1. Linux
Software RAID
& LVM
Patrick Ladd
5/2/2007

2. Terminology : RAID RAID: Redundant Array of Inexpensive Disks
Combine 2 or more smaller disks into one larger disk
Why?
Redundancy
Reduce or eliminate exposure to disk failure
Performance
More data under the drive heads
Ease of management
Single disk simplifies file system layout
Capacity:
n: Number of disks
s: Size of disks

3. Software RAID vs Hardware RAID
Implemented at kernel level in OS
Pros:
More flexible
Hardware independence
Cheap
Cons:
Requires properly configured kernel
Uses system resources
Hardware RAID
Built into card or motherboard
Pros:
Faster
Battery backed option
Cons:
Hardware dependency
Drivers?
Expensive

4. RAID 0: Striping Not really RAID No redundancy Performance: Best
Capacity: Best (n x s)
Reliability: Worst

5. RAID 1: Mirroring Fastest Least efficient disk usage
Disks added in pairs
Performance: Best
Capacity: Worst (n x s) / 2
Reliability: Best

6. RAID 5: Striping with Distributed Parity
Efficient space usage
Good balance of features
Requires n >= 3
Performance: Good
Capacity: Good (n-1) x s
Reliability: Good

7. Layered RAID RAID 51 RAID 10 RAID 01 RAID made up of RAID disks
Examples:
RAID 01 / 0+1 – RAID 1 of RAID 0's
RAID 10 – RAID 0 of RAID 1's
RAID 05 – RAID 0 of RAID 5's
RAID 50 – RAID 5 of RAID 0's
RAID 51 – RAID 5 of RAID 1's
Combine benefits of multiple RAID setups
Hopefully to offset the drawbacks
Tends to be storage inefficient
RAID 10
RAID 01

8. LVM – What & Why LVM: Logical Volume Manager
Abstract the interface to storage devices
Separate physical layer from file system layers
Why?
Ease of Management
Flexible partitioning
Human readable disk names
Data migration flexibility
Snapshots
Alternate way to do RAID 0 (Striping)

9. LVM Objects Physical Volume (PV)
Represents physical hard drive partition
Broken up into Physical Extents (PE)
Logical Volume (LV)
Virtual “partition” for O/S
Broken up into Logical Extents (LE)
Volume Group (VG)
Collection of PVs and LVs
Maps LEs to PEs

10. LVM Architecture OS LV LV LV LV LV VG VG PV PV PV PV PV Hard Drive

11. LVM Metadata UUID - Unique identifier stored in every VG, LV, PV
Header contained at start of every PV
UUIDs of all other PVs
UUIDs of all LVs
Allocation map of PE-->LE

12. Mapping Modes Linear: Data from LV laid out sequentially in PVs
Striped:
Data laid out alternately among 2 or more PVs
Possibly some performance improvement if PVs are on separate physical hardware
Concatenation:
LVM2 allows extending LVs using one of the above two mapping modes with more data in another layout

13. Ubuntu Install Has LVM / RAID support
Must be done from alternate ISO image (i.e. ubuntu-6.10-alternate-i386.iso)
Must be done in text mode

14. Fedora Core 6 Install LVM / RAID support
Includes Anaconda graphical installer support

15. LVM Common Commands Display object information - [pv|vg|lv]display
Create object - [pv|vg|lv]create
Delete object – [pv|vg|lv]remove
Change object – [pv|vg|lv]change
Display metadata– [pv|vg|lv]s
Rename – [vg|lv]rename

16. pvdisplay Output [root@laptop ~]# pvdisplay --- Physical volume ---
PV Name /dev/hda3
VG Name MainVG
PV Size GB / not usable 9.59 MB
Allocatable yes (but full)
PE Size (KByte)
Total PE
Free PE
Allocated PE
PV UUID XH408a-y8Pz-RoHs-rEOD-RZcW-AFwB-vytZ5I

17. vgdisplay Output [root@laptop ~]# vgdisplay --- Volume group ---
VG Name MainVG
System ID
Format lvm2
Metadata Areas
Metadata Sequence No 8
VG Access read/write
VG Status resizable
MAX LV
Cur LV
Open LV
Max PV
Cur PV
Act PV
VG Size GB
PE Size MB
Total PE
Alloc PE / Size / 7.78 GB
Free PE / Size / 0
VG UUID JYEdd7-mKC2-qAqS-1jsP-EWD4-UGVC-ZGn07n

18. lvdisplay Output [root@laptop ~]# lvdisplay --- Logical volume ---
LV Name /dev/MainVG/RootLV
VG Name MainVG
LV UUID DgwUgn-wb2F-K9Kn-PKcj-Ra0p-CwoO-FHpH44
LV Write Access read/write
LV Status available
# open
LV Size GB
Current LE
Segments
Allocation inherit
Read ahead sectors 0
Block device :0

19. Resizing a Partition Pre LVM2 – e2fsadm
Nice tool to automatically take filesystem offline, resize logical volume, then resize filesystem, put volume back online
Now - has to be done manually umount /tmp lvresize +10M /dev/MainVG/TmpLV ext2resize /tmp mount /tmp

20. Adding a drive Give drive a partition table
Safer – prevents other OS that don't recognize LVM from doing strange things to it
Setup PV on partition (pvcreate)
Add PV to VG (vgextend)

21. Converting to LVM Add 2nd Drive Set up LVM on it Mount it
Copy contents of existing drive to LVM
Boot to new LVM image
Ensure things work
Add old drive as LVM physical volume
Extend LVM onto old drive

22. Snapshots Create an exact copy of a LV
Can be read-only or read-write (default)
Caution: copy on write means that snapshot must be large enough to handle writes
lvcreate -L 500M -s -n BackupLV /dev/MainVG/TmpLV

23. Uses for Snapshots Read only Good for backups
Make sure applications / file system are consistent
Force write of data to disk
sync file system
Read/Write
Speculative experiments
Try something out, if it doesn't work, remove snapshot and try again
Virtual Machine images?

24. Sources Packages Needed RAID Support in kernel mdadm
LVM - Current incarnation is LVM2
Device mapper in kernel (most kernels built in)
Device mapper in user space (device-mapper)
Device mapper support library (libdevmapper)
LVM Tools (lvm2)
HOWTOs

25. EVMS
EVMS – Enterprise Volume Management System
Competitor to LVM
Integrated support for RAID 0,1,4,5
Handles LVM & LVM2 volumes
Better user-space tools
LVM has better internals
Not included in most pre-built kernels
Has a nice GUI – works with LVM

26. EVMS GUI

27. EVMS Text GUI