[Beowulf] RAID for home beowulf

[Tomislav Maric]

Mark Hahn wrote:
>> 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.

I can't wrap my mind around the RAID config, because I'm using software
RAID: it supports linear mode, and RAID level 0,1,4 or 5. Since it acts
as if a partition is a device, this gives me way too much freedom (more
to think about). :))

So, maybe the bold question to ask would be: what would be the best RAID
config for 3 HDDS and a max 6 node HPC cluster? Should I just use RAID 1
for the system partitions on one disk,  and RAID 0 for the simulation
data placed on the same partitions on other two disks: after
post-processing, the data is gone anyway... and with a good backup
strategy, I don't have to worry about RAID0 not recovering from a disk
fail...

> 
>>> 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/...
> 

Well, I want to configure the net boot for all diskless nodes and use
the master node and it's RAID for a performance gains with writing CFD
simulation data against network communication and to be able to scale
more easily.

>>> 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.

Excellent, thank you very much!

Best regards,
Tomislav

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