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Re: Software-RAID and partitioning

> Christian Pernegger wrote:
> >
> > I have 3 18GB SCSI disks I want to use in a "new-style" Soft-RAID-5
> > configuration. At the moment I have
> >
> > 1. partition    sd.4    swap
> > 2. partition    sd.3    ext2            (for squid)
> > 3. partition    sd.1    raid-auto
> >
> > This of course means I have to have the whole md device under one
> > mountpoint (the directories under which are the targets of symlinks
under / .)
> >
> > I'd rather be able to have a real seperate partition on the array for at
> > /home.
> >
> > Is it feasible to split the raid partitions (sd.1) on the disks and
create an
> > md0 and an md1, or does this hamper performance?
> >
> > Thanks
> >
> > Christian
> >
> > --
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Mike Fedyk wrote:
> I'm looking into making a raid setup also, and I'm not familiar with the
> "new-style" you're talking about.
> You would have to resize all of the partitions on each drive.
> I'm not too familiar with Raid5.  Does the parity partition have to be the
> size as the data partitions?

In my experience with software RAID (on Solaris with Solstice DiskSuite),
partitions need to be the same size (and *geometry*:
cylinders/heads/sectors) for each RAID slice. Remember, in *any* redundant
RAID level (1 and higher), there has to be parity for each bit. So therefore
one can reasonably conclude the partitions must be exactly the same size.

Now, one doesn't have to use the whole drive for a RAID 5 partition. You
could partition each of those 18 GB drives in half, say, and RAID 5 the
first half across the three for a 18 / 2 * (3 - 1) = 18 GB RAID 5. Then use
the leftover 3 partitions for straight filesystems. Or mirror two of them
with RAID 1, or stripe two of them with RAID 0, etc...

Clearly, it can get a little academic with these configurations. I'm not
sure what the performance impact is of having one or more partitions on a
drive devoted to RAID while having one or more just normal. Anymore, if I
want large filesystems, I tend to simplify my life and use each whole drive
in my RAID 0 or 5.

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