[GLLUG] RAID1 and Debian 7.2.0 installer

John Edwards john at cornerstonelinux.co.uk
Mon Oct 14 16:07:42 UTC 2013

On Mon, Oct 14, 2013 at 03:54:51PM +0100, James Roberts wrote:
> On 14/10/13 14:14, John Edwards wrote:
>> The first thing to check is whether a machine that old can detect the
>> 3TB disks correctly. Then check whether the 3TB disks are partitioned
>> using the DOS style partitions or the newer GPT partitions.
>> Also I assume that the machine can not boot using EFI and so uses a
>> conventional PC BIOS.
> I have had all sorts of "interesting" issues with accessing
> partitions/disks >> 2TB and have not yet got a successful general
> solution using single partitions (or equivalents) and any form of
> software RAID. The tools overall are limited in number and there's
> not much up to date info around.

Basically for physical disks over 3TB in size you really should use
GPT partitions. You can write straight to an un-partitioned disks as
some people have suggested, but then you have no way of telling the
difference between that and a blank disk.

> I guess ZFS/BtrFS would eventually be a comprehensive answer.

I think that both are a little too bleeding-edge for most people.

> The solution I have adopted for some machines is as follows:
> - partitions or disks >2TB using LVM on RAW on hardware RAID for
> data mounted into...
> - a separate <2TB partition (or sometimes a USB key) for the boot
> and/or system...

Having the boot and/or root filesystems on a smallish (say 10GB)
partition at the start of the disk can be useful if things go wrong
because it isolates problems and is also a lot quicker to fsck.

> - but that does not help you.
> It's possible to do Software RAID on LVM partitions but... complex.
> Horrid even.

And wrong. Sistina and RedHat say that LVM should be placed on top of
RAID and not the other way around.

> The only times I have lost data on Linux it has involved LVM +
> Software RAID (together - separately, they are just fine to fix)

I've used LVM on top of Linux software RAID (RAID1, RAID10, RAID5 and
RAID6) on many servers for many years without any problems between
them (just the usual disk failures). They do quite separate jobs and as
long as you understand that then I think things go alot smoother.

Having said that 'pvmove' can still be quite scary to run (it
"magically" moves parts of the logical volumes from one physical
entity to another).

|    John Edwards   Email: john at cornerstonelinux.co.uk    |
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