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I know this might sound like a silly question, but here I go. I have CentOS 7 running in a server with a disk using LVM, according to the vendor the disk could achieve 500 iops, the disk is under /dev/sda and I can see the LVM DM devices showing up as dm-0 and dm-1. I'm monitoring the server with telegraf and plotting the metrics on Grafana to see the behavior of multiple metrics when I run my applications, up to this point all is good. The question I have is regarding the iops count, should I count the both the sda and dm-x io request or just sda to measure my average io request usage?

Grafana sample

Escanoru
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The LVM devices starting with DM are an abstraction on top of the disk device.

The disk device is /dev/sda.

Your vendor offered IOPS values for the disk itself.

You probably should measure the performance of /dev/sda if you want to evaluate the claims of the vendor.

Note that the "average" I/O performance is unlikely to closely approach the peak rate unless your disk usage is consistent and saturated, at which point your experience of the performance of the system is unlikely to be positive (it implies that some process is waiting for I/O literally all_the_time.).

Slartibartfast
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  • Thanks for providing your feedback, it sounds like the iops from the LVM DM devices are "artificial", are the DM iops actually presenting a load to the system? – Escanoru Oct 26 '21 at 17:47
  • Looking at the metrics shared in the following screenshot I see quite the activity for both RIOPS and WIOPS on the both devices: https://imgur.com/a/hYbDUTF – Escanoru Oct 26 '21 at 17:55
  • Those images are interesting. It looks to me like the LVM setup is providing some level of write reduction (could be coalescing multiple writes into one?) but not much read reduction (you only show one of the two LVM devices in the graph, so hard to be sure). – Slartibartfast Oct 29 '21 at 01:10