Hi Adrian,
Clearly I am a NetApp employee, and as such my views are clearly hugely biased !
In the past, I have also been a backup administrator, which I have to say tainted those opinions as well.......
Firstly, glad to hear you have been having a good (& I like to imagine, fairly typical !) experience with Snapshots and SnapMirror 🙂
So my view is that you need to examine your RPO/RTO needs, & for longer term retention, the RPO in particular. RPO (recovery point obvjective) is really about where you need to be able to recover from. Hopefully for most customers reading this, they use our Snapshot copies to recover data for recent needs (eg, up to the 35 days you mention).
Beyond that, you mention taking tapes weekly: does that mean there is a "business imperative" to be able to restore to weekly points forever, or can you drop to perhaps monthly recovery points ? This impacts how we can use Snapshot copies to help: if you need to keep backups for 7 years, that's "only" 84 copies on a monthly basis, whereas for weekly ones you are at 364. The former drops easily into an ability to maintain those snapshots in a single volume, the latter would need some mechanism (either FlexClone-inspired, and space efficient, or "new baseline copy" to new volume media) to extend beyond the ~250 copies you might keep.
I have done a fair amount of customer-data analysis in the UK over the past 18 months, and for the majority of data I see weekly change rates below 2%....there are some exceptions, but it should be fairly simple for you to identify your change rate simply by looking at the "df" output of your volumes and seeing what space those 35 days worth of Snapshot copies take up. One customer had data over 2 years, with a weekly change rate averaging below 1%. Starts to get easy to keep online backups for longer when these numbers are so low (>50:1 space savings for subsequent backupsis tremendously efficient compared to a tape-based mechanism).
Oh, & of the systems I looked at, none were yet using our deduplication (which clearly integrates best with SnapVault from 7.3 onwards): that adds some "icing" to what I would consider to be a great cake 😉
You can also consider whether you wish to "lock" those secondary copies, perhaps not for "compliance" reasons but simply to avoid adminstrator error (or malicious damage !) - SnapLock could be a consideration here.
If you are considering a secondary system to replace tape, I generally also recommend a tertiary, to cover the "unthinkable" disaster of losing an entire secondary system for whatever reason. Sometimes this design can enable you to have "active active" secondary systems cross-replicating, making best use of bandwidth and locations.
Hopefully you will get some customer thoughts in response to this. In addition, I would recommend you speak with your SE, we can perhaps help look closer at your data to see what "size" you might need for longer retentions.
cheers
Mike
