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VTL capacity considerations

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I really like Virtual Tape Libraries (VTLs). They are fast, they are easy to implement and they have huge space saving capabilities with their inbuild deduplication engine.

What I dislike is the way vendors talk about the capacity a given VTL model has. Often capacity is given in form of RAW capacity. This way it seems that even small VTL systems have plenty of storage in the backend. Well, it is plenty of storage in the backend but the really USEABLE capacity is far less then the RAW capacity. There are a few point you have to consider to come to the real useable capacity of a VTL.

  1. RAID level. All VTL data pools reside on disks configured in either RAID5 or more often RAID6. This will reduce the amount of useable capacity dramatically as the disk type often used in VTLs are high capacity MDL SAS or SATA disks with 2-4TB in size. So talking about RAID6 you will loose 2x the capacity of a single disk.
  2. Hot spares. Sometimes one or more of the disks are configured as hot spare. This is good in terms of reliability and data safety but will also affect useable capacity.
  3. Operating systems disks. All VTLs are running some kind of basic OS the VTL software runs on. This OS has to be installed on local disks. Most professional VTLs have dedicated disks for OS but if you don't have these, the capacity for the OS will also be substracted from the overall capacity.
  4. Disk size itself. A 2TB disk doesn't provide 2048GB of disk space. It only offers 1863GB useable capacity. A 3TB disk only offers 2794GB and a 4TB 3725GB. In combination with the RAID penalty a 12x4TB 48TB RAW capacity VTL can quickly be reduced to 37253GB of space you can really use to store your data on.
  5. Deduplication workspace. One of the most important but often forgotten capacity factor. Nearly all VTLs on the market offer deduplication in any form (hardware, software, inline, post-processing etc.). They all have one in common: they need disk space to store temporary deduplication data on. This working space is essential as the dedup process will dramatically slow down or even fail at all if this area is depleted. Common settings for the work space (or sometimes call meta-data area) is 8-15% of the useable disk space. So take any RAID config, hotspare, OS and disk size conversion factor into account and then substract another 8-15% from it and you will have the REAL useable capacity of a VTL.

It is not a surprise that VTLs that are announced by marketing with e.g. 48TB of capacity offer only 20-25TB useabel capacity in the end.

So don't rely on marketing information or any size the model name or any data sheet will give you and ask the vendor or your favorite distributor how much space you REALLY get to store your data on.

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