What is RAID and LVM

RAID is usually defined as Redundant Array of Inexpensive disks. It is normally used to spread data among several physical hard drives with enough redundancy that should any drive fail the data will still be intact. Once created a RAID array appears to be one device which can be used pretty much like a regular partition. There are several kinds of RAID but I will only refer to the two most common here.

The first is RAID-1 which is also known as mirroring. With RAID-1 it’s basically done with two essentially identical drives, each with a complete set of data. The second, the one I will mostly refer to in this guide is RAID-5 which is set up using three or more drives with the data spread in a way that any one drive failing will not result in data loss. The Red Hat website has a great overview of the RAID Levels .

There is one limitation with Linux Software RAID that a /boot partition can only reside on a RAID-1 array.

Linux supports both several hardware RAID devices but also software RAID which allows you to use any IDE or SCSI drives as the physical devices. In all cases I’ll refer to software RAID.

LVM stands for Logical Volume Manager and is a way of grouping drives and/or partition in a way where instead of dealing with hard and fast physical partitions the data is managed in a virtual basis where the virtual partitions can be resized. The Red Hat website has a great overview of the Logical Volume Manager .

There is one limitation that a LVM cannot be used for the /boot.


Initial set of a RAID-5 array

I recommend you experiment with setting up and managing RAID and LVM systems before using it on an important filesystem. One way I was able to do it was to take old hard drive and create a bunch of partitions on it (8 or so should be enough) and try combining them into RAID arrays. In my testing I created two RAID-5 arrays each with 3 partitions. You can then manually fail and hot remove the partitions from the array and then add them back to see how the recovery process works. You’ll get a warning about the partitions sharing a physical disc but you can ignore that since it’s only for experimentation.

In my case I have two systems with RAID arrays, one with two 73G SCSI drives running RAID-1 (mirroring) and my other test system is configured with three 120G IDE drives running RAID-5. In most cases I will refer to my RAID-5 configuration as that will be more typical.

I have an extra IDE controller in my system to allow me to support the use of more than 4 IDE devices which caused a very odd drive assignment. The order doesn’t seem to bother the Linux kernel so it doesn’t bother me. My basic configuration is as follows:

hda 120G drive
hdb 120G drive
hde 60G boot drive not on RAID array
hdf 120G drive
hdg CD-ROM drive

The first step is to create the physical partitions on each drive that will be part of the RAID array. In my case I want to use each 120G drive in the array in it’s entirety. All the drives are partitioned identically so for example, this is how hda is partitioned:

Disk /dev/hda: 120.0 GB, 120034123776 bytes
16 heads, 63 sectors/track, 232581 cylinders
Units = cylinders of 1008 * 512 = 516096 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/hda1   *           1      232581   117220792+  fd  Linux raid autodetect

So now with all three drives with a partitioned with id fd Linux raid autodetect you can go ahead and combine the partitions into a RAID array:

# /sbin/mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 \
	/dev/hdb1 /dev/hda1 /dev/hdf1

Wow, that was easy. That created a special device /dev/md0 which can be used instead of a physical partition. You can check on the status of that RAID array with the mdadm command:

# /sbin/mdadm --detail /dev/md0
        Version : 00.90.01
  Creation Time : Wed May 11 20:00:18 2005
     Raid Level : raid5
     Array Size : 234436352 (223.58 GiB 240.06 GB)
    Device Size : 117218176 (111.79 GiB 120.03 GB)
   Raid Devices : 3
  Total Devices : 3
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Fri Jun 10 04:13:11 2005
          State : clean
 Active Devices : 3
Working Devices : 3
 Failed Devices : 0
  Spare Devices : 0

         Layout : left-symmetric
     Chunk Size : 64K

           UUID : 36161bdd:a9018a79:60e0757a:e27bb7ca
         Events : 0.10670

    Number   Major   Minor   RaidDevice State
       0       3        1        0      active sync   /dev/hda1
       1       3       65        1      active sync   /dev/hdb1
       2      33       65        2      active sync   /dev/hdf1

The important lines to see are the State line which should say clean otherwise there might be a problem. At the bottom you should make sure that the State column always says active sync which says each device is actively in the array. You could potentially have a spare device that’s on-hand should any drive should fail. If you have a spare you’ll see it listed as such here.

One thing you’ll see above if you’re paying attention is the fact that the size of the array is 240G but I have three 120G drives as part of the array. That’s because the extra space is used as extra parity data that is needed to survive the failure of one of the drives.

Initial set of LVM on top of RAID

Now that we have /dev/md0 device you can create a Logical Volume on top of it. Why would you want to do that? If I were to build an ext3 filesystem on top of the RAID device and someday wanted to increase it’s capacity I wouldn’t be able to do that without backing up the data, building a new RAID array and restoring my data. Using LVM allows me to expand (or contract) the size of the filesystem without disturbing the existing data.

Anyway, here are the steps to then add this RAID array to the LVM system. The first command pvcreate will “initialize a disk or partition for use by LVM”. The second command vgcreate will then create the Volume Group, in my case I called it lvm-raid:

# pvcreate /dev/md0
# vgcreate lvm-raid /dev/md0

The default value for the physical extent size can be too low for a large RAID array. In those cases you’ll need to specify the -s option with a larger than default physical extent size. The default is only 4MB as of the version in Fedora Core 5. The maximum number of physical extents is approximately 65k so take your maximum volume size and divide it by 65k then round it to the next nice round number. For example, to successfully create a 550G RAID let’s figure that’s approximately 550,000 megabytes and divide by 65,000 which gives you roughly 8.46. Round it up to the next nice round number and use 16M (for 16 megabytes) as the physical extent size and you’ll be fine:

# vgcreate -s 16M <volume group name>

Ok, you’ve created a blank receptacle but now you have to tell how many Physical Extents from the physical device (/dev/md0 in this case) will be allocated to this Volume Group. In my case I wanted all the data from /dev/md0 to be allocated to this Volume Group. If later I wanted to add additional space I would create a new RAID array and add that physical device to this Volume Group.

To find out how many PEs are available to me use the vgdisplay command to find out how many are available and now I can create a Logical Volume using all (or some) of the space in the Volume Group. In my case I call the Logical Volume lvm0.

# vgdisplay lvm-raid
   Free  PE / Size       57235 / 223.57 GB
# lvcreate -l 57235 lvm-raid -n lvm0

In the end you will have a device you can use very much like a plain ‘ol partition called /dev/lvm-raid/lvm0. You can now check on the status of the Logical Volume with the lvdisplay command. The device can then be used to to create a filesystem on.

# lvdisplay /dev/lvm-raid/lvm0 
  --- Logical volume ---
  LV Name                /dev/lvm-raid/lvm0
  VG Name                lvm-raid
  LV UUID                FFX673-dGlX-tsEL-6UXl-1hLs-6b3Y-rkO9O2
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                223.57 GB
  Current LE             57235
  Segments               1
  Allocation             inherit
  Read ahead sectors     0
  Block device           253:2
# mkfs.ext3 /dev/lvm-raid/lvm0
# mount /dev/lvm-raid/lvm0 /mnt
# df -h /mnt
Filesystem            Size  Used Avail Use% Mounted on
                       224G   93M  224G   1% /mnt

Handling a Drive Failure

As everything eventually does break (some sooner than others) a drive in the array will fail. It is a very good idea to run smartd on all drives in your array (and probably ALL drives period) to be notified of a failure or a pending failure as soon as possible. You can also manually fail a partition, meaning to take it out of the RAID array, with the following command:

# /sbin/mdadm /dev/md0 -f /dev/hdb1
mdadm: set /dev/hdb1 faulty in /dev/md0

Once the system has determined a drive has failed or is otherwise missing (you can shut down and pull out a drive and reboot to similate a drive failure or use the command to manually fail a drive above it will show something like this in mdadm:

# /sbin/mdadm --detail /dev/md0
     Update Time : Wed Jun 15 11:30:59 2005
           State : clean, degraded
  Active Devices : 2
 Working Devices : 2
  Failed Devices : 1
   Spare Devices : 0
     Number   Major   Minor   RaidDevice State
        0       3        1        0      active sync   /dev/hda1
        1       0        0        -      removed
        2      33       65        2      active sync   /dev/hdf1

You’ll notice in this case I had /dev/hdb fail. I replaced it with a new drive with the same capacity and was able to add it back to the array. The first step is to partition the new drive just like when first creating the array. Then you can simply add the partition back to the array and watch the status as the data is rebuilt onto the newly replace drive.

# /sbin/mdadm /dev/md0 -a /dev/hdb1
# /sbin/mdadm --detail /dev/md0
     Update Time : Wed Jun 15 12:11:23 2005
           State : clean, degraded, recovering
  Active Devices : 2
 Working Devices : 3
  Failed Devices : 0
   Spare Devices : 1

          Layout : left-symmetric
      Chunk Size : 64K

  Rebuild Status : 2% complete

During the rebuild process the system performance may be somewhat impacted but the data should remain in-tact.