I have a HP DL320e running the custom HP build of ESXi 5.5.Guest is a Ubuntu server 14.04 with vmware tools installed. Write speeds to the two SSDs in RAID1 (disk added with thin provision) are great (about 440MB/s) after boot. But over some time it gradually slows down to half the speed (220MB/s).
The test command I am running is
dd if=/dev/zero bs=1024k of=/tmp/test-file count=1024 conv=fdatasync
If I reboot the machine I get the same behavior again. Fast in beginning and slow after a while.
I found one post here on serverfault that said having multiple sockets could be a problem. I never had multiple sockets but I had multiple cores... but I tried to run with just one core, and I still get the same problem.
I don´t see any build up under the performance tab in esxi or in the Ubuntu machine.
Any one got any idea on what this might be?
Ok, so I did some more testing and this seems to be a esxi problem. When I run Ubuntu on the exact same hardware I don´t get this gradual slowness in I/O.
I know it is a advantage to have a RAID card with a battery/cache when running esxi, but I dont see why this problem should show up with just one guest running.
Any one got any idea on what I can try out next... should there be some sort of log that tells me why it is going slower!?