[ovirt-users] NFS sync vs async

[Jean-Francois Courteau]

Hello there,

At first I thought I had a performance problem with virtio-scsi on 
Windows, but after thorough experimentation, I finally found that my 
performance problem was related to the way I share my storage using NFS.

Using the settings suggested on the oVirt website for the /etc/exports 
file, I implemented the following line:
    /storage    
*(rw,sync,no_subtree_check,all_squash,anonuid=36,anongid=36)

The underlying filesystem is ext4.

In the end, whatever the VM I am running through this NFS export, I get 
extremely poor write performance, like sub-100 IOPS (my disks usually 
can do 800-1k). Under the hood, iotop shows that my host IO is all taken 
up by jbd2, and if I understand correctly, it is the ext4 logging 
process.

I have read that using the "async" option in my NFS export is unsafe, 
like if my host crashes during a write operation, it could corrupt my VM 
Disks.

What is the best combination of filesystem / settings if I want to go 
with NFS sync? Is someone getting good performance with the same options 
as me? if so, why do I get such abysmal IOPS numbers?

Thanks!

J-F Courteau