/etc/exports & mount option

Question

In my Linux server I have the following options in /etc/exports

/home *(rw,sync,no_subtree_check,no_root_squash)

And I mount from a Mac using

mount -t nfs -o resvport,rw,noatime,soft,intr,rsize=32768,wsize=32768,timeo=900,retrans=3,proto=tcp,vers=3,async 192.168.1.121:/home /Volumes/home

As you can see the server is specifying the sync but my mount option is using async, so which one will be used?

Solution

sync and async have different meanings for the two different situations.

sync in the client context makes all writes to the file be committed to the server. async causes all writes to the file to not be transmitted to the server immediately, usually only when the file is closed. So another host opening the same file is not going to see the changes made by the first host.

Note that NFS offers "close to open" consistency meaning other clients cannot anyway assume that data in a file open by others is consistent (or frankly, that it is consistent at all if you do not use nfslock to add some form of synchronization between clients).

sync in the server context (the default) causes the server to only reply to say the data was written when the storage backend actually informs it the data was written. async in the server context gets the server to merely respond as if the file was written on the server irrespective of if it actually has written it. This is a lot faster but also very dangerous as data may have a problem being committed!

In most cases, async on the client side and sync on the server side. This offers a pretty consistent behaviour with regards to how NFS is supposed to work.

Comments

[Brigo]

You local filesystem will use sync (/home) and the remote, will use async, afaik.

[Ryan]

Updated my question: my local setting is set in /etc/exports *, not *fstab