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9-8 Chapter 9, Mass Storage Configuration
Addendum 108431-001 (11-88) to
Manual No. 108033-003
□ SFT II (disk duplexing) goes beyond disk
mirroring in both fault tolerance and perfor-
mance. It helps ensure continuous system
operation if either a fixed disk drive or a
controller fails. It improves performance by
simultaneously servicing two disk write or
read requests and by selecting the fixed disk
drive that is best able to service a given read
request. Duplexed drives typically perform
read operations twice as quickly as mirrored
drives. Disk duplexing requires two fixed
disk drive controllers and two identical fixed
disk drives.
IMPORTANT
Do not use CPQDSK or NVCPQDSK for non-
dedicated servers.
The EXTDISK diskette contains two versions of
the Value Added Disk Driver (VADD) for Novell
NetWare: CPQDSK and NVCPQDSK. Both ver-
sions operate in dedicated servers running Novell
NetWare (V2.1x). The two versions are identical
except the CPQDSK performs a read-after-write
verification and NVCPQDSK does not.
□ NVCPQDSK executes disk write operations
over two times faster than CPQDSK because
it does not perform read-after-write verifica-
tion.
NOTE: The probability of unrecoverable
disk write errors with mirrored or
duplexed drives is very low, since
erroneous data on one fixed disk
drive can usually be recovered from
the second. Disk configurations that
do not use mirroring or duplexing
depend on the inherent reliability of
the fixed disk drive and tape backup
units as the only protection against
disk errors.