Linux – How to generate different levels of disk load on a Linux (2.4) system

benchmarkhard drivelinuxperformance

I need to find out about the impact different levels of disk I/O have on my application. To do so I am looking for a way to generate different amounts of disk load while I am running the application in a benchmark mode.

We currently get complaints from users about slow performance, however the information we get is very thin, so we cannot yet determine where exactly the problem is. I/O is the primary suspect, however we'd like to know for sure.

For starters I would be fine with something to create a configurable continuous read or write load, e. g. from /dev/zero to a tmp file or something. I know I can use dd, but this will push the disk to the limit immediately whereas I'd like some kind of throttle to make different test runs with increasing amounts of background traffic.

To make things a little more challenging, this has to run on RedHat9 boxes, which means kernel 2.4. So ideally this would be some kind of script that makes use of the default tools present anyway.

Best Answer

You could make a big file (eg 1GB) and then use rsync to copy the file. rsync has a built in option to limit the bandwith. So you could copy the file at 10KiB/sec, or 50KiB/sec or 2MiB/sec, etc.

Related Solutions

Sql-server – Pay no attention to that SAN behind the curtain

Without making enemies on the SAN team, how can I reassure myself and the application developers that our SQL servers aren't suffering from poorly configured storage? Just use perfmon stats? Other benchmarks like sqlio?

In short, there probably isn't a way to be truly sure. What I would say (I am a SAN admin), is that if your applications are performing up to your expectations, don't worry about it. If you start to see performance issues that you believe could be related to SAN/Disk IO performance, then it might be wise to inquire. I do not use much HP storage like you do, but in the IBM/NetApp world I can say from experience that there aren't many options which would allow you to configure it "poorly". Most enterprise storage these days takes a lot of the guesswork out of building raid arrays, and doesn't really let you do it wrong. Unless they are mixing drive speeds and capacities within the same raid groups you can rest-assured in most cases that your disk is performing fine.

If I load test on these SAN drives, does that really give me a reliable, repeatable measure of what I will see when we go live? (assuming that the SAN software might "dynamically configure" differently at different points in time.)

Load testing should be plenty reliable. Just keep in mind that when you are load testing one box, that being on a shared SAN/Disk Array that its performance can (and will) be affected by other systems using the same storage.

Does heavy IO in one part of the SAN (say the Exchange server) impact my SQL servers? (assuming they aren't giving dedicated disks to each server, which I've been told they are not)

It can. It is not all about the disks, or which disks, the servers are on. All of the data is being served up via a disk controller, and then a SAN switch. The performance you will see greatly depends on how the disk controller is connected to is corresponding disk shelves, and the corresponding SAN. If the entire array connects to the backbone SAN on one single strand of 4gbps fiber, then clearly the performance will be impacted. If the array is connected across two redundant SAN's which are load balanced, using trunked links, then it would impossible for exchange alone to suck up too much bandwidth. Another thing which needs to be considered is how many IO/sec the array is capable of. As long as the array and the SAN it is connected to are scaled correctly, heavy IO in other parts of the SAN environment should not impact your SQL performance.

Would requesting separating logical drives for different functions logical drives (data vs log vs tempdb) help here? Would the SAN see the different IO activity on these and optimally configure them differently?

That is probably a matter of preference, and also greatly depends on how your storage admins configure it. They could give you three LUNs in the same array or volume, in which case its all the same anyway. If they gave you individual LUNs on different arrays, in different volumes (physically different disks), then it might be worth it for you to separate them.

We're in a bit of a space crunch right now. Application teams being told to trim data archives, etc. Would space concerns cause the SAN team to make different decisions on how they configure internal storage (RAID levels, etc) that could impact my server's performance?

I don't imagine your storage admin would change the raid level in order to free up space. If he would, then he should probably be fired. Space concerns can lead things to be configured differently, but not normally in a performance-impacting way. They might just become a little more tight about how much space they give you. They might enable features such as data de-duplication (if the array supports it) which can hinder the performance of the array while the process runs, but not around the clock.

Linux Hard Disk Load – How to Monitor Hard Disk Load on Linux

You can get a pretty good measure of this using the iostat tool.

% iostat -dx /dev/sda 5

Device:         rrqm/s   wrqm/s     r/s     w/s   rsec/s   wsec/s avgrq-sz avgqu-sz   await  svctm  %util
sda               0.78    11.03    1.19    2.82    72.98   111.07    45.80     0.13   32.78   1.60   0.64

The disk utilisation is listed in the last column. This is defined as

Percentage of CPU time during which I/O requests were issued to the device (band-width utilization for the device). Device saturation occurs when this value is close to 100%.

Best Answer

Related Solutions

Sql-server – Pay no attention to that SAN behind the curtain

Linux Hard Disk Load – How to Monitor Hard Disk Load on Linux

Related Topic