Is it possible to force NFS storage into asynchronous mount

[Matthew Weiner]

I'm using Hypervisor 8.2 with an NFS mount on Dell storage, currently an FS8600 but likely moving to a PowerStore 1200T at some point in the future.  Being in synchronous mode the performance, well, not so good.  The problem I have is that according to the documentation Hypervisor depends on the storage advertising the mount as async, but on my storage there's no way to configure it to do so.  According to Dell it's a client thing, so I have a bit of a catch-22, the server says the client should do it, the client says the server should. 

 

Is there any way with mount options, like via the CLI, to just tell Hypervisor to mount the NFS volume async?  I have UPSes, standby generators, battery backed cache, all that stuff, so the infentesimal risk of data loss vs. a potential significant performance gain is worth it.

[Mark Syms]

Unfortunately Dell are not correct in this assertion. The sync/async setting is a server side setting, possibly one that their filer software does not support. It is not possible for a client to request async from a server which is configured with sync.

 

See https://linux.die.net/man/5/exports and the explanation of General Options.

[Matthew Weiner]

Well that's unfortunate.  Dell confirmed there is no way to do this on the FS8600 family and from the documentation I found on the PowerStore that is the same.  Performance is pretty bad in sync mode, at least on the FS8600s, and the experiments I've done with FreeNAS, etc. it's not too much better.  I'll have to try to lean on Dell to see if they'll add async mount mode to the PowerStore's capabilities.

[Kyle Peterson]

We use a netapp A250 and netapp wants nfs to be async as well but citrix defaults to sync.

 

So in this scenario can you change it from sync to async on the hypervisor side?

[Mark Syms]

12 hours ago, Kyle Peterson said:

We use a netapp A250 and netapp wants nfs to be async as well but citrix defaults to sync.

No, no it doesn't. The setting is server side in the NFS storage, there is no setting that Citrix Hypervisor/XenServer can set to force Asynchronous I/O to the Filer.

[Mark Syms]

The async mode supported by the Linux Kernel NFS server technically violates the NFS Specification (and is documented as such) so it's not entirely surprising that vendors do not supoprt it in their expensive hardware filers as it reduces the guarantees for data integrity.

 

XenServer quite rightly asserts that I/O must be confirmed completed by the remote storage before those I/O operations are reported as completed to the VM guests and if this results in the filer having to complete expensive operations then the fault lies in the filer.

 

For the NetApp in particular this should not be the case as all write operations go first to non-volatile memory (and are thus safe) and then are flushed to the backing storage in the background.

[Kyle Peterson]

Ok makes sense, I was more just curious. Every time I update the netapp part of the pre-check tells me to make sure all mounts are async if possible which is why I thought I'd ask.

 

All good.