□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
□NVCPQDSK executes disk write operations over two times faster than CPQDSK because it does not perform
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.
