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Title
Tuning Shared Memory, Tranfer buffer settings for better Netvault Performance. -
Description
Tuning Shared Memory, Tranfer buffer settings for better Netvault Performance.
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Cause
Retuning the "Amount of Memory to Assign for Transfer Buffers (Kb)", sometimes referred to as the “Shared Memory Setting”, is a critical factor in maximizing backup/restore performance.
This value represents the amount of buffer space (shared memory) allocated for transferring data between a data-reading process (the plugin or APM reading the data) and a data-writing process (a Device Manager process). Increasing this setting will allocate a larger amount of transfer buffers and result in better backup/restore performance. The objective is to buffer enough data to maintain a constant stream to the tape drive thus avoiding unnecessary rewinding and repositioning of the tape during backup (shoe shining). -
Resolution
For more information on this topic, please see the Module 4-Advanced Administration Part Two-Web-based Training
Shared memory is allocated in 32Kb blocks. The default of 257, which is the minimum shared memory size, is equal to 8 buffers of 32 Kb plus one Kb.. Therefore, the formula used to determine a valid setting for this field is as follows:
( x 32Kb) + 1 byte
As a general rule, 2x the physical drives buffer/cache size is ideal, providing the server has the physical memory to allocate.
Examples of the optimal values, in KB, that can be specified for some of the drives are given here:- T10000C Tape drives
1048577 (1GB + 1 KB) - LTO-5 and 6 and T10000B Tape Drives
524289 (512 MB + 1 KB) - LTO-3 and 4 and T10000A Tape Drives
262145 (256 MB + 1 KB) - First generation modern Tape Drives (e.g., LTO 1 or 2, SDLT or SAIT)
65537 (64 MB + 1 KB) - Medium Tape Drives (e.g., DLT8000, DLT7000 and AIT-3)
32769 (32 MB + 1 KB) - Older Professional Tape Drives (e.g., DLT2000, DLT4000, AIT-2, etc.)
16385 (16 MB + 1 KB) - Older Low Capacity Low-end Drives (e.g., EXB-8505, AIT-1, DAT, etc.)
8193 (8 MB + 1 KB) - On Virtual Drives, VTL's
8197
Linux/UNIX-based Machine Controlling the Target Drive - These machines must not only have available RAM to allow for an increased setting, but also enough memory configured for Shared Memory. To allow for larger Shared Memory settings for a drive, the following parameters must be checked on the Linux/UNIX machine:
- 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 maximum shared memory is determined by the formula: SHMMAX * SHMSEG
These values are often limited by the “ulimit” setting, and the command “ulimit -a” should be used to allow vieiwing of the values for these system parameters.
Windows-based Machine Controlling the Target Drive - It is recommended that at least eight gigabytes (8 Gb) of RAM be installed and additional Virtual Memory be configured (i.e., more than what is set up with a default installation of Windows). - T10000C Tape drives