Johannes Schlüter

With ZFS you may not only do "file system" stuff but ZFS may also provide raw block devices ("zvol") which benefit from ZFS space accounting, snapshotting, checksumming, etc. A purpose of these is to use these zvols and exporting them via iSCSI or give them to applications which can store data on them. One application for this I'm using s VirtualBox and as I always forget the exact commands needed to create a zvol and making it available to VBox I decided to write it down.

Reasons for me for using zvols instead of regular VBox disks are that I can easily snapshot them (every 15 minutes a snapshot ...) individually and can easily clone them (around one second and barely any disk space needed to get a clone of a VM to do some experimental stuff...) and incremental backups using snapshots and zfs send. That said there's at least one - possibly - negative factor: A regular virtual disk file can be shared with other people and other operating systems, a zvol has to be dumped into a regular vmdk first.

Anyways here are the steps needed:

# zfs create -V 10G tank/some_name
# chown johannes /dev/zvol/rdsk/tank/some_name
# VBoxManage internalcommands createrawvmdk \
   -filename /home/johannes/VBoxdisks/some_name.vmdk \
   -rawdisk /dev/zvol/rdsk/tank/some_name
# VBoxManage registerimage disk /home/johannes/VBoxdisks/some_name.vmdk

So first we create the zvol with a size of 10G. This won't be allocated but everybody asking for the size of the device will get this information back and this is the maximum size that will be used - as one can use compression and dedup there this often might be way less usage. Then, as I'm running VBox under my user account, I give my user all the rights needed by making the regular user the owner. The third step creates a vmdk file pointing to the raw device location which is then registered with VirtualBox so a VM can be configured for using it.

Works nicely.

So, I use OpenSolaris on most of my boxes as native operating system. VMs run other OSs. My choice for OpenSolaris was driven by the availability of DTrace. One of the greatest tools for system/program analysis ever created. By running OpenSolaris I've also got ZFS which is Oracle's über file system. I never really cared about ZFS, at least not until I missed it. So ZFS integrates all the different storage layers in one system - RAID-controller, logical volume manager, POSIX file system layer, ... Really nice to have that integrated, eases management. Now I don't change my disks that often and the file system silently runs underneath. From time to time I looked into my auto snapshot to restore some stuff and got used to snapshot my VMs (running on ZFS-powered "virtual" zvol devices) before updating them which over time became a habit about which I didn't really think.

Then I've got myself a netbook. Some cheap up to date ASUS EeePC. On that system I choose to install Ubuntu - which was troublesome enough (had to compile my owned manually patched wireless driver) so I didn't bother to try OpenSolaris. Works like a charm, even without ZFS. Some time after I configured the netbook a new Ubuntu release came out and since then I'm in trouble. I read on too many sites that things broke with this release so I dare to update the system. On my OpenSolaris boxes updating to a update, even to a dev build, is a no-brainer: The packaging system automatically creates a ZFS snapshot and configures the boot loader in a way that the old as well as the new system can be booted. So I can click the update button, reboot and either it works (typical case) or I can revert. Really nice.

Now back to Ubuntu: If I press the button and something goes wrong I have to reinstall the system (or use a backup) which I don't want. I just want to use the netbook as a mobile browser, presentation system etc. There are other systems I use to play/experiment with...

At the recent PHP Barcamp Salzburg we got to a discussion about ZFS, too. In the discussion there was talk about the auto snapshotting and a claim was "well, I won't need it, I have everything in a version control software and I know what I delete" that might be true but once you have ZFS you change your way to operate and you don't have the whole system in a version control thing. It's so great to be able to clone a VM in less than a second to play with some stuff. It is cool to be able to enable compression with one short shell command. It's fantastic to have a fully checksummed filesystem with RAID-Z. Man how did we live in the old days? Nice to e aware of the luxury I'm used to

P.S. This blog is running on ZFS, too - of course, gave a good feeling to be able to revert during today's update, too.

So, this website moved. It isn't the citizen of a Linux box anymore but is running inside a zone on an OpenSolaris host. The only non-default software powering this server I compiled myself is a current svn snapshot of PHP 5.3.2-dev. Let's see if I can keep this system clean or whether it becomes such a mess as the old Linux box. For now I'm happy about the isolation using zones, snapshots with ZFS before playing around and DTrace in case something goes wrong