VTL - Is compression on disk and virtual tape the same?

Asset ID:	1-71-1132963.1
Update Date:	2011-07-17
Keywords:

Solution Type Technical Instruction Sure

Solution 1132963.1 : VTL - Is compression on disk and virtual tape the same?

Applies to:

Sun StorageTek VTL Plus Storage Appliance - Version: 1.0 - Build 1323 to 2.0 - Build 1656 - Release: 1.0 to 2.0
Sun StorageTek VTL Prime System - Version: 1.0 - Build 1813 to 1.1 - Build 2076   [Release: 1.0 to 1.0]
Sun StorageTek VTL Storage Appliance - Version: 4.0 - Build 1221 to 4.0 - Build 1221   [Release: 4.0 to 4.0]
Sun StorageTek VTL Value System - Version: 1.0 - Build 1323 to 1.0 - Build 1323   [Release: 1.0 to 1.0]
Information in this document applies to any platform.
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***Checked for relevance on 14-07-2011*** (dd-mm-yyyy)

Goal

Is compression on disk and virtual tape the same?
What are the benefits of compression?
Is compression used for replication?

Solution

Standard VTL compression (disk or virtual tape compression, it is the same thing?)

VTL incorporates software compression via an LZO algorithm that runs on the VTL server. You can enable or disable compression for an entire virtual library, including all virtual tape devices. Using compression allows backup software products to use their compression options. Backup software products commonly support the following two compression methods:

Device-side compression: performed inside the tape drives.
Client-side compression: performed by backup client software running on a backup client.

If you disable the VTL Server compression option, enabling device-side compression on the backup software will have no effect.

Benefits of enabling compression:

Enabling compression on virtual tape drives increases capacity. The increase in capacity is directly related to the compressibility of the data being backed up. If you can compress the data being backed up by a factor of 2:1, you can store twice as much information on the virtual tape.

Engineering comments:

Compression with this method is greater than allowing a tape device to compression. Actual observed compression over device side compression is about 20% greater, or my guess is 2.5:1. The drawback to this is about a 30% hit in maximum performance. If a stream to a drive was running at 50 MB/sec, turning on compression will decrease this to ~35 MB/sec. the bigger the system, the more devices writing concurrently can max out an appliance. This is an extreme case, but wanted to give you an indication of what I have observed.

Another consideration when using compression and using backend tape drives to have VTL write the data to a physical tape. Create tapes with a size 20 to 25 % smaller. For example if you use lto gen 2. the default size is 197 GB. If you use compression and backend or inline tape devices, the size of the virtual needs to be 160- 170 GB in size. This size is changed during the library creation in VTL. When a tape is exported to physical tapes the data stream is uncompressed and the device uses hardware compression. Since the compression is greater, all the data on a virtual will not fit on a physical tape. If the backup software is doing this function, it handles the devices and it will write 1 compressed Vtape to 2 physical volumes.

Replication compression:
This option is used only with the replication option. This option will compress blocks of data before sending it out the network. This compression uses a similar algorithm. This will reduce the network traffic by about half, (data dependent). The data is compressed at the source VTL and uncompressed and stored at the target VTL.

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