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Fractional Reservation

January 18th, 2009

I’ve been talking customers through this a lot recently as I have to explain where all their space disappears to! And I’ve got a much better understanding of what it actually is now. It makes more sense as NetApp have changed it’s description, in Operations Manager 3.7 it’s now referred to as “Overwrite Reserved Space”. It also helps understand how A-SIS can help to reduce this.

This is easiest to explain with pictures. We usually use something similar to the below graphic to explain snapshots, and I’m sure everyone that has had some dealings with NetApp will have seen a graphic like this before.


Snapshot Functionality

Snapshot Functionality


So basically a snapshot locks the data blocks of the data referenced by it in place. This means that any new or changed blocks (C1 in the above graphic) in the active file-system are written to a different location. This concept is fundamentally the same as what Fractional Reservation is.


So the LUN gets filled up with data, we take a snapshot and that data is locked in place. Potentially all this data could change, and we need to guarantee not only this existing data, but also the potential that all this data could change. Any changed data gets written into the Fractional Reservation area rather than into the area that the existing LUN data is in. (I know that in reality this is spread across all the disks and these areas don’t actually exist, but it makes it easier to visual and understand explaining it this way).


So the reason a LUN will be switched offline if the fractional reservation area is set to 0, is that the filer needs to protect the existing data that is locked between the active file system and the most recent snapshot, plus any additional changes that happen to the active file system. If the volume / LUN / frac-res and snap reserve are full, then this space is not available and the filer needs to take action to prevent these writes from failing. The filers guarantee no data loss, but with no space free and no-where to write the new data, it has to offline the lun to prevent the writes from failing.


So fractional reservation is in constant use by the filer as an over-write area for the LUN. Without it, you need to make sure that sufficient space is free to allow for the maximum rate of change you would expect. Defaults are good, but trimming down on these you need to monitor the rate of change and make sure the worst case scenario is within a buffer of free space that you allow.


And that is Fractional Reservation!


A-SIS won’t make any difference to the Fractional Reservation area as such, but it can help as the data blocks within the LUN will get de-duped, but the Fractional Reservation area per-se would always be required as you need this LUN over-write area for changing data. If you reduce the footprint of the non-changing data with A-SIS, you reduce the potential reservation area required. For this reason, I still don’t think we should ever totally reduce frac-res to 0, but keep it at a minimal of about 10% with snapshot reservation on top of this. But most of all monitor it!


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