[linux-lvm] How to best use LVM?

[William Blunn]

> 	- Can I use it to extend a RAID 5 set, 
> 	  or concatenate 2 RAID 5 sets together?

I think the core answer to a lot of your questions is that LVM doesn't do RAID (other than a form of RAID0) itself. Most of the time what LVM does is concatenation, and relies on RAID being done at a lower level, i.e. you use a RAID set as a PV.

LVM won't actually coalesce two RAID5 sets together into a single RAID5 set, but you can get the effect of concatenation. 

If you have two RAID5 sets, you can make each one into a PV, and then add both of those PVs into the VG. You still have two distinct RAID5 sets, but LVM will present it to you as one big area.

You can then create LVs in the VG, and create filesystems on the LVs. You can cheerfully create an LV which is bigger than the largest PV, or even one big LV which fills the entire VG.

If you already have data on a RAID5 set, and you want to convert it to LVM use, then you can't keep the data that is already on there.

Composing your VG out of RAID devices (rather than simple disk partitions) is the way to go. If you lose one PV, you tend to lose the whole VG. So it's a good idea to find some way of making your PVs resilient.

Re extending RAID5 sets, LVM won't help you to do that per se, i.e. it won't help you extend an existing RAID 5 set per se.

However if you already have a RAID5 set which you have put in a VG, and you get a new RAID5 set on your system (e.g. adding a new RAID5 system, or making a new RAID5 set out of some newly available disk space), then you can add that new RAID5 set to the VG. You can then extend your existing LVs into the new space in the VG, or create new LVs.

(( A lot of people seem to want LVM to do magic on their existing systems which are full of data, which is something it doesn't do. You are going to need to backup-and-restore, or set up LVM on new disks.

However, once you have subscribed to the LVM philosophy, bitten the bullet, and migrated your existing system over to LVM, then adding new space is trivially easy.

The real trick is to put in LVM on day one even if you're not sure you need it.

Like putting data cabling into a building, it's easy to put in at the beginning when the building is being built, but harder to put in later when you realise you need it. ))

> 	- Can I use LVM to combine different types of RAID, such as 1 
> 	  and 0?

Yes. You can have several PVs of different RAID types.

You can even tell LVM on which PV you want it to put each LV. You can even move parts of LVs or whole LVs to a different PV, after creation, without losing data.

However if you have some PVs with redundancy (e.g. RAID1 (mirrored)) it would probably not be sensible to mix them with non-redundant PVs (e.g. RAID0 (striped)). This is because the VG is only as strong as the weakest link.

> 	- Can I easily "grow" a RAID set using LVM?

Same question as the first part of your first question?

> The other question I had more directly relates to the Linux 
> implementation for LVM:
> 
> 	- Which version do I use?

Can't give you a complete answer because I have never used LVM2. I use LVM1 on a number of machines and it works fine for me. I use kernel 2.4.18 with LVM1.0.x patch, with Debian 3.0 (Woody) lvm10 (LVM1.0.4) userland tools.

Although you would need to give some more information about your situation. You are using Debian unstable which suggests you want to live on the bleeding edge, but you are also using RAID5 which suggests you want to make a stable reliable system. AFAICT these two goals are incompatible.

> Is lvm10 the 1.x series and lvm2 the 2.x? Or does lvm10 come after lvm2?

The former.

> If the former, is 2.x stable enough to use,

Same question as "Which version do I use?".

> and is it better to 
> compile lvm from source or use the pre-compiled debian packages?

For LVM1: I have found the pre-compiled debian packages to be OK.

> I've tried both the lvm10 and lvm2 packages, but continually recieve 
> errors when I do things like run lvdisplay (it complains about not 
> being able to flush the device several times), and also something 
> about not being able to open the device-mapper.

For LVM1: I believe the errors about flushing are fine.

Bill
-- 
William H. Blunn - <bill+s.9sfuw6uo at tao-group.com> - Developer Support
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