Monday, June 1, 2015

Bad Idea #8 - BTRFS Subvols

Bad Idea #8 - BTRFS Subvols

Got your attention? 
If you like Btrfs then I expect so. 
But take heart: the problem here is not subvolumes. This is not a slam on subvols nor on Btrfs itself. The problem is subvolume abuse. Hopefully you'll keep reading and not click away.
Like so many other things, Btrfs subvols are more good than bad, but side effects can have unexpected results, unintended consequences. Here's a quick story of one case of a particularly annoying side effect. 
I'm embracing Btrfs for a lot of reasons, so I chose it as the root filesystem type when doing a recent Linux installation. Snapshot capability was also selected by default. I knew about that feature but did not know about some of its consequences. Then when signing on to the shiny new system, I found more than a dozen subvols mounted in support of snapshot. 
This broke my RSYNC plan. 
My usual habit is to install the new system in a virtual machine, then copy ... er, uh ... take a snapshot (ha!). The copy gets put into a dedicated logical volume. I then copy the new kernel and initrd to a common boot partition used by all systems. This works really well: I get a variety of distros and releases and yet can 'zypper' or 'yum' as needed within each. 
This time, there were Btrfs subvolumes. The subvols showed as individual mount points, so the "-x" flag did the right thing by not descending across mounts. We wouldn't want to snag /proc or /sys when snapping that copy of the new op sys, so we really need that "-x". But we do want the normal files, all of them. We don't want to have to construct a myriad 'rsync' invocations to make it work.
The system I installed, not knowing to de-select snapshots, wound up with 14 subvols. It looks like a case of some developer trying to do with subvols what we all used to do with LVM. (Scaled up just a bit since that number of "volumes" is beyond what anyone bothered with before we had subvols.) Now ... the layering violation (subvols versus LVM) is a whole nutha topic. But you can see from this count: 15 invocations of 'rsync' versus just one.

Perhaps just a nuisance?
Fairly, though, a nuisance with questionable requirement.
These "bad idea" rants sound horribly whiny. There is an upside. Reader, please understand, it's really about KISS. In this particular complaint, subvolume abuse goes against the grain of simpler filesystem handling. 
Where has simplicity gone? 
We in the Linux and open source world love to pick on Windows, yet we're now creating the same complicated crap, monolithic mechanisms with unforeseen flaws. 
-- R; <><

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