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Nas Backup Repository

NAS Backup Repositories need to be sized differently than standard repositories due to the fact that the backup file format is different, there are two levels of primary retention (short term and short term copy) and an archival retention.

Regarding the archival retention, only older file versions or deleted data are eligible to be moved to the Archive Repository as they age out and, as you can image, this constraint introduces a few challenges when sizing a NAS Backup Repository.

Also, compared to a standard Backup Repository a NAS Backup Repository consumes more RAM.

File Format

NAS Backup does not use VBK/VIB file format for storing data into the Backup Repository. A new file format has been developed specifically for this purpose and to properly treat the multitude of files to be protected. Its structure can be compared to an object storage, where data is divided into chunks and arranged into groups of files. The extension of those files is .vblob and their size is limited to 64 MB. The backup mode is always incremental forever.

Best Practice

Sizing Short Term repositories

The Short Term Backup Repository will always hold a copy of all source file, pulled from the network share.

NAS devices and file servers usually have a daily change rate between 1% and 3%. Please note that the percentage itself can vary a lot depending on the total capacity of the NAS: smaller units can have higher change rate with the change of only a minor subset of data, while larger devices may have a very small percentage with a huge amount of data is changed daily.

The achievable compression on NAS data depends on many factors. The data type is the most important, affecting how much compression is effective (eg: plain text or office data are very easy to compress, binary files are more difficult); some kinds of data (like PDFs) are compressed by default, as are most media files and, along with files protected by encryption, will give very bad to zero compression ratios. In average, total compression varies between 20% and 50% of the source size.

Given the information above, to size the Short Term repository you can refer to the following formula. Calculate first the initial backup (A) and then the increments size (B)

A = ((100 - data reduction) / 100) * source size
B = ((change rate / 100) * source size) * retention

Then sum up the two values:

Data size = A + B

Metadata and free “workspace” need to be taken into account as well. Add 10% for each:

Grand total = Data size + ((Data size * 0.1) * 2)

Considering the new backup file format, deduplicating storage appliances may be not the right choice for storing NAS backup data. Check carefully the limitations following the link at the bottom of the page. Appliances may struggle rehydrating the multitude of files, for which the size is limited to 64 MB, that VBR uses to store NAS data.

Sizing Short Term repositories (copy)

No additional sizing is required here as this is a complete mirror of the primary repository.

Compute resources

The Short Term Backup Repository should have half the proxy cores and 6GB of memory per-core.


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