On Fri, Apr 11, 2003 at 01:08:49PM +0200, Johnny Billquist wrote:
> On Fri, 11 Apr 2003, Jukka Marin wrote:
> > The biggest surprise was that RAID1 is slower than the disks made me
> > expect.  When the RAID is 100% busy, the disks are not, neither is the
> > CPU.
> 
> Isn't that to be expected?
> You are distributing disk I/O through one device to several other
> devices. When that device is 100% busy, the underlying disks ought to be
> (1/n)*100 % busy. Thus, if you only have one disk, it should be 100% busy,
> but if you have two disks, they should only be 50% busy, both of them.

Hmm.  If I write to RAID1, doesn't it write to both disks simultaneously,
making both disks 100% busy when RAID1 is 100% busy?  How about reads, does
it read the same data from both disks, compare the data and then pass it to
user process or does it read only one disk (doesn't seem so) or does it
read part of the data from disk 1 and part from disk 2?  If it uses both
disks, then why is RAID1 50% slower than a single disk?

  -jm