Can this be causing very slow IO?

[leonardorame]

Hi, this is a TrueNAS-12.0-U4 server running on an i7-5820K CPU with 32Gb of RAM and a RAIDZ1 (3x4tb HDD drives).

I has an Ubuntu Server virtual machine running PostgreSql that reads/writes to a ZVOL. Most of the time this setup runs very well, but today I was called because the UI of an web application that runs on that Virtual Machine got stuck on a grid (that operation runs a select query against the PostgreSql database).
I didn't find anything wrong inside the virtual machine, but this got my attention:

While I was trying to figure out what's happening the folks at the other side rebooted the system, and the problem disappeared for now.

Can ARC Requests prefetch_data be the culpit?, is there a "tunning" for it?.

 

[sretalla]

Can ARC Requests prefetch_data be the culpit?

I doubt it, more likely a symptom of a poorly designed query in the DB.

is there a "tunning" for it?

Yes, but don't bother.

Get the owner of the DB to shout at their DB admin or programmer to do a better job.

 

[leonardorame]

Thanks. Yes it looks like a process was leaking memory (this process also connects to the DB), usually this VM uses lees than 10% of the assigned RAM, an when the slow down appeared the RAM was at 65%.