The term Block Padding is used in backup to tape environments when there is a need for tape copy and backup mirroring. Tape still remains a sequential medium, using different tracks to store data. Therefore, there are slight variations in the overall capacity of individual tapes. This can pose a significant challenge when attempting to make an exact copy from a tape which is slightly larger than the destination tape. Planning for this eventual issue must be done before the initial tape initialization.
HP Data Protector uses a parameter called “OB2BLKPADDING” in the omnirc file on each system (media agent) connected with a tape device. The parameters’ value defines the number of empty blocks to be added on the specific tape. This empty space allows tape content to be copied to another tape of the same type without any problem as the empty space is not copied from the source.
Best practice as described in the omnirc file. The optimum value of the variable differs from one medium to another. The number of blocks should be calculated according to the set block size. Normally, the empty blocks should take up approximately 1 percent of the length of the entire tape. If this is still not enough for successful copying, increase the value by 0.5 percent until you find the most suitable value.
Block Padding is defined for each device type. The parameter is defined with a postfix: “OB2BLKPADDING_<DeviceType>=<number_of_empty_blocks>“.
- OB2BLKPADDING_1 DAT/DDS
- OB2BLKPADDING_2 QIC Quarter Inch Cartridge
- OB2BLKPADDING_3 8mm – ExaByte
- OB2BLKPADDING_4 AIT – Advanced Information Technology
- OB2BLKPADDING_5 3480 Cartridge
- OB2BLKPADDING_6 Raw Magnetic Disk
- OB2BLKPADDING_7 Regular Disk File
- OB2BLKPADDING_8 STK 9840
- OB2BLKPADDING_9 Generic Magnetic Tape Device
- OB2BLKPADDING_10 DLT – Digital Linear Tape
- OB2BLKPADDING_11 StorageTek SD-3 – Redwood
- OB2BLKPADDING_12 3590 Cartridge (Magstar)
- OB2BLKPADDING_13 LTO Ultrium
- OB2BLKPADDING_14 Quantum SuperDLT
It’s important to mention this can pose a potential problem when using “media copy” or backup “mirroring” (read once, write-out twice). It should not pose a problem when using “session copy” or “object copy”.
CommVault Simpana uses auxiliary copies to move data from one tape to another one. These auxiliary copies can be compared with the way how HP Data Protector addresses “Session Copy”. Hence there is no need to implement block padding when using CommVault Simpana.
Thanks for reading,