> [mailto:[EMAIL PROTECTED]]On Behalf Of Juan Piernas
> C�novas
> Sent: Saturday, July 10, 1999 4:04 PM
>
> I have two questions:
>
> 1.- Why are RAID-1 writes slower than writes to one of the
> RAID-1 disks?. Software overhead?. Hardware problems?.
For every write request, two physical writes are performed, under RAID1.
One write to each mirror. Under SCSI, with asynchronous access, this can
be overlapped to some degree, but not entirely in parallel. Under IDE,
parallel writes are next to impossible and even asynchronous writes are
difficult. This is somewhat ameliorated with multiple disk controllers,
evenso not entirely. The end result is that write times are 25% to 100%
longer in duration, depending on hardware configuration..
> 2.- Why are RAID-1 reads slightly better than reads from one
> of the RAID-1 disks?. If a RAID-1 is made up of two disks,
> I've got two copies of data and two request can be made in
> parallel. Why isn't read performance much better?. With
> RAID-0 and a single file I get nearly a two fold
> performance.
RAID0 is faster becasue of overlapped IO, on a striped volume-set,
theoretically. This would not be true with two RAID0 disks, on the same
IDE bus, becasue IDE can not overlap IO. With RAID1, on SCSI or separate
IDE controllers, there should be a 0% to 40% improvement, in read
access, due to over-lapped IO, depending on the exact hardware
configuration.
I hope this helps.
