HP-UX System Administrator's Guide: Logical Volume Management (762803-001, March 2014)

D Striped and Mirrored Logical Volumes
This appendix provides more details on striped and mirrored logical volumes. It describes the
difference between standard hardware-based RAID and LVM implementation of RAID.
D.1 Summary of Hardware Raid Configuration
RAID 0, commonly referred to as striping, refers to the segmentation of logical sequences of data
across disks. RAID 1, commonly referred to as mirroring, refers to creating exact copies of logical
sequences of data. When implemented in a device hardware, RAID 10 (or RAID 1+0) and RAID
01 (or RAID 0+1) are nested RAID levels. The difference between RAID 0+1 and RAID 1+0 is the
location of each RAID system: RAID 0+1 is a mirror of stripes whereas RAID 1+0 is a stripe of
mirrors. Figure 6 shows the RAID 10 and RAID 01 configurations (A1, A2...Ax are stripe chunks
of a logical volume).
With a hardware-based RAID 10 configuration, I/O operations are striped first then each strip is
mirrored. With hardware-based RAID 01, I/Os are mirrored first then striped. RAID 10 and RAID
01 can have the same physical disk layout.
Figure 6 Hardware Configuration: Raid 10 and Raid 01
The advantages of hardware-based RAID 10 over RAID 01:
• When one disk fails and is replaced, only the amount of data on this disk needs to be copied
or re- synchronized.
• RAID 10 is more tolerant to multiple disk failures before data becomes unavailable.
The advantages of hardware-based RAID 01 over RAID 10:
• Simpler to configure striped volumes and then extend mirroring.
• Able to split the mirror copy and have two usable volume sets
D.2 LVM Implementation of RAID Levels in HP-UX
LVM implementation of RAID management is different from the hardware based solutions because
it does not nest the RAID levels, but processes them simultaneously. Typically with hardware
solutions, you create a LUN with a RAID level and the RAID functions are stacked. LVM provides
more flexibility on how logical volumes are created amongst a set of disks as compared to hardware
solutions.
LVM allocates the physical extents for striped and mirrored logical volumes in sets of stripe width
multiplied by the number of copies of the data. For example, if the logical volume is 1-way mirrored
and striped across two disks, extents are allocated to the logical volume, four at a time. LVM
enforces that the physical extents of a single set are from different physical volumes. Within this
D.1 Summary of Hardware Raid Configuration 171