NetVault 12.4 - Administration Guide

On Windows, you might be required to change the registry setting MaximumSGList to use block sizes larger than 64KB. Before changing this setting, check 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:

Start the Registry Editor.

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).

Create the Parameters key, if it is not present.

Under Parameters, create the Device key, if it is not present.

Under the Device key, add the DWORD registry value MaximumSGList, if it is not present.

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 is:

(256KiB/4KiB) + 1 = 65

The decimal value is 65 and the hexadecimal value is 0x41.

You can set the block size to any value 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

Thus, the maximum value of 255 corresponds to a maximum media block size of 2MiB.

Reboot the system to apply the changes.

Drive Transfer Buffer Size

The transfer buffer or the shared memory is allocated in blocks of 32KiB. The default value is 8193KiB.Changing the default Drive Transfer Buffer Size option.

Increasing the transfer buffer size can improve backup 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:

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Maximum size of a shared memory segment (SHMMAX)

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Minimum size of shared memory segment (SHMMIN)

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Maximum number of shared memory identifiers in the system (SHMMNI)

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Maximum number of shared memory segments a user process can attach (SHMSEG)

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Maximum number of semaphore identifiers in the system (SEMMNI)

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Maximum number of semaphores in a set (SEMMSL)

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Maximum number of semaphores in the system (SEMMNS)

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Maximum number of operations per semop call (SEMOPM)

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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.

To enable software compression for virtual tape drives, configure the following options.

Table 97. Software compression options for virtual tape drives

Option

Description

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 percent, one of the following occurs:

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If the compressed data size is less than 80 percent of the original data size, the data is backed up in its compressed form.

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If the compressed data size is more than 80 percent of the original data size, the data is backed up in its uncompressed form.

If you specify 80 percent, a file size of a 100MB must be <= 80MB after compression however, the file size can be >80MB (like 81MB, 82MB, etc.), as the compression is done at block level.

When the specified compression level is not achieved for any block, in that case NetVault Backup backs up that block in its uncompressed form. Where, some blocks may be compressed and some blocks may remain in uncompressed form. Hence, in some case, the file size after compression can be greater than 80 percent.

The extent that data can be compressed depends on the data contents. Also the 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.

Click OK to save the settings and close the dialog box.

Optimal transfer buffer size

The following table provides examples of the optimal values that can be used for some drive types.

Table 98. Optimal transfer buffer size of different drive types
Drive type	Optimal transfer buffer size (in KiB)
Fast Modern Tape Drives For example, LTO series, SDLT, and SAIT	65537 (64MiB + 1KiB)
Medium Speed Tape Drives For example, DLT8000, DLT7000, and AIT-3	32769 (32MiB + 1KiB)
Older Professional Tape Drives For example, DLT2000, DLT4000, and AIT-2	16385 (16MiB + 1KiB)
Older Low Capacity Low-end Drives For example, EXB-8505, AIT-1, and DAT	8193 (8MiB + 1KiB)