Random Wisdom

On a default Fedora installation, Gnome will automatically mount (with some default options) any recognized volumes as soon as an external storage device is plugged in. While the defaults might be fine in general, sometimes it is necessary to supply some more options.

The tool responsible for automating the mount (gnome-volume-manager) gets additional options from the /system/storage/default_options/$fstype$/mount_options key of gconf — the Gnome “system registry”. One can check/modify existing entries (one a per user basis) by running:

$ gconf-editor /system/storage/default_options &

This should launch the graphical configuration editor with the default_options key selected. Under this key are the entries for different filesystems. Default mount options can then be changed by selecting the desired filesystem and editing the mount_options key.

If a required filesystem type is not listed, then it can be added by using the gconftool-2 utility. E.g. if we wanted to add the ext3 filesystem to the configuration database with the options “sync” and “uid=“, we would run the command:

$ gconftool-2 -t list –list-type string \

                  -s /system/storage/default_options/ext3/mount_options “[sync,uid=]”

Next time an external volume is plugged in, it will be mounted with the additional options specified! It should be noted that regardless of the options supplied via gconf, some mount options are always present: r(o|w),noexec,nosuid,nodev. At the moment I do not know how to change them.

The main motivation for finding out about the defaults is that I wanted to add “sync” as a default option. This causes data to be written immediately to the device, instead of being buffered first — a useful option to have for external devices. It should minimized data loss in case of an accidental removal (without running umount first). However, it should also be noted that for solid state drives (e.g. flash), this may result in a shortening of service life and poorer performance.

Update [Sun Mar 18, 2007]:
I’ve done some rudimentary throughput performance testing with both the sync and async modes and the performance hit with sync appears to be quite severe (at least 20 times slower than async) — even with a high performance HDD as the target. In light of this, I am removing sync from the list of default options. Given the type of data I’m likely to store on the device, speed is certainly more valuable than data integrity.

Test results:
Testing was carried out on a FAT32 volume. For each mode, both the transfer time and the subsequent un-mount time (indicating the time needed to flush the buffer) are shown.

async:

$ time dd if=/dev/zero of=/tmp/usb/dummy bs=8k count=130000 && time sudo umount /tmp/usb/
130000+0 records in
130000+0 records out
1064960000 bytes (1.1 GB) copied, 45.9035 seconds, 23.2 MB/s

real    0m46.026s
user    0m0.035s
sys     0m2.915s

real    0m11.680s
user    0m0.003s
sys     0m0.263s

sync:

$ time dd if=/dev/zero of=/tmp/usb/dummy bs=8k count=130000 && time sudo umount /tmp/usb/
130000+0 records in
130000+0 records out
1064960000 bytes (1.1 GB) copied, 969.384 seconds, 1.1 MB/s

real    16m13.525s
user    0m0.048s
sys     0m12.156s

real    0m0.842s
user    0m0.004s
sys     0m0.199s