I am reproducing an internal twiki post that I put together when we decided to give the Sun Thumper (x4500) a try as an iSCSI target running ZFS.
Introduction
Inspired by a nice paper on utility the following model aims at providing quantitative justifications to choose a SAN solution. This approach relies on the ability to create a “utility” function that takes into account a certain number of factors (explained below) and come up with a mono-dimensional measure (no unit).
Factors are:
- Performance (measured in iops)
- Capacity (here assumed to be usable, typically raid5 for dev/test, expressed in TB)
- Availability (aggregate in %)
- Power (in kWh)
- Acquisition cost (in $, depreciated over n years)
- Revenue (in $, if we can tie that to some business figure)
- Management (in hours per TB)
- Reliability (fractional and total, in %), a measure of the chances to lose some or all of the datasets
Definitions
Utility = Revenue - Cost(downtime) - Cost(data loss) - Cost(management) - Acquisition
Cost(downtime) = (1 - Availability) * SAN size * #developers per TB * hourly developer rate
Cost(data loss) = Hourly Failure * SAN size * restore hr per TB * (hourly IT rate * #IT staff per TB + hourly developer rate * #developers per TB)
and
Acquisition = capex and opex over the lifetime of the SAN
We will assume that revenue is 0 for development and testing. This is of course not true but since any scenario yields the same basic functionality this is a safe assumption to make.
We also assume that we have a backup of everything. In case this is impossible, the restore time becomes that of recreating data from scratch.
The winner is the solution with the higher utility.
