The benefits of flexible space usage in filesystems

January 11, 2016

My home and work Linux machines are very similar. They have about the same collection of filesystems, and they both have significant amounts of free disk space (my home machine more than my work machine, because I've got much bigger disks there). But despite this, the filesystems on my work machine have lots of free space while the filesystems on my home machine tend to run perpetually relatively close to being out of space.

At one level, the difference is in how the disk space is managed. At work, I've migrated to ZFS on Linux; at home, everything is ext3 on top of LVM (on top of a software RAID mirror). But the real answer is that shrinking an extN filesystem and a LVM logical volume is kind of a pain, and also kind of dangerous (at least as far as I know). If I grew filesystems wildly at home, it'd be a pain to shrink them later if I needed the space elsewhere, so for the most part I only expand filesystems when I really need the space.

In theory this shouldn't make any difference; if I need the space, I'll grow the filesystem. In practice it makes me irrationally reluctant to do things that need substantial chunks of space temporarily. I would probably be better off if I adopted a policy that all of the filesystems I used actively should have, say, 40 GB of free space more or less at all times, but I'm not that sensible.

(There's some irrational bit of me that still thinks that disk space is in short supply. It's not; I have more than a TB free, and that's after extravagantly using space to store more or less every photograph I've ever taken. In RAW format, no less.)

This doesn't happen at work because ZFS dynamically shares the free pool space between all of the filesystems. Unless you go out of your way to set it up otherwise, there is no filesystem specific free space, just general pool free space that is claimed (and then released) as you use space in filesystems. Filesystems are just an organizational thing, not something that forces up-front space allocation. So I can use however much space I want, wherever I want to, and the only time I'll run out of space in a filesystem is if I'm genuinely out of all disk space.

This is a really nice feature of ZFS, and I wish I had it at home. It would clearly make my life easier by entirely removing one current concern; I just wouldn't have to manage space on a per-filesystem basis any more. Space would just be space.

(Someday I will have this at home, by migrating my home system to ZFS. But that probably won't be for a while for various reasons. Not having ZFS at home is still tolerable, so I suspect that I won't migrate until I'm migrating hardware anyways and that probably won't be for a while for various reasons.)

PS: btrfs is not what I consider a viable option. At this point I'd probably only consider btrfs once a major Linux distribution has made it their default filesystem for new installs and has survived at least a year of that choice without problems. And I'm not holding my breath for that.

Sidebar: Why I believe shrinking a LVM logical volume is dangerous

To grow a filesystem inside a LVM volume, you first grow the volume and then grow the filesystem to use the new space. To shrink a volume, you do this in reverse; you first shrink the filesystem and then shrink the volume. However, as far as I know there is nothing in LVM that prevents you from accidentally shrinking the volume so that it is smaller than the filesystem. Doing this by accident will truncate the end of your filesystem, almost definitely lose some of your data, and quite probably destroy the filesystem outright. Hence the danger of doing this.

It would be really great if LVM knew about common filesystem types, could read their superblocks to determine the FS-level size, and by default would refuse to shrink a logical volume below that size. But as far as I know it doesn't.

(As a practical matter I probably never want to shrink a filesystem without a current backup of it, which brings up the awkward subject of my home backup strategy.)

Written on 11 January 2016.
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Last modified: Mon Jan 11 00:48:16 2016
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