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Re: lvm with raid

## Russell Coker (russell@coker.com.au):

> > Yes. Given the price of RAID controllers (ServerRAID, for example) and
> > the problems of software RAID, I strongly suggest getting a decent
> > controller and do whatever RAID level you need.
> Hardware RAID is more expensive.

Yes. It's not a cheap solution.

> Some of the SCSI hardware RAID devices have 
> bottlenecks on throughput (I suspect to help sell the high-end models without 
> the bottleneck).  The 3ware hardware RAID devices are well known for 
> saturating the PCI bus, but to get maximum performance out of them some 
> people used to use two 3ware cards in two PCI buses with software RAID-0 to 
> beat the PCI bottleneck (I can't remember if it was 32bit-66MHz or 
> 64bit-33MHz, but the end result was that performance didn't go much better 
> than about 210MB/s on a single 3ware card).

PCI-X rulez :)
Seriously, as I need more disk space and CPU than disk IO, I went for
RAID 5. If level 0 or 1 fits your application better, software RAID
might be an option. But why burn CPU on RAID when your controller
brings it's own CPU? And for mirroring disks, why not take the
on-board controller?

> The vast majority of hardware RAID devices are too slow to handle more than 4 
> disks at full speed, the way they lay the data on the disk is not documented 
> (so if they mess up it will be really bad for you), and they really aren't 
> that cheap (not anything that's worth using).

If your storage messes up, it will take the filesystem with it.
And given the speed of TSM in our installation, you really
do not want to think about desaster recovery. I want a new
tape library...

> If you want just two disks for reliability then software RAID-1 is the
> easiest and most reliable.


>   The cheapest RAID often doesn't support telling 
> Linux about the status, or has so many bugs in the driver software that you 
> can't rely on it.

Without well chosen and tested hardware, you are lost anyway.
So it boils down to: use software for level 0 (and 1, if it has
to be cheap), spend big money, if you want level 5.


Spare Space

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