[Nottingham] LVM and load balancing and ReiserFS

[Peter Chang]

[There's two approaches to software RAID: using the multiple disks (md) 
driver or using LVM. The latter doesn't do parity based redundacy.]

On Fri, 26 Oct 2007, Martin wrote:
[...]
> Can LVM be used whereby directory trees are 'sensibly' spread across 2
> or 3 HDDs?

Depends on what you mean by sensible. LVM is based on aggregating physical 
volumes (which can be partitions) into volume groups that can then be 
split into logical volumes. Within each type of volumes, there are extents 
which match in size. So a 1-to-1 mapping can be made between an LE and a 
PE or a 1-to-N mapping for mirroring. The mapping of extents in a LV can 
be between different PVs for striping. See
  http://tldp.org/HOWTO/LVM-HOWTO/

> Or is LVM 'dumb' and just provides an extended linear LBA address range?

The LV is presented as a flat space so there is no way for any filesystem 
to know where its directories and files are going to end up.

> What happens if you have 3 HDDs and you remove one of the HDDs?

Boom!?! You'll have to dig up the dirty details yourself but with striping 
you're obviously SOL. With mirroring, it will find that some PV of a LV is 
missing and you can replace that PV.

> What is the ReiserFS strategy for physically placing directory trees and
> files across a disk?

Don't know, I don't use it.

hth,
  Peter


This message has been checked for viruses but the contents of an attachment
may still contain software viruses, which could damage your computer system:
you are advised to perform your own checks. Email communications with the
University of Nottingham may be monitored as permitted by UK legislation.