[linux-lvm] Do I loose all my data if one disk crashes?

[linux-lvm] Do I loose all my data if one disk crashes?

[Dave Newman]

Hi --

I just installed lvm on three 160 GB IDE hard drives, giving me a singe 
480 GB partition.

I followed the instructions:
http://www.sistina.com/lvm_howtos/lvm_howto/Setting_up_LVM_on_three_S.html
and the install went exactly as described.

I did a "vanilla" install (doing nothing special).  

My question is: Do I loose all my data if any of the three disks 
crashes?  And if so, what can I do to improve the "restore-ability" of 
my configuration.

Thanks,
Dave

Re: [linux-lvm] Do I loose all my data if one disk crashes?

[lembark]

-- Dave Newman <newman@uci.edu> on 05/01/02 15:26:51 -0700

> Hi --
> 
> I just installed lvm on three 160 GB IDE hard drives, giving me a singe 480
> GB partition.

Not possible. You have combined the storage from 3 Phyiscal Volumes
("PV") into a single Volume Group ("VG"). After this point you can 
create Logical Volumes ("LV") w/in the VG.

Whether you loose data when a VG croaks depends on whether
your LV's are spread across multiple VG's. This is often
done either for the increased space or to improve performance
(by striping the LV in kernel-page sized chunks). If your
filesystem is striped and any disk it's on croaks then you
will loose whatever data is on the dead disk [until it's 
restored from the backup that you obviously intend to make
if you're dealing with have a TB of data]. 

If you have multiple LV's and they are all kept on a single
drive then only the LV's on the one PV will have to be restored.

I'd suggest if you're dealing with this much data that
a RAID system would be worth it. RAID5 isn't all that 
expensive to install (one more drive in this case) and 
can save you many, many hours restoring data.

In this case you'd want a RAID5 w/ 4k stripe (4 x 1K chunk)
(4k is standard page size for linux ext[23], change this
number to match whatever file system page you really use).
The RAID will combine the disks into a single unit for you,
so the VG will use a single PV. LVM is still useful here for
managing the individual LV's (e.g., growing them as time 
goes along). It would also be useful to have LVM in the case
where your physical storage has to grow. Just add another
RAID5 setup, add it to the VG and expand onto it.

--
Steven Lembark                              2930 W. Palmer
Workhorse Computing                      Chicago, IL 60647
                                           +1 800 762 1582

Re: [linux-lvm] Do I loose all my data if one disk crashes?

[Adrian Head]

On Thu, 2 May 2002 08:26, Dave Newman wrote:
> My question is: Do I loose all my data if any of the three disks
> crashes?  And if so, what can I do to improve the "restore-ability" of
> my configuration.

I have not read the LVM instructions so I'm unsure what config you have ended 
up with.

One thing to remember is that LVM does not implement RAID5 only RAID0 and 
linear.  Therefore, if a drive dies you will lose data.

To give the system a little extra survivability (yes you 
_still_need_backups_) you can have LVM ontop of a Linux software RAID5.  This 
will allow one drive to fail but the array will still be accessable.  Of 
course with this config you effectively lose one of your drives because of 
parity information.

-- 
Adrian Head

(Public Key available on request.)