Question : Problem: Drive Performance Questions

I wonder if someone might be able to throw some light on the numbers in the Drive Performance table attached below.  These numbers were collected by Performance Monitor in Windows Server 2003.  The Drive is a RAID 0 set of four Seagate Savvio 10K.2 disks. The load comes from Microsofts Video Load Simulator, which lets you specify a number, n, and will then read n video files concurrently from a target drive from beginning to end at the videos play rate, which is 6.7mbps here.  Each request to the disk is for 256KB.  So for each file in n,  the Load Simulator will make 256KB requests to the disk at a rate needed to pull in data at 6.7mbps, which would be one request every 308ms (256KB/308ms = 6.7Mbits/s) for a single stream.

The first thing puzzling about these numbers is that the elapsed time reported for each read -  Avg Disk Sec per Read -  goes up with the number of streams requested. I could understand this if queues were forming so that requests had to wait longer in a queue before getting  serviced, but for the 10 and 40-stream loads no queue is reported. Plus, if queue time were included I would expect the queue of 5 reported for the 110 stream case to insert something like  5 x 9 = 45ms, but we only see 26ms for the entire read.   So whats happening here? What are the numbers telling us?

Also puzzling are the values obtained by dividing the read size by the seconds per read.  For the 10-stream load we have 256KB/9ms = 28MB/s, which is three times the actual throughput of the drive.  This one is easy: the Load Simulator is inserting  time gaps between the requests because it doesnt need to use the drive 100% of the time to get 10 x 6.7mbps from it. But look at the 40-stream case.  256KB/12ms = 21MB/s, but the drive is delivering 35MB/s.  How can that be?   And for the 110-stream case we have  256KB/26ms = 9.8MB/s but the drive is delivering 96MB/s!  

Thanks for any ideas on what might be going on here.

steva

Answer : Problem: Drive Performance Questions

The setting is set via the Adaptec Storage Manger (or via contoller BIOS). There are settings on controller level, and read / write cache is set on virtual disc (array) level. I have no access to my system until tomorrow, but if you right click the controller node and the graphical symbol of the array on the right side, you see the options.

The hint with the performance mode is from the help of Adaptec Storage Manager. On the root level you finf somthing like performance settings, there is a (small) description of this.

There is not a lot I can find about the real background of this function, have a look here:
http://www.adaptec.com/NR/rdonlyres/B4350D45-E31A-429E-8058-3BE8E2F07E86/0/Series5PerformanceWhitepaperv7.pdf
They point out the different behaviour for different file types and usages.

There is also the feature "Dynamic caching algorithm", which seems to be the same, but no further explanation, what it is and how it works.