Chart 4 - Null Device Dumps : Reading Data From Disk Arrays - Compaq 219700-001 - ProLiant - 1500 White Paper

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The tests were done as striped dumps using two null devices (2 threads) for the first test, four devices
(4 threads) for the second test, and six devices (6 threads) for third test. The results are shown below:

Chart 4 - Null Device Dumps : Reading Data from Disk Arrays

In each case the throughput is determined by the rate of read requests to the disk drives, and is limited
by a combination of controller saturation (including SCSI bus bandwidth) and software overhead.
Thus, throughput increases as we increase the number of controllers and SCSI channels used,
until a maximum rate of 100 GB/hr is seen with the database implemented across both channels of two
Smart-2 controllers. Note that both controllers were configured on a single PCI bus; the server has a
second PCI bus which is still idle. In many system layouts however, multiple disk controllers are not
combined into a single array using software striping, so that SQL Server will rarely be reading data
from more than one controller at a time during a dump. Therefore the maximum dump throughput
achieved will usually be limited to around 50 GB/hr (second bar in above chart). We will expect that
to be the case in the following sections, where striped dumps are done to various hardware storage
devices.
SQL Server allows the use of "disk dump devices" for backup purposes, as well as tape. The key
benefit of using disk storage is performance: a backup process (dump) to disk can often be completed
very fast, especially if sending the data to a disk array controller that can spread the writes across
multiple disk drives (spindles). Of course disk storage will always be limited by capacity, for the disk
media cannot be conveniently replaced and stored like a tape cartridge. The disk solution should
generally be used in conjunction with an off-line backup utility which can later transfer the dump
image files onto tape, thus allowing normal database transactions to continue on the SQL Server with
minimal time spent on dump activity. If the preservation period of your archives is relatively short
however (i.e: the backup sets are completely replaced quite frequently), then a large, dedicated disk