RAID expansion comes to OpenZFS at last
Don't get too excited, filesystem fans – there are limits
ZFS has been around for 16 years and has a solid reputation, but it does have limitations compared to its rivals. One of these is in the process of being lifted: soon it will be possible to add space to an existing ZFS array. As long as you're using FreeBSD, anyway.
Filesystems are one of the core differentiators between enterprise Linux distributions (and other Unix and Unix-like OSes). That means, like anything in the Unix world, there's "advocacy" – in other words, bitter squabbling about which is best or worst.
ZFS was originally developed by Sun for Solaris. At one point, Apple was going to adopt it, but changed its mind. Now, of course, Oracle owns it – but there is OpenZFS, and a very small chance that the situation might change.
You can use OpenZFS on Linux; the problem is building it into the kernel because Sun's Common Development and Distribution Licence is incompatible with Linux's GPL2. Despite this, Ubuntu added ZFS support in 2016 and enabled ZFS on root in 2019.
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Btrfs is not so encumbered. Also, it's quite straightforward to add new devices to a Btrfs filesystem, or resize one. This has led several commentators to praise Btrfs's flexibility. Naturally, others have long claimed the reverse.
There are still significant caveats with the new online expansion functionality, but even so, this new ability is useful and counters one criticism of OpenZFS. ®