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Re: lvm with raid

On Fri, 2 Jul 2004 05:09, Christoph Moench-Tegeder <cmt@burggraben.net> wrote:
> 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?

Does software RAID-5 really burn CPU?  See the below web page for the speed of 
an old machine in doing the RAID-5 checksum calculations.  Given that data to 
be written to disk will already be in the cache it seems that there won't be 
any significant overhead for this.

The advantage of hardware for RAID-1 is that there are bottlenecks for IO 
speed.  Doing only half the writes on the motherboard side will help things.

> > 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.

Not necessarily.  Sometimes you just have the RAID devices refuse to recognise 
the storage.  If you know the block layout then writing a program to 
reconstruct a RAID-5 from the set of disks (or even the set minus one disk) 
should not be difficult.

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