Chart 2 - Throughput Variance With Block Size & Data Patterns - Compaq 219700-001 - ProLiant - 1500 White Paper

Compaq Backup and Recovery for Microsoft SQL Server 6.x
Chart 2 - Throughput Variance with Block Size & Data Patterns
The above discussion is relevant to the subject of this paper in helping to explain performance
differences that may be seen between versions of SQL Server. The blocking factor scheme used in
SQL Server 6.0 was improved in SQL Server version 6.5 specifically for DLT drives. Whereas SQL
Server 6.0 used an 8KB block size, version 6.5 now uses a much larger block size of 64KB.
Furthermore, 6.5 uses variable block mode, in which only one block is sent to (or received from) the
drive with each data transfer, but the size of the block can be varied during the dump (or load)
operation. SQL Server 6.5 does not vary the block size though, so that the 64KB block is always used,
thus providing for optimal performance with the DLT Tape drives.
will become apparent in the later section entitled Single Tape Device Dump.
25
SQL Server versions prior to 6.0 would use a hard-coded block size of 512 bytes, which had serious
performance repercussions in the case of DLT tape drives. As a result, Compaq recommends that DLT drives
not be used with these earlier (i.e: version 4.21x) SQL Servers. 512 byte blocks work fine for DAT and QIC
drives.
26
Exceptions are when writing headers, trailers, or file-marks to the tape, where the size of the data sent may be
less.
¤
1997 Compaq Computer Corporation, All Rights Reserved
%OR FN6 L ] H
26
The advantages of this scheme
Page 21
25
Doc No 444A/0797