LVM on software RAID
Although RAID and LVM may seem like analogous technologies they each present unique features. This article uses an example with three similar 1TB SATA hard drives. The article assumes that the drives are accessible as
/dev/sdc. If you are using IDE drives, for maximum performance make sure that each drive is a master on its own separate channel.
|LVM Logical Volumes||
|LVM Volume Groups|
Many tutorials treat the swap space differently, either by creating a separate RAID1 array or a LVM logical volume. Creating the swap space on a separate array is not intended to provide additional redundancy, but instead, to prevent a corrupt swap space from rendering the system inoperable, which is more likely to happen when the swap space is located on the same partition as the root directory.
GRUB supports the default style of metadata currently created by mdadm (i.e. 1.2) when combined with an initramfs, which has replaced in Arch Linux with mkinitcpio. Syslinux only supports version 1.0, and therefore requires the
Some boot loaders (e.g. GRUB Legacy, LILO) will not support any 1.x metadata versions, and instead require the older version, 0.90. If you would like to use one of those boot loaders make sure to add the option
--metadata=0.90 to the
/boot array during RAID installation.
Obtain the latest installation media and boot the Arch Linux installer as outlined in Getting and installing Arch.
Load kernel modules
Load the appropriate RAID (e.g.
raid10) and LVM (i.e.
dm-mod) modules. The following example makes use of RAID1 and RAID5.
# modprobe raid1 # modprobe raid5 # modprobe dm-mod
Prepare the hard drives
Each hard drive will have a 200 MiB
/boot partition, 2048 MiB
/swap partition, and a
/ partition that takes up the remainder of the disk.
The boot partition must be RAID1; i.e it cannot be striped (RAID0) or RAID5, RAID6, etc.. This is because GRUB does not have RAID drivers. Any other level will prevent your system from booting. Additionally, if there is a problem with one boot partition, the boot loader can boot normally from the other two partitions in the
Partition hard drives
We will use gdisk to create three partitions on each of the three hard drives (i.e.
Name Flags Part Type FS Type [Label] Size (MB) ------------------------------------------------------------------------------- sda1 Boot Primary linux_raid_m 200.00 # /boot sda2 Primary linux_raid_m 2000.00 # /swap sda3 Primary linux_raid_m 97900.00 # /
To create partitions, see GPT fdisk#Create a partition table and partitions.
After creating the physical partitions, you are ready to setup the /boot, /swap, and / arrays with
mdadm. It is an advanced tool for RAID management that will be used to create a
/etc/mdadm.conf within the installation environment.
Create the / array at
# mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/sd[abc]3
Create the /swap array at
# mdadm --create /dev/md1 --level=1 --raid-devices=3 /dev/sd[abc]2
- If the only reason you are using RAID is to prevent stored data loss (i.e. you are not concerned about some running applications crashing in the event of a disk failure), then there is no reason to RAID the swap partitions -- you can use them as multiple individual swap partitions.
- If you plan on installing a boot loader that does not support the 1.x version of RAID metadata make sure to add the
--metadata=0.90option to the following command.
Create the /boot array at
# mdadm --create /dev/md2 --level=1 --raid-devices=3 --metadata=1.0 /dev/sd[abc]1
After you create a RAID volume, it will synchronize the contents of the physical partitions within the array. You can monitor the progress by refreshing the output of
/proc/mdstat ten times per second with:
# watch -n .1 cat /proc/mdstat
Further information about the arrays is accessible with:
# mdadm --misc --detail /dev/md
Once synchronization is complete the
State line should read
clean. Each device in the table at the bottom of the output should read
active sync in the
active sync means each device is actively in the array.
It is good practice to regularly run data scrubbing to check for and fix errors.
To initiate a data scrub:
# echo check > /sys/block/md0/md/sync_action
As with many tasks/items relating to mdadm, the status of the scrub can be queried:
# cat /proc/mdstat
$ cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4] [raid1] md0 : active raid1 sdb1 sdc1 3906778112 blocks super 1.2 [2/2] [UU] [>....................] check = 4.0% (158288320/3906778112) finish=386.5min speed=161604K/sec bitmap: 0/30 pages [0KB], 65536KB chunk
To stop a currently running data scrub safely:
# echo idle > /sys/block/md0/md/sync_action
When the scrub is complete, admins may check how many blocks (if any) have been flagged as bad:
# cat /sys/block/md0/md/mismatch_cnt
The check operation scans the drives for bad sectors and mismatches. Bad sectors are automatically repaired. If it finds mismatches, i.e., good sectors that contain bad data (the data in a sector does not agree with what the data from another disk indicates that it should be, for example the parity block + the other data blocks would cause us to think that this data block is incorrect), then no action is taken, but the event is logged (see below). This "do nothing" allows admins to inspect the data in the sector and the data that would be produced by rebuilding the sectors from redundant information and pick the correct data to keep.
General Notes on Scrubbing
/sys/block/md0/md/sync_actionbut this is ill-advised since if a mismatch in the data is encountered, it would be automatically updated to be consistent. The danger is that we really do not know whether it is the parity or the data block that is correct (or which data block in case of RAID1). It is luck-of-the-draw whether or not the operation gets the right data instead of the bad data.
It is a good idea to set up a cron job as root to schedule a periodic scrub. SeeAUR which can assist with this.
RAID1 and RAID10 Notes on Scrubbing
Due to the fact that RAID1 and RAID10 writes in the kernel are unbuffered, an array can have non-0 mismatch counts even when the array is healthy. These non-0 counts will only exist in transient data areas where they do not pose a problem. However, since we cannot tell the difference between a non-0 count that is just in transient data or a non-0 count that signifies a real problem. This fact is a source of false positives for RAID1 and RAID10 arrays. It is however recommended to still scrub to catch and correct any bad sectors there might be in the devices.
This section will convert the two RAIDs into physical volumes (PVs). Then combine those PVs into a volume group (VG). The VG will then be divided into logical volumes (LVs) that will act like physical partitions (e.g.
/home). If you did not understand that make sure you read the LVM Introduction section.
Create physical volumes
Make the RAIDs accessible to LVM by converting them into physical volumes (PVs) using the following command. Repeat this action for each of the RAID arrays created above.
# pvcreate /dev/md0
Confirm that LVM has added the PVs with:
Create the volume group
Next step is to create a volume group (VG) on the PVs.
Create a volume group (VG) with the first PV:
# vgcreate VolGroupArray /dev/md0
Confirm that LVM has added the VG with:
Create logical volumes
In this example we will create separate
/home LVs. The LVs will be accessible as
Create a / LV:
# lvcreate -L 20G VolGroupArray -n lvroot
Create a /var LV:
# lvcreate -L 15G VolGroupArray -n lvvar
/swapLV with the
-C yoption, which creates a contiguous partition, so that your swap space does not get partitioned over one or more disks nor over non-contiguous physical extents:
# lvcreate -C y -L 2G VolGroupArray -n lvswap
Create a /home LV that takes up the remainder of space in the VG:
# lvcreate -l +100%FREE VolGroupArray -n lvhome
Confirm that LVM has created the LVs with:
Update RAID configuration
Since the installer builds the initrd using
/etc/mdadm.conf in the target system, you should update that file with your RAID configuration. The original file can simply be deleted because it contains comments on how to fill it correctly, and that is something mdadm can do automatically for you. So let us delete the original and have mdadm create you a new one with the current setup:
# mdadm --examine --scan >> /etc/mdadm.conf
mdadm.conffile from within the installer.
Prepare hard drive
Follow the directions outlined the in #Installation section until you reach the Prepare Hard Drive section. Skip the first two steps and navigate to the Manually Configure block devices, filesystems and mountpoints page. Remember to only configure the PVs (e.g.
/dev/VolGroupArray/lvhome) and not the actual disks (e.g.
mkfs.xfswill not align the chunk size and stripe size for optimum performance (see: Optimum RAID).
Once it is complete you can safely reboot your machine:
Install the bootloader on the Alternate Boot Drives
Once you have successfully booted your new system for the first time, you will want to install the bootloader onto the other two disks (or on the other disk if you have only 2 HDDs) so that, in the event of disk failure, the system can be booted from any of the remaining drives (e.g. by switching the boot order in the BIOS). The method depends on the bootloader system you are using:
Log in to your new system as root and do:
# /usr/sbin/syslinux-install_update -iam
Syslinux will deal with installing the bootloader to the MBR on each of the members of the RAID array:
Detected RAID on /boot - installing Syslinux with --raid Syslinux install successful
Attribute Legacy Bios Bootable Set - /dev/sda1 Attribute Legacy Bios Bootable Set - /dev/sdb1 Installed MBR (/usr/lib/syslinux/gptmbr.bin) to /dev/sda Installed MBR (/usr/lib/syslinux/gptmbr.bin) to /dev/sdb
Log in to your new system as root and do:
# grub grub> device (hd0) /dev/sdb grub> root (hd0,0) grub> setup (hd0) grub> device (hd0) /dev/sdc grub> root (hd0,0) grub> setup (hd0) grub> quit
Archive your filesystem partition scheme
Now that you are done, it is worth taking a second to archive off the partition state of each of your drives. This guarantees that it will be trivially easy to replace/rebuild a disk in the event that one fails. See fdisk#Backup and restore partition table.
- What is better LVM on RAID or RAID on LVM? on Server Fault
- Managing RAID and LVM with Linux (v0.5) by Gregory Gulik
- 2011-09-08 - Arch Linux - LVM & RAID (1.2 metadata) + SYSLINUX
- 2011-04-20 - Arch Linux - Software RAID and LVM questions
- 2011-03-12 - Arch Linux - Some newbie questions about installation, LVM, grub, RAID