HP Data Protector Software Performance White Paper

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Figure 43 shows the default configuration parameters for Windows file systems. The
asynchronous reading is set OFF.
Figure 43. Backup configuration – advanced WinFS filesystem options – asynchronous reading
On the client, the Data Protector template Data_Protector_home\omnirc keeps parameters
for asynchronous reading:
OB2DAASYNC=not set|0|1 (default: not set)
If set to one, the Disk Agent will always use asynchronous reading. If set to zero, the Disk
Agent will never use asynchronous reading. By default, asynchronous mode is not forced
and is governed by the option in the backup specifications. If this variable is set, the setting
takes precedence over the option in the backup specification or the command line.
OB2DAASYNC_READERS=2-256 (default: 64)
This variable controls the number of concurrent reads from the disk (APCs) when
asynchronous reading is used. Generally, performance improves with higher values, but a
plateau is typically quickly reached. Note that good performance will be achieved more
easily with contiguous files, while higher values may be beneficial with fragmented files.
However, setting the value too high may cause some performance degradation with certain
types of disks or RAID arrays due to an excessively high number of outstanding I/O
requests.
OB2DAASYNC_SECTORS=1-1024 (default: 128)
This variable tells how many sectors are read in one read request when asynchronous
reading is used. The sector size of the disk being backed up is determined at runtime
(typically 512 bytes). Generally, performance improves with higher values. Some types of
disks are less sensitive to this parameter and reach the maximum throughput with lower
values than others. Some types of disks do not handle reads well with sizes greater than a
certain value (for example, 256), which degrades performance. Setting the value slightly
lower (such as 248) may yield the best performance for such a disk.