 drive for one chunk of data while asking the other
for the next chunk of data UNLESS the chunks are large enough
to offset the time required for the drive to physically reach the next
piece of desired data.
Even if the transfer size is 64Kbytes, alternating from drive
to the other will not yield higher throughput because each
drive will have the seek to the next 64K chunk. How do they
reach the next chunk? By passing directly over the 64K chunk
that is not desired (because its coming from the other drive).
The time required to skip that 64K chunk is the same as the
time required to read that 64K chunk.
However, if you construct a test where there are two independent
streams of data to be read and the software raid1 implementation
is written properly, then you should see double the read performance.
Steve.
