HP NetRAID 1Si Controller - Manual (Page 106)

Chapter 5 Reconstructing and Rebuilding Drives
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there is no risk of losing any data. The Power Fail Safeguard feature is an option
that you can enable or disable by using HP NetRAID Express Tools (if you are
using Windows NT or Windows 2000) or HP NetRAID Config utilities (if you are
the Novell NetWare, IBM OS/2, SCO OpenServer, or SCO UnixWare). With or
without PFS enabled, a reconstruction restarts automatically where it was
interrupted by a power failure.
NOTE The Power Fail Safeguard feature is enabled by default.
When a new logical drive is initialized, the last 1 MB of each physical disk is
reserved for use by the adapter. Three blocks (sectors) are used to store
configuration information, and the remaining area is used for the Power Fail
Safeguard feature. (The 1 MB of space is reserved even if PFS is not enabled.)
When the PFS feature is enabled. The reserved space in the 1-MB segment on
each disk is used to temporarily store data which will be reorganized and written
during a reconstruction. Thus, there is always a copy of data which resides on disk
during the reconstruction process so there is no risk of losing any data.
When the PFS feature is disabled. The data is read from the original set of disks
into memory, and the reorganized data is written to the disks. Because the data
transitions through memory, there is a brief exposure where data could be lost if a
power loss occurs. The reconstruction process is not able to recover data from
memory after a power loss (even with battery backup).
Although PFS protects the reconstruction operation, it decreases reconstruction
performance. When PFS is enabled, the reconstruction time increases by a factor
of about 2.5 times due to the extra overhead of always having data saved on disk.
However, there is no effect on normal array performance (when no reconstruction
is going on) with PFS enabled. A typical reconstruction time for a 4-drive RAID 5
array using 2-GB disks to a 5-drive RAID 5 array is about 90 minutes with PFS
disabled and no other data transfers. With PFS enabled, the same reconstruction
requires about 230 minutes.
PFS is enabled/disabled through either the HP NetRAID Express Tools or
HP NetRAID Config utilities. The default setting is Enabled. Since there is a
performance penalty only during the reconstruction operation itself, it is
recommended that you leave this feature enabled unless there are some rigid time
constraints on reconstruction activities.
Using HP NetRAID Express Tools to Change the PFS Setting
To change the PFS setting by using HP NetRAID Express Tools:
1. Log off all users, close all applications, and power down the HP NetServer.