To configure performance options for a tape drive 1 In the Navigation pane, click Manage Devices.
3 Click Performance, and in the Drive Performance Options, configure the following settings.
Table 5. Drive performance options
Option Description
Drive Block Size This option specifies the block size for reading and writing data to the media. The default value is 256KiB.
The media block size can be increased in increments of 1KiB, but many devices may only accept a value in the multiples of 4KiB or 32KiB.
NOTE: The changes to the media block size settings are only applied to blank media. If you are reusing media, blank it first for these changes to take effect.
Large media blocks enable reading and writing of larger blocks of data, thereby reducing the number of times a backup needs to read data and write it to media. However, this does not always imply an overall faster backup. The maximum media block size is limited by several factors, including the operating system, SCSI adapter, and make, model, and type of drives.
On Linux and UNIX systems, you can increase the media block size for optimum performance.
On Windows, you might be required to change the registry setting MaximumSGList to use block sizes larger than 64KB. Before changing this setting, ensure that the SCSI bus is only used by the tape devices. If other devices also use the SCSI bus, this registry change might prevent them from working. If you want to apply these changes only to a specific channel on the HBA, consult the hardware vendor.
To change the registry setting on Windows, follow these steps: 1 Start the Registry Editor.
2 Open the key
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\ Services\<HBA Vendor>\Parameters (where <HBA Name> is specific to your SCSI card – for example, QL2200 for a Qlogic 2200 card).
3 Create the Parameters key if not present.
4 Under Parameters, create the Device key if not present. 5 Under the Device key, add the DWORD registry value
MaximumSGList if not present.
6 Calculate the hexadecimal value of MaximumSGList: On 32-bit systems:
MaximumSGList = (Maximum Block Size / 4KiB) + 1 For example, if the block size is set to 256KiB, the value for this key will be (256KiB/4KiB) + 1 = 65. The decimal value will be 65 and the hexadecimal value will be 0x41. The block size can be set from 64KiB through 1012KiB. The maximum value 255 is internally converted to 257 to make a block size of 1 MiB (1024 KiB). On 64-bit systems:
On 64-bit systems, the default OS page size is 8KiB. The formula for calculating MaximumSGList is:
MaximumSGList = (Maximum Block Size / 8KiB) +1 Therefore, the maximum value of 255 corresponds to a maximum media block size of 2 MiB.
4 Click OK to save the settings and close the dialog box.
Drive Transfer Buffer Size The transfer buffer or the shared memory is allocated in blocks of 32KiB. The default size for this option is 8MiB.
The transfer buffer size can be increased for faster performance. To calculate the buffer size, use the following formula:
(<Total number of buffers> x 32KiB) + 1 byte
On Linux and UNIX systems, you require sufficient RAM and large Shared Memory segment. Before increasing the transfer buffer size, check the following settings on these platforms:
• Maximum size of a shared memory segment (SHMMAX) • Minimum size of shared memory segment (SHMMIN)
• Maximum number of shared memory identifiers in the system (SHMMNI)
• Maximum number of shared memory segments a user process can attach (SHMSEG)
• Maximum number of semaphore identifiers in the system (SEMMNI) • Maximum number of semaphores in a set (SEMMSL)
• Maximum number of semaphores in the system (SEMMNS) • Maximum number of operations per semop call (SEMOPM) • Semaphore maximum value (SEMVMX)
The total allowed shared memory is determined by the formula
SHMMAX * SHMSEG. These values are often limited by the ulimit setting, and the command ulimit -a can be used to view these system settings. On Windows, you require at least 2GB RAM and large virtual memory. You might also have to change the MaximumSGlist setting on the SCSI card. For examples, see the Optimal transfer buffer size.
Software Data Compression
To perform software compression, select this check box. The data is compressed when it is transferred to the device during backup.
Compression Threshold The value set for this option determines the minimum level of compression that must be achieved when data is compressed during a backup. For example, if you set the value to 80%, one of the following occurs:
• If the compressed data size is less than 80% of the original data size, the data is backed up in its compressed form.
• If the compressed data size is more than 80% of the original data size, the data is backed up in its uncompressed form.
If you enter 80%, a file size of a 100MB must be <= 80MB after compression. If the specified level is not achieved, NetVault Backup backs up the file in its uncompressed form. The extent that data can be compressed is dependent on the data contents. Encrypted data cannot be compressed. With some files, compression may actually result in a file that is larger than the original uncompressed file.
Compression Data Blocks Type or select the number of data blocks per compression unit. The default block size is 8KiB.
Table 5. Drive performance options