Advanced Backup to Disk Performance White Paper

• Transfer Size
Transfer size is the overall total size of the SCSI transfer within a single SCSI command. In some
operating systems, there is limit set on this. For example, in Microsoft Windows the default transfer
size is 64 KB and to increase the overall transfer size above 64 KB in Windows, a registry entry
called “MaximumSGList” (associated with the HBA) must be changed. Many modern HBAs already
install their drivers with this registry value set appropriately. Check the registry entries.
• Fragmentation
The more fragmented the files are on disk, the more random will be the disk access method, hence
the backup will take longer. If your system does have a de-fragmentation utility, it is advisable to
run it before full backups or on a regular scheduled basis to ensure files are largely contiguously
arranged on disk.
• SAN Inter Switch Links
In the switched fabric, SAN Inter Switch Links (ISLs) ensure that SAN connections have sufficient
bandwidth to support the backup traffic going through them. Trunk multiple ISLs together where
possible.
•
2 Gigabit Fibre Channel
Expect no more than 180 MB/s maximum from a single 2 GB/s Fibre Channel connection.
• SCSI Burst Rate
Beware of disk drives quoted as Ultra320; Ultra 320 refers to the burst rate not the sustained rate.
The typical sustained rate from a 15K rpm SCSI Ultra320 disk drive is around 80 MB/s for raw
sequential I/Os (that is, without file system read overhead).
• SATA Disks
These types of disks are lower cost, lower performance, and lower reliability than the SCSI disk
previously listed but are useful for staging backups because they offer high capacity. A typical
7200K rpm SATA disk drive has lower seek times than an equivalent SCSI disk drive, a burst rate
of around 150 MB/s, and a sustained transfer rate of around 50 MB/s.
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