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Circling Back To Littles Law Now that we have tools to gather information.

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Presentation on theme: "Circling Back To Littles Law Now that we have tools to gather information."— Presentation transcript:

1 Circling Back To Littles Law Now that we have tools to gather information

2 2 Latency – A measure of time delay experienced in a system, the precise definition of which depends on the system and the time being measured. In storage, latency is generally referred to as response time, in ms. Throughput – The amount of material or items passing through a system or process. In storage, IO/s in units of 4k

3 Latency & Throughput Random SQL SERVER example: Latency starts to spike as near saturation

4 4 Latency & Throughput Latency starts to spike as near saturation

5 5 Disk IOPS versus Latency

6 throughput A B LATENCY throughput B True in Real Life Too

7 Littles Law Review & Example Littles Law: Restated: N = L * W N = # Cars in Jam T = Lanes (Throughput) Wait = time from A->B Assume 4 cars arrive every second (lanes) A->B is 30 seconds N = 4*30 = 120

8 8 Littles Law - Review We can use this with Latency & Throughput on a Netapp system too. Standard version: Re-written for Netapp: Translating into IO terms: N = # of outstanding IOs T = Throughput of IOs R = Response time of each IO

9 9 Littles Law - An Example Typical situation: – An user complains of poor performance: My dd/cp/tar/Oracle query (for example: full table scan) etc. process isnt fast enough – A casual look at sysstat shows the filer is not very busy – NetApp Service returns with a statement of thread-limited What does this mean?

10 Littles Law - An Example Compute Wait for Storage Read Request Data Return Time In this example, the process is either computing or reading. It is always busy. But the CPU and the storage are not, on average, fully used. Client side tools would be needed to determine this: debugger, strace, dtrace, etc.

11 11 Littles Law - An Example Using stats show volume: volume:dwhprod1:san_read_data: b/s volume:dwhprod1:san_read_latency:4.23ms volume:dwhprod1:san_read_ops:653/s How many threads (on average) are running here? From Littles Law: (N threads) / (service time per op) = throughput

12 12 How many threads (on average) are running here? (N threads) / (service time per op) = throughput N threads = throughput × (service time) Service Time: volume:dwhprod1:san_read_latency:4.23ms Throughput: volume:dwhprod1:san_read_ops:653/s Littles Law - An Example

13 13 Littles Law - An Example How many threads (on average) are running here? throughput × (service time) N threads 653 × What are the performance implications of having only 2.8 concurrent requests (on average)?

14 14 Littles Law - An Example This example is a concurrency-limited workload – Each thread is always busy – Not enough threads to keep the system busy Implications: – Storage system not fully utilized – High I/O wait times at the server

15 15 Littles Law - An Example Solution: Add more threads – Sometimes you cannot, for example if there is a mapping of 1 thread to each application user, you cannot increase the user population – Fix Client Inefficiencies FCP/iSCSI - Increase queue depth NFS - Poor IO concurrency due to inefficient NFS client design, use an updated NFS client or 3rd party product (ex. Oracle DirectNFS) and/or Make the IO subsystem/disks faster – Including fixing client filesystem caching – PAM/Hybrid Aggregates


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