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 modified 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 is more RAM hungry.
NAS Backup does not use VBK/VIB file format for storing data into the Backup Repository. A new file format has been developed spcifically for this purpose and to properly treat the moltitude of file to be protected. Its structure can be compared to an object storage, where data are divided into chunks and arranged into groups of files. The extention of those files is .vblob and their size is fixed to 64 MB. The backup mode is always incremental.
The Short Term Backup Repository will always hold a copy of all source file, pulled from the network share.
Usually NAS device and file server have a daily change rate between 1% and 3%.
The average compression on NAS data is usually 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 need to be taken into account as well, so:
Grand total = (Data size * 0.1) * 2
Considenring 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 in rehydrate the moltitude of files, for which the size is fixed to 64 MB, that VBR uses to store NAS data.
No additional sizing is required here as this is a complete mirror of the primary repository.