Sep 132017

After reorganizing my ZFS datasets a bit, I suddenly noted I couldn’t copy any file larger than a few MB. A bit of investigation later and I figured why it was so.

My ZFS data sets were as follows:

# zfs list
NAME                            USED  AVAIL  REFER  MOUNTPOINT
Data                           2.06T   965G    96K  none
Data/Users                      181G   965G    96K  none
Data/Users/User1               44.3G  19.7G  2.23G  /Data/Users/User1
Data/Users/User2               14.7G  49.3G   264K  /Data/Users/User2
Data/Users/User3                224K  64.0G    96K  /Data/Users/User3
And my Samba share was pointing to /Data/Users/.

Guess what? Path /Data/Users was not pointing to any dataset as my parent dataset for Data/Users was not mounted. Instead it pointed to memory disk md0 which had just a few MB free. Samba doesn’t check full path for disk size but only its root share.

The easiest way to workaround this would be to simply mount parent dataset. But why go for easy?

A bit more complicated solution is getting Samba to use custom script to determine free space. We can then use this script to return available disk space for our parent dataset instead of built-in samba calculation.

To do this, we first create script /myScripts/sambaDiskFree:

DATASET=`pwd | cut -c2-`
zfs list -H -p -o available,used $DATASET | awk '{print $1+$2 " " $1}'

This script will check current directory, map its name to dataset (in my case it is as easy as stripping first slash character) and return two numbers. First is total disk space, followed by available diskspace – both in bytes.

Once script is saved and marked as executable (chmod +x), we just need to reference it in Services > CIFS/SMB > Settings under Additional parameters:

dfree command = /myScripts/sambaDiskFree

This will tell Samba to use our script for disk space determinations.

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