On Thu, 27 Apr 2000, Corin Hartland-Swann wrote:
>
> Holger,
>
> On Thu, 27 Apr 2000, Holger Kiehl wrote:
> > On Thu, 27 Apr 2000, Corin Hartland-Swann wrote:
> > > I was hoping that RAID-1 would 'stripe' reads between the disks,
> > > increasing read performance to RAID-0 levels, but leaving write
> > > performance at single-disk levels. Does anyone know why it doesn't do
> > > this?
> >
> > I think there is a patch for this, search the archive of this list.
>
> Thanks - where can I find the archive?
>
One is at http://kernelnotes.org/lnxlists/linux-raid/
> Also - is it a pretty stable patch? (This is a production server)
>
I don't know, sorry.
> > > If RAID-1 doesn't 'stripe' the reads, then it seems that my best bet is
> > > going for RAID-5 over 4 disks. Does anyone have any comments and/or
> > > suggestions about doing this?
> >
> > It depends on what you want to do with /. Since /var/log is also
> > located here and if you have big log files, I would make this also
> > a raid 5 partition. ftpd and httpd are doing lots of small writes
> > here, so raid1 will slow you down if its being used as a web or ftp
> > server.
>
> Can RAID-1 _ever_ be slower than RAID-5? Isn't it much simpler to write
> out data to 2 disks at identical offsets etc, rather than spread over
> various disks, plus parity calculations?
>
In writes raid5 will always be faster because the writes gets spread
across multiple disks. Whereas with raid1 each write will be written
to all disks and the raid code must wait until all disks have finished. So
with raid1 the more disk you put in the faster will be your reads, but
the writes will become slower. With raid5 both reads and writes will
increase. Someone correct me if I am wrong here.
Most the time raid1 is good for / since you mostly only read from here
except for /var and /tmp. And raid1 gives you a much better redundancy
then raid5. You might want to put /var and /tmp in separate raid5
filesystems. But I am not sure how the raid code performs if you have
lots of raid partitions on the same set of disks. Another problem
with lots of partitions is choosing the correct size. Thus, I always
make two raid5 partitions, one for / and the other for /home.
Holger
