What exactly "snap delta" counts?

[aborzenkov]

I do not think there is clear definition of "snap delta" anywhere. Specifically, right now I had a case of delta between two VSM snapshots of 290GB, but transferred size was somewhere around 600GB. The difference is too large to be explained by metadata. So far my only explanation is that "snap delta" shows size of changed data, while VSM of course transfers both changed and new data.

It is not the first time I noticed somewhat great discrepancy between estimation using snap delta and real amount of transfereed data.

So - what exactly is counted in "snap delta"?

[alapati]

Hi,

Please check this.

http://now.netapp.com/NOW/knowledge/docs/ontap/rel733/html/ontap/cmdref/man1/na_snap.1.htm

If you believe there is a discrepency, I highly recommend you open a case and have them investigate.

Thanks

Srinath

[aborzenkov]

Well, it just tells about "rate of change of data", without defining how "data change" is actually calculated. And how it is calculated is exactly my question

[mwalters]

Hi there,

Given that a VSM replication will take all changed data since the previous VSM update, you *should* see that it transfers the changed amount shown in the "snap delta" output, give or take some extra 'metadata' that it would also require.   

Your scenario sounds somewhat at odds with this, and I would suggest you open a case with support (as Srinath suggested).   I do not believe you should see snap delta indicate 290GB yet the transfer itself send 600GB !

cheers

Mike