Choosing the Right Disk Technology in a High Availability Environment DRAFT Version 2.0, August 1996

DRAFT -- Revision 2.0
August 22, 1996Page 36
Data Protection
Mirroring can also be done in software or in hardware. Mirroring in software is
accomplished via LVM, and uses more CPU cycles than does hardware mirroring. LVM
mirroring appears to provide better read performance than does hardware mirroring
using RAID level 0/1. This is due to LVM queuing the read onto the disk drive that has
the shortest queue. Read performance has been seen to improve by as much as 40%
with LVM mirroring. A new version of the firmware for the HADA disk arrays should
improve mirrored read performance by allowing reads from both sides of the mirror
instead of always reading from one disk.
However, RAID level 0/1 appears to offer better write performance than does LVM
mirroring which degrades write performance by about 10% due to the dual writes. With
RAID 0/1, a single write operation is sent to the disk array and is handled very efficiently
when a write cache is configured.
RAID levels 3 and 5 also provide data protection. The data protection is parity, so this
solution is less expensive than mirroring which requires double the disk space.
However, RAID 3 provides consistently lower performance as compared to standalone
disks except when the I/O size is >= 64 KB. RAID 5 performance is high for large I/Os
(>= 64 KB) but poor for smaller I/O sizes.
AutoRAID combines RAID 5 and RAID 0/1 in the same disk array and trades off the
increased storage requirements of RAID 0/1 with the lower cost of RAID 5. AutoRAID
automatically migrates data between the two RAID modes as data access patterns
change. The most frequently accessed data in RAID 0/1 mode while the rest of the
data is kept in lower cost RAID 5. The amount of space used for RAID 0/1 is
configurable.
I/O Size
On a Unix system used for business applications, including HP-UX, most I/Os are
relatively small, between 2 KB (raw I/O) and 8 KB (file system). I/O performance is
linear in relation to size using standalone LVM mirrored disks if I/O setup and seek time
is ignored. HP-UX version 10.0 implements a new feature called I/O merging. During
the disk sort operation that sorts I/Os in the disk queue by cylinder location, the driver
will also look for I/Os that are contiguous on the disk and will merge the I/Os into a
single I/O. This feature can improve disk-intensive application performance when I/Os
are sequential.
With disk arrays configured in RAID 3 or 5 mode, performance varies in a number of
ways. RAID 3 provides consistent I/O performance for reads and writes at small I/O