Tuesday Oct 09, 2007

Performability analysis of T5120 and T5220

In complex systems, we must often trade-off performance against reliability, availability, or serviceability. In many cases, a system design will include both performance and availability requirements. We use performability analysis to examine the performance versus availability trade-off. Performability is simply the ability to perform. A performability analysis combines performance characterization for systems under the possible combinations of degraded states with the probability that the system will be operating the degraded states.

The simplest performability analysis is often appropriate for multiple node, shared nothing clusters which scale performance perfectly. For example, in a simple web server farm, you might have N servers capable of delivering M pages per server. Disregarding other bottlenecks in the system such, as the capacity of the internet connection to the server farm, we can say that N+1 servers will deliver M\*(N+1) performance. Thus we can estimate the aggregate performance of any number of web servers.

We can also perform an availability analysis on a web server. We can build Markov models which consider the reliability of the components in a server and their expected time to repair. The output of the models will provide the estimated time per year that each web server may be operational. More specifically, we will know the staying time per year for each of the model states. For a simple model, the performance reward for an up state is M and a down state is 0. A system which provides 99.99% (four-nines) availability can be expected to be down for approximately 53 minutes per year and up for the remainder.

For a shared nothing cluster, we can further simplify the analysis by ignoring common fault effects. In practice, this means that a failure or repair in one web server does not affect any other web servers. In many respects, this is the same simplifying assumption we made with performance, where the performance of a web server is dependent on any of the other web servers.

The shared nothing cluster availability model will contain the following system states and the annual staying time in each state: all up, one down (N-1 up), two down (N-2 up), three down (N-3 up), and so on. The availability model inputs include the unscheduled mean time between system interruption (U_MTBSI) and mean time to repair (MTTR) for the nodes. We often choose a MTTR value by considering the cost of service response time. For many shared nothing clusters, a service response time of 48 hours may be reasonable – a value which may not be reasonable for a database or storage tier. Model results might look like this:

System State

Annual Staying Time (minutes)

Cumulative Uptime (%)

Performance Reward

All up

521,395.20

99.2

M \* N

1 down

4,162.75

99.992

M \* (N - 1)

2 down

39.95

99.9996

M \* (N - 2)

3 down

2.00

99.99998

M \* (N - 3)

> 3 down

0.11

100

< M \* (N - 4)

Total

525,600.00

100


Now we have enough data to evaluate the performability of the system. For the simple analysis, we accept the cumulative uptime result for the minimum required performance. We can then compare various systems considering performability.

We have modeled the new Sun SPARC Enterprise T5120 and Sun SPARC Enterprise T5220 servers against the venerable Sun Fire V490 servers. For this analysis we chose a performance benchmark with a metric that showed we needed 6 T5120 or T5220 servers to match the performance of 9 V490 servers. We will choose to overprovision by one server, which is often optimum for such architectures. The performability results are:

Servers

Units

Performability (%)

Sun SPARC Enterprise T5120

6 + 1

99.99988

Sun SPARC Enterprise T5220

6 + 1

99.99988

Sun Fire V490

9 + 1

99.99893

You might notice that the T5120 and T5220 have the same performability results. This is because they share the same motherboard design, disks, power supplies, etc. It is much more interesting to compare these to the V490. Even though we use more V490 systems, the T5120 and T5220 solution provides better performability. Fewer, faster, more reliable servers should generally have better performability than more, slower, less reliable servers.

 

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