[Beowulf] RAID for home beowulf

Mark Hahn hahn at mcmaster.ca
Sat Oct 3 14:19:30 PDT 2009

> It depends on your workload. RAID5 is good for large sequential writes,
> but sucks at small sequential writes because for every write it has to
> do a read to compare parity.

well, it's bad at small random writes.  small _sequential_ writes would 
be able to avoid reads for all but the first transaction.

IMO, raid5 is often unappealing because raid10 avoids the write penalty,
and raid6 is a lot more survivable.  ultimately it depends on your taste
in trading off performance, space efficiency, risk.

>> 2) I want to put the /home at the beginning of the disks go get faster
>> write/seek speeds, if the partitions are the same, software RAID doesn't
>> care where they are?
> I don't think this will buy you much performance. There probably is a
> measurable difference, but I don't think it's enough to worry about.

inner tracks are normally about 60% of the speed of outer tracks - 
that's for a normal density-optimized disk, not a latency-optimized 
(and therefore inherently small) "enterprise" disk.

>> 3) I'll leave the /boot partition on one of the 3 disks and it will NOT
>> be included in the RAID array, is this ok?
> Sure, but /boot is actually trivial to mirror. Just make sure your boot
> loader is on each disk in the mirror and that each disk is partitioned
> identically, and all you have to do if a drive dies is change the device
> you boot off of if a drive dies.

or better yet, don't bother booting of the local disk.  simply make your 
head/admin/master server reliable and net-boot.  it's likley that nodes 
won't be functional without the master server anyway, and net-booting 
doesn't mean you can't use the local disk for swap/scratch/...

>> 4) I've read about setting up parallel swaping via priority given to
>> swap partitions in fstab, but also how it would be ok to create RAID 1
>> array of swap partitions for the HA of the cluster. What should I choose?
> Any swapping at all will kill performance. I would get enough RAM to
> make sure you don't swap.

well, using swap space is harmless as long as you're not actually swapping
_in_ any nontrivial amount.

unless you have some very extreme parameters (uncheckpointable long jobs,
flakey hardware or power, banking-level reliability expectations),
I wouldn't bother raiding swap.

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