[6.7] Weird write performance problem, VM reports half of a host write speed

[hardwired666]

Hi,

I've setup a server with 2 WD RED's in RAID1 configuration (LSI 9361). When started to write to a guest operating system, I can't get the full gigabit write speed.

The guest VM reports around 50MB/s of write speed and HUGE latency as below:

however, host reports full write speed (around 100MB/s):

esxtop shows ~100MB/s as well:

I have no idea what is the problem there I'm using paravirtual driver of course.

Checked with SSD RAID1 array on the same controller and VM - full write speed.

Thanks in advance for any help.

[vijayrana968]

Not too relevant but check Power Management settings on Host.

[hardwired666]

Checked and set to High performance - nothing changed

I've re-installed to 6.5 and prepared a disk as eagerly zeroed - still have huge latencies (~1000)...

[sk84]

What performance do you expect from 2 SATA hard disks in RAID-1?

I guess the host sees both disks and because it is RAID-1, all data written to disk 1 is also replicated to disk 2.

So if the virtual machine writes to disk 1 at 50 MB/s, the host sees the incoming 50 MB/s on disk 1 and since all data is replicated to disk 2, the host summarizes this.

What surprises me, is that the host obviously sees both disks. With a hardware raid controller it would only have to present one logical device to the ESXi host...

[hardwired666]

The host does not see 2 disks. It see one Virtual Drive. That's a full HW RAID controller.

The half of the speed was caused by zero'ing the disk before the data being written. With eager zeroed - it's not the issue anymore.

Can you show your performance graphs for similar setups? I mean HDD drives in RAID1 setup under some HW RAID controller. I expect it to write ~100MB/s. It should. However, I see aroung 1000ms od latency 😕