As all know disk performance may effect overall performance. Connection performance , ram , cpu have no means if disk perfomance gets low.
For example all SQL queries have a weight for returning results for physical disk just as /tmp directory.
We can make IO test for what is happening.
Via SSH:
dd if=/dev/zero of=test bs=64k count=16k conv=fdatasync
Do not forget:
If test bytes are gets low, and you have single disk; your disk going to die soon,
And if you get unstable results and you are using raid; check your disks status. Maybe one of your disk alredy died.
Happy coding.