Software RAID and LVM on Linux

(the joy of disk virtualization)

David W. Aquilina

dwa@trilug.org

October TriLUG Presentation

Introduction

• http://www.trilug.org/~dwa/md-lvm-presentation/
• All standard disclaimers apply.
• Thank you Red Hat and Global Support Services.
• Software RAID
• Supported RAID types
• How to create an array
• Common maintenance tasks
• Logical Volume Management
• LVM Structure
• Creating logical volumes
• Common maintenance tasks

Platforms

• Red Hat Enterprise Linux v.4 will be used for most of the demonstrations.
• Software RAID and LVM are reasonably mature, so most distributions should work.
• Fedora Core 4 will be used for the Xen/LVM demonstration.

Experimenting without a SAN

• Loopback devices

dd if=/dev/zero of=/tmp/disk/1 bs=1M seek=1023 count=1 &&
losetup /dev/loop1 /tmp/disk/1

• Whole-system Virtualization
• Xen
• VMware
• Qemu

Software RAID

• Supported via the MD (multiple device) driver
• Adds protection against disk failures (except RAID0)
• NOT a replacement for good backups
• Can increase performance in certain scenarios.

RAID Levels

• 0 - Provides striping, which can increase performance. A single disk failure will destroy the array.
• 1 - Provides mirroring. All but one device is allowed to fail.
• 4 - Striped array with a parity device. One device is allowed to fail.
• 5 - Striped array with distributed parity. One device is allowed to fail.
• 6 - Striped array with dual distributed parity. Two devices are allowed to fail.
• Multipath - Provides multiple pathways to the same disk.

MD demonstrations

• General Syntax:

mdadm	-C /dev/mdN	--level=foo	--raid-devices=X	/dev/bar /dev/baz /dev/qux
	Create /dev/mdN	Raid level foo	Total number of devices in the array	Devices to include in the array. 'missing' is a possible device

• To create a RAID1 array, one would use mdadm -C /dev/md0 --level=raid1 --raid-devices=2 /dev/sda1 /dev/sdb1

MD Demos Continued

• Dealing with failed disks:
• mdadm /dev/md0 -r /dev/sda1 to remove the failed device from the array, at which point you can deal with the failed hardware
• mdadm /dev/md0 -a /dev/sda1 to add the fixed drive back.
• mdadm /dev/md0 --fail /dev/sda1 can be used to simulate a drive failure.
• Spare disks:
• Can be specified when the array is created with the --spare-devices=N option. For example, mdadm --create /dev/md1 --level=raid5 --raid-devices=3 --spare-devices=1 /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1.
• Can also be added at a later time with the hotadd option. For example, mdadm /dev/md1 -a /dev/sde1.

Logical Volume Management

• Similar functionality exists in many other Unixes.
• Linux's LVM was originally developed by Sistina Software (also the creators of GFS, a cluster filesystem) who was later acquired by Red Hat.
• LVM abstracts physical disks to increase flexibility and functionality.
• LVM also adds features such as snapshots, resizable volumes, and persistent naming.

LVM Structure

Creating a Logical Volume

1. Create the physical volume: pvcreate /dev/md0
2. Create a volume group and include the PV: vgcreate TriVG /dev/md0
• PVs can be added to existing volume groups with the vgextend command.
3. Create the logical volume out of the above volume group: lvcreate -n lv01 -L 512M TriVG
4. Create a filesystem on the device as you would any other block devices: mkfs -j /dev/TriVG/lv01

LVM Features

• Persistent Naming - A particular volume will always be /dev/[VG]/[LV], even if the underlying physical disk changes.
• The ability to move or otherwise change any underlying physical disks without dataloss.
• Snapshots - Creates a new block device which is an exact copy of the original, while allowing the original to still be accessed. Useful for backups, for example. When a snapshot is created, a user-defined amount of space is allocated to it to store changes made to both the original and the snapshot since it was created.

LVM Snapshot Tricks

• Xen virtual machines can use LVM volumes (including snapshots) as their disks.
• By installing a system onto a logical volume and then snapshotting it, multiple Xen virtual machines can be created using only a fraction of the space.
• Caveats:
• System specific information (such as /etc and /var) should be stored separately.
• If more changes are made to the original or the snapshot than has been allocated for, the Xen VMs will die horrible deaths.
• Snapshots are created with the lvcreate command: lvcreate -s -n [New LV Name] -L [Space to allocate to changes] [VG]/[Source LV]