NFS synchronous or asynchronous?

[mcparlandj]

Does anyone know if the default NFS for Proxmox using an NFS server is synchronous or asynchronous?

 

[mir]

man nfs:

ac / noac Selects whether the client may cache file attributes. If
neither option is specified (or if ac is specified), the
client caches file attributes.

To improve performance, NFS clients cache file
attributes. Every few seconds, an NFS client checks the
server's version of each file's attributes for updates.
Changes that occur on the server in those small inter‐
vals remain undetected until the client checks the
server again. The noac option prevents clients from
caching file attributes so that applications can more
quickly detect file changes on the server.

In addition to preventing the client from caching file
attributes, the noac option forces application writes to
become synchronous so that local changes to a file
become visible on the server immediately. That way,
other clients can quickly detect recent writes when they
check the file's attributes.

Since default does not set ac or noac the mount is asynchronous.

 

[mcparlandj]

That's great news! Vmware sets to synchronous, and using ZFS backed NFS, even with a fast ZIL performance is awful. I did some testing with Proxmox and performance was amazing and I was thinking maybe I was looking at the numbers wrong

 

[czechsys]

Well, setting async for VM storage isn't good way...

 

[dswartz]

That depends. In general I agree, but one might have no real choice. Depending on the setup, it might be perfectly safe...

 

[mcparlandj]

As far as i know iscsi does asynchronous writes as well. So that makes me think doing asynchronous with NFS is just as sketchy as using vanilla iscsi.