Date: Mon, 12 Feb 2007 11:05:53 -0600 From: Tillman Hodgson <tillman@seekingfire.com> To: freebsd-questions@freebsd.org Subject: Mounting multiple NFS shares to the same point Message-ID: <20070212170553.GA543@seekingfire.com>
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A bit of background: I run backup scripts (dumps piped through gzip to a fileshare) out of periodic on a daily, weekly and monthly basis. In the script I mount the NFS share, perform the dumps, and then umount the share. I was worried that if a daily backup took a long time (more than twice the normal time) then the weekly would bomb out because the filesystem was already mounted. So I was going to write some checks to see if it existed before mounting it. Which is when I discovered that you can mount multiple NFS shares to the same directory :-) Here's an example of it in action: [root@athena ~]# mount /exports/srvbackup/ [root@athena ~]# mount | grep srvbackup nas:/srvbackup on /exports/srvbackup (nfs) [root@athena ~]# mount /exports/srvbackup/ [root@athena ~]# mount | grep srvbackup nas:/srvbackup on /exports/srvbackup (nfs) nas:/srvbackup on /exports/srvbackup (nfs) [root@athena ~]# umount /exports/srvbackup/ [root@athena ~]# mount | grep srvbackup nas:/srvbackup on /exports/srvbackup (nfs) [root@athena ~]# umount /exports/srvbackup/ [root@athena ~]# mount | grep srvbackup [root@athena ~]# man mount_nfs Further, you can mount /different/ shares to the same directory: [root@athena ~]# mount /exports/srvbackup/ [root@athena ~]# mount_nfs nas:/pub /exports/srvbackup/ [root@athena ~]# mount | grep srvbackup nas:/srvbackup on /exports/srvbackup (nfs) nas:/pub on /exports/srvbackup (nfs) I then cd'ed to /exports/srvbackup, and only saw files from the second mount (nas:/pub). So it's not doing a union mount or anything like that. Is this normal behaviour? Are there any problems with (performance, perhaps) that might occur if an NFS share is mounted twice? What if my backup job is still running, would it be interrupted by the second mount 75 minutes later (according to the `periodic` entires in crontab) or will it be fine? This definitely seems odd to me, I would've expected mount to express an error to me. -T
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