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ep:labs:03:contents:tasks:ex1 [2020/08/04 20:26] gheorghe.petre2608 [01. [10p] Rotational delay - IOPS calculations] |
ep:labs:03:contents:tasks:ex1 [2025/02/11 23:27] (current) cezar.craciunoiu |
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| - | ==== 01. [10p] Rotational delay - IOPS calculations ==== | + | ==== 01. [30p] Vmstat ==== |
| + | The **vmstat** utility provides a good low-overhead view of system performance. Since **vmstat** is such a low-overhead tool, it is practical to have it running even on heavily loaded servers when it is needed to monitor the system’s health. | ||
| - | <note tip> | + | === [10p] Task A - Monitoring stress === |
| - | Every disk in your storage system has a maximum theoretical IOPS value that is based on a formula. Disk performance and IOPS is based on three key factors: | + | Run **vmstat** on your machine with a 1 second delay between updates. Notice the CPU utilisation (info about the output columns [[https://medium.com/@damianmyerscough/vmstat-explained-83b3e87493b3|here]]). |
| - | * **Rotational speed**. Measured in RPM, mostly 7,200, 10,000 or 15,000 RPM. A higher rotational speed is associated with a higher-performing disk. | + | In another terminal, use the **stress** command to start N CPU workers, where N is the number of cores on your system. |
| - | * **Average latency**. The time it takes for the sector of the disk being accessed to rotate into position under a read/write head. | + | Do not pass the number directly. Instead, use command substitution. |
| - | * **Average seek time**. The time (in ms) it takes for the hard drive’s read/write head to position itself over the track being read or written. | + | |
| - | * **Average IOPS**: Divide 1 by the sum of the average latency in ms and the average seek time in ms (1 / (average latency in ms + average seek time in ms). | + | Note: if you are trying to solve the lab on //fep// and you don't have **stress** installed, try cloning and compiling [[https://github.com/ColinIanKing/stress-ng|stress-ng]]. |
| - | </note> | + | |
| - | To calculate the **IOPS range** divide 1 by the sum of the average latency in ms and the average seek time in ms. The formula is: | + | <solution -hidden> |
| - | <code>average IOPS = 1 / (average latency in ms + average seek time in ms). | + | **Task A:**\\ |
| + | <code bash> | ||
| + | $ vmstat -w -n 1 | ||
| + | $ stress -c $(nproc) | ||
| </code> | </code> | ||
| + | </solution> | ||
| - | Let's calculate the Rotational Delay - RD for a 10K RPM drive: | + | === [10p] Task B - How does it work? === |
| + | Let us look at how **vmstat** works under the hood. We can assume that all these statistics (memory, swap, etc.) can not be normally gathered in userspace. So how does **vmstat** get these values from the kernel? Or rather, how does any process interact with the kernel? Most obvious answer: //**system calls**//. | ||
| - | * Divide 10000 RPM by 60 seconds: **''10000/60 = 166 RPS''** | + | <code bash> |
| - | * Convert 1 of 166 to decimal: **''1/166 = 0.006 seconds per Rotation''** | + | $ strace vmstat |
| - | * Multiply the seconds per rotation by 1000 milliseconds (6 MS per rotation). | + | </code> |
| - | * Divide the total in half (RD is considered half a revolution around a disk): **''6/2 = 3 MS''** | + | |
| - | * Add an average of 3 MS for seek time: **''3 MS + 3 MS = 6 MS''** | + | |
| - | * Add 2 MS for latency (internal transfer): **''6 MS + 2 MS = 8 MS''** | + | |
| - | * Divide 1000 MS by 8 MS per I/O: **''1000/8 = 125 IOPS''** | + | |
| - | === [10p] Task A - Calculate rotational delay === | + | //"All well and good. But what am I looking at?"// |
| - | Calculate the rotational delay (RD) for a 5400 RPM drive. | + | What you //should// be looking at are the system calls after the two **write**s that display the output header (hint: it has to do with **/proc/** file system). So, what are these files that **vmstat** opens? |
| - | <solution -hidden> | + | <code bash> |
| - | As shown in the //"Calculating IOs Per Second"// section: | + | $ file /proc/meminfo |
| - | <code> | + | $ cat /proc/meminfo |
| - | 5400 / 60 = 90 RPS | + | |
| - | 1/90 = 0.011 seconds per Rotation | + | $ man 5 proc |
| - | 0.011 * 1000 = 11ms per Rotation | + | |
| - | 11 / 2 = 5.5ms RD (Rotational Delay = half a revolution around a disk) | + | |
| - | add approx. 3ms seek time => 8.5ms | + | |
| - | add 2ms latency => 10.5ms | + | |
| - | Divide 1000ms by 10.5ms per I/O => approx. 95 IOPS | + | |
| </code> | </code> | ||
| - | </solution> | ||
| + | The manual should contain enough information about what these kernel interfaces can provide. However, if you are interested in //how// the kernel generates the statistics in **/proc/meminfo** (for example), a good place to start would be [[https://elixir.bootlin.com/linux/v4.15/source/fs/proc/meminfo.c|meminfo.c]] (but first, [[https://ocw.cs.pub.ro/courses/so2|SO2 wiki]]). | ||
| + | === [10p] Task C - USO flashbacks (1) === | ||
| + | Write a one-liner that uses **vmstat** to report complete **disk statistics** and sort the output in **descending** order based on **total reads** column. | ||
| + | |||
| + | <note tip> | ||
| + | You can eliminate the first two header lines from the **vmstat** output using ''tail -n +3''. | ||
| + | </note> | ||
| + | |||
| + | <solution -hidden> | ||
| + | <code bash> | ||
| + | $ vmstat -wdn | tail -n +3 | sort -nrk 2 | ||
| + | </code> | ||
| + | </solution> | ||
