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I want to setup a debian server mainly used as data storage. I have 4 devices:

  • /dev/sda (160GB) - installed debian on it
  • /dev/sdb, /dev/sdc, /dev/sdd (all 500GB) - created a raid5 array with it

Now I am not sure, how to go on. Is it usefull to create a LVM on the raid5 /dev/md0? How can I do this? Is there a good HowTo?

Or can I just create filesystem on the raid5 and create different partitions?

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You have seen the LVM HOWTO? tldp.org/HOWTO/LVM-HOWTO –  Zoredache Feb 8 '11 at 19:58

2 Answers 2

up vote 2 down vote accepted

Create the mdadm raid5 array. Create a volume group (I used the complete array). Create LVM2 logical volumes at whatever size you deem necessary.

I used redhat's documentation when building my 8TB NAS on Debian Lenny.

http://www.redhat.com/magazine/009jul05/features/lvm2/

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The LVM2 logical volumes are my partitions then? Or do I need to partitioning them respectively installing a filesystem on them? –  Develman Feb 8 '11 at 19:48
    
Most likely what you want is for LVM to be directly on top of the /dev/md* device. So use pvcreate on /dev/md* and pass /dev/md* to vgcreate. Within the volume group you create your LVs, and on your LVs, you put a filesystem. –  Zoredache Feb 8 '11 at 19:57
    
@Zoredache: I was to fast ;) Everything was written in redhat's documentation posted by PG Puters. But thanks for your response. –  Develman Feb 8 '11 at 20:02

If you are running a server, what will happen if your root device (on /sda) goes down?

It is normal to have at least a RAID1 configuration for your root partition, to help keep the server up should a single disk fail. If uptime isn't an issue perhaps this isn't a consideration. Note that you have to be careful to get grub installed on both disks' boot sectors if you are using software raid for a boot partition.

Raid 5 for your data storage is sensible, although Raid 6 is better! You asked

Is it usefull to create a LVM on the raid5 /dev/md0?

If you want to use up the whole of the software raid device, it isn't useful to create LVM logical devices. If you think you might want to, for instance, move /var off your boot device to a new device created in a logical volume, then yes. Naturally if you are going to do that you should be careful not to use the whole of your md0 device with a single logical volume as then the reason for using lvm is somewhat obviated.

Other important stuff to know about lvm:

  • While it is fine to fill up your PV with a VG, don't fill up your VG with LVs if you want flexibility in future (!)
  • Choose a filesystem that can expand if you choose to extend an LV (XFS is a good choice)
  • Filesystems in general aren't good at contracting to match an LV contraction
  • LVM snapshots are best avoided as they have terrible IO.

Check out the tldp Howto for some more interesting info.

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