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Re: lvm with raid

On Thu, 1 Jul 2004 17:43, Christoph Moench-Tegeder <cmt@burggraben.net> wrote:
> ## Russell Coker (russell@coker.com.au):
> > > ## Gustavo Polillo (gustavo@ib.usp.br):
> > > >   Is it possible to make lvm with raid ?? Is there anyone here that
> > > > make it?
> > >
> > > Works as expected. RAID appears as a simple SCSI drive.
> >
> > Only for hardware RAID.
> 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.  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).

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 you want just two disks for reliability then software RAID-1 is the easiest 
and most reliable.  You can mount a RAID-1 file system as a non-RAID device 
if you wish.  One big advantage of Linux software RAID is that you know 
what's going on.  With every hardware RAID system I've ever seen or heard of 
(including the biggest ones that Sun sells) there have been situations where 
the administrator finds themselves without a way of discovering what's 
happening with their data.  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.  The more expensive RAID hardware has a computer in it 
which can have protocol errors when talking to the host or simply crash.

> > Software RAID looks quite different to the OS and
> > there are still some minor quirks in getting it working for boot devices.
> > One of which is that for LILO you need the MBR to be provided by the
> > debian-mbr program and have the LILO block inside the RAID, as well as
> > having identical block numbers in both disks in the RAID-1 (RAID-5 and
> > RAID-0 is not supported).
> That's why I don't use software RAID. Thanks for the summary.

The summary was not for your benefit.  It was for the benefit of people who 
actually want to get work done.

http://www.coker.com.au/selinux/   My NSA Security Enhanced Linux packages
http://www.coker.com.au/bonnie++/  Bonnie++ hard drive benchmark
http://www.coker.com.au/postal/    Postal SMTP/POP benchmark
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