Re: Byte blasts Linux?

Paul Doom (pauldoom@webcreate.net)
Wed, 09 Jun 1999 20:42:52 +0000


> > I always thought that Linux striped through all configured swap
> > partitions, and I didn't need to do swap over RAID. In fact, if I
> > remember well, the documentation of the Raid tools says that swapping
> > over a striped device won't give better performance, because the kernel
> > already stripes swap partitions...
>
> Well, you certainly want to put the swap space on the RAID array because
> otherwise, you are doomed if one of your drives blows up. ;)

Linux only writes swap evenly accross multiple disks (stripe) if you set the
swap priority of each swap
partition to an equal value. If one is set higher than another (which is
what swapon defaults to doing if
you do not specify priorities), then you get the same effect as in using the
"linear" (also called "append")
type md driver, where the kernel only writes to one swap partition till said
partition is full, then it writes to
the next, following the swap priorities. See the swapon man page for
details. Using a RAID 0 device for swap instead of using multiple swap
partitions may or may not be a win. Using RAID 0 will lose on overhead to
direct swap, but since you can tune the stripe size on a RAID device, it
might come out ahead with
certain hardware. (I would be interested to see this tested.) Using swap on
RAID levels other than 0 may be of use to some for fault tolerance.

-Paul

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