Note: nfsv3, kernel 3.10.0-1160.62.1.el7.x86_64

Could someone explain the magic behind the mounting of what I would call "submounts" from a Netapp filer? I see mounted filesystems in /etc/mtab (or, /proc/mounts) that are subdirectories- actually, additional filesystems- on the Netapp side. But I have no reference to those mounts in the automounter or /etc/fstab.

A little background: We have a 100 TiB filesystem on our NetApp filer. That's the maximum for this type of filesystem. But we are hauling in data at the rate of 1 TiB per day. So once we filled up, we had to start arranging the data in other filesystems, named by month, which are somehow joined on the Netapp side. I'm not too worried about how it's joined- that's Netapp magic. It works.

But how does NFSd know to create a new mount on our Linux clients? The mount is visible as soon as someone references the path.

For example, we have our 100 TiB mounted at /remote/datafeed. Our NetApp admin has created other 40 TiB filesystems at, for example /remote/datafeed/2022/07. So as soon as the first of July rolls around, our apps start dumping data to that path.

Our client machines mount /remote/datafeed, thus:

netapp01:/datafeed01 /remote/datafeed nfs rw,hard,intr,bg,tcp,rsize=32768,wsize=32768 0 0

So the /etc/mtab shows: netapp01:/datafeed01 /remote/datafeed nfs rw,relatime,vers=3,rsize=32768,wsize=32768,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,local_lock=none,addr= 0 0

df on a Linux client will show /remote/datafeed but no other mount with that in its name. Now, I cd /remote/datafeed/2022/07 and- voila!- both /remote/datafeed and /remote/datafeed/2022/07 now appear in its output (as well as in /etc/mtab and /proc/mounts).

What sort of sorcery is this? Again, there is nothing in any automounter config files nor in /etc/fstab that references this directory. Not only that, but I tried it with automounter off and it still works. I think it's some sort of communication between the Netapp and nfsd that makes the latter create a mount, but I don't know what it's called nor where it's documented. I looked, but I must be using the wrong search terms.


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