[linux-lvm] Safeguarding LMV data?

[linux-lvm] Safeguarding LMV data?

[Craig Hagerman]

Hi,

I am using LVM to make two SATA discs (200GB and 250GB respectively)
into one big logical volume. As well I have an ATA (80GB) disc which
has 3 partitions (Debian /, and /usr, and another partition with
Ubuntu installed)

If I understand things correctly, if one of those drives fails I will
lose all the data spread across both drives. Is this correct? When I
set this up (over a year ago) I guess I kind of assumed that if one
drive failed I could shrink the LV and still have access to the data
on the good drive.

I think I have read that the best way to safeguard my data would be to
set up the hard drives as RAID 5 (which would require an additional
drive - minimum 3, right). I didn't consider this when I first set up
the LVM.

I am wondering if there is anything I can do now to mitigate the
chance of one drive failing and losing all data? I have thought about
buying another drive (say a 400GB), moving the data on there, setting
up LVM with RAID 5 and then moving the data back .... except that the
new drive would be intended to be used in that RAID 5 configuration. I
don't think I have a good enough understanding of either RAID 5 or LVM
and would really appreciate some advice for what I can do.

Craig Hagerman

Re: [linux-lvm] Safeguarding LMV data?

[Matthew Gillen]

Craig Hagerman wrote:
> If I understand things correctly, if one of those drives fails I will
> lose all the data spread across both drives. Is this correct?

Essentially.

> When I
> set this up (over a year ago) I guess I kind of assumed that if one
> drive failed I could shrink the LV and still have access to the data
> on the good drive.

I've heard of people that have been able to save data in that situation, but
it's sort of black magic, and you never know what exactly you'll be able to
recover.  From the filesystem's perspective, it's like half your blocks went
bad.  Filesystems really aren't built to handle those situations gracefully.

> I think I have read that the best way to safeguard my data would be to
> set up the hard drives as RAID 5 (which would require an additional
> drive - minimum 3, right). I didn't consider this when I first set up
> the LVM.

Why not RAID 1?  RAID 5 requires 3 disks of the same size (with the RAID
device having the capacity of 2 disks).  RAID 1 (mirror) requires two disks of
the same size (with the RAID device having the capacity of one disk).

> I am wondering if there is anything I can do now to mitigate the
> chance of one drive failing and losing all data? I have thought about
> buying another drive (say a 400GB), moving the data on there, setting
> up LVM with RAID 5 and then moving the data back .... except that the
> new drive would be intended to be used in that RAID 5 configuration. I
> don't think I have a good enough understanding of either RAID 5 or LVM
> and would really appreciate some advice for what I can do.

Read the RAID howto first, and figure out what you want to do at the RAID
level, taking into consideration how many drives you want to buy, the level of
redundancy you need, etc:
http://tldp.org/HOWTO/html_single/Software-RAID-HOWTO/

Once you have the specifics of a particular setup, ask again what to do at the
LVM layer (include at least how big your existing LV is, and how big your new
drive(s) are).

Matt