The next term I want to take a look at is fractional reserve. This term is often misunderstood.
Fractional reserve is applied to the space reserved blocks. Without space reservations, fractional reserve has no meaning. In the previous discussion, I described how space reservation is used to make sure there are enough free blocks in the volume to overwrite all the blocks in a LUN which are bound by snapshots. Let’s take a look at a sample volume in my simulator:
Notice the fractional reserve is set to 100%. This is the default. It means that all of the blocks (100%) protected by space reservations will be shielded with free blocks.
If we were to fill up all the blocks in the LUN and then take a snapshot, space reservations would apply to all of the blocks in the LUN and the snapshot would fail unless the number of free blocks in the volume were at least equal to the number of blocks (in this case, all of them) that would be needed for overwrites in case the entire LUN were to be changed.
But this is a worst case scenario. This is the default behavior and if we implement this solution then writes to the LUN will always succeed. We will never be without free blocks, no matter how high the rate of change in the LUN.
In some scenarios, this solution is overkill. We may know that the rate of change for this LUN is predictable and is relatively slow. It is reasonable to set the fractional reserve to less than 100% in such a situation.
For example, suppose we are keeping 5 daily snapshots online for a database that is updating at a rate of 5% per day. We can get a very accurate estimate of the historical rate of change by using the snap delta command. Here is an example of the snap delta command:
Notice we are given the change rate in both KB changed between snapshots and also the rate of change in KB/hour.
Using SnapDrive, we can change the fractional reserve to 50% instead of 100%. Now snapshots will succeed as long as we have blocks necessary to overwrite half of the block in the LUN. Since we are only updating 5% per day this provides enough block for 10 days. As long as we are automatically deleting snapshots after 5 days, we should never run out of space and we have an adequate safety margin even if the rate of change were to double for two or three days. SnapDrive also has the ability to monitor and notify the storage administrator if thresholds for free space or rate of change are exceeded.
Leaving fractional reserve set to 100% is the most conservative solution and will ensure the LUN remains writable in even the worst case scenario. In some situations this is unnecessary and we can recover that space for other purposes.
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