"Roeland M.J. Meyer" wrote:
>
> > [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..
I see.
>
> > 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
I know.
> IDE controllers, there should be a 0% to 40% improvement, in read
> access, due to over-lapped IO, depending on the exact hardware
> configuration.
Yes but, why don't you have a 70% to 80% improvement like with RAID0?.
You also have overlapped IO, don't you?.
>
> I hope this helps.
Yes, thanks.
--
D. Juan Piernas C�novas
Departamento de Ingenier�a y Tecnolog�a de Computadores
Facultad de Inform�tica. Universidad de Murcia
Campus de Espinardo - 30080 Murcia (SPAIN)
Tel.: +34968364633 Fax: +34968364151
email: [EMAIL PROTECTED]
