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Is it possible to split a filesystem between multiple drives? Like if one fills up, it starts using the other drive. But it should appear as only one big directory. For example, ext4 has a max volume limit of 1EiB, do they really think we are going to be getting 1EiB or bigger drives?

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You can't split a filesystem between multiple block devices (i.e. disks or partitions), but you can use technologies like RAID or LVM to combine multiple block devices into a larger virtual block device that can be the basis for a large filesystem. RAID in particular is commonly used to build very large disk arrays — think racks and racks of hard drives all working together. But to the filesystem it just looks like one very large disk.

Exception: ZFS and btrfs have built-in multi-disk functionality similar to RAID or LVM. These filesytems directly support spanning multiple disks. But ZFS isn't well-supported on Linux (license incompatibilities prevent it from being being integrated into the official kernel), and btrfs is a work-in-progress that may not be ready for production use yet.

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  • Excellent coverage. One thing you may wish to add is that if ANY of the drives fail, you'll no longer be able to access the entire filesystem.
    – ChrisInEdmonton
    Feb 24, 2015 at 16:07
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    Unless you're using RAID, other than RAID0. Higher RAID "levels" incorporate redundancy, reducing the available storage space in exchange for resilience against disk failure. RAID1 stores two copies of everything (so you need twice as many disks as your desired capacity), and RAID5 gives you n-1 disks worth of capacity and can withstand any single disk failing.
    – Wyzard
    Feb 24, 2015 at 16:11
  • ZFS on Linux has been supporting kernel mode for a while and has a PPA that allows easy install in many distros.
    – Alex
    Feb 24, 2015 at 17:37
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mhddfs should do the trick - does a fairly naive load balance (prefers disks in order, unless less free space is available than a set limit - then it picks the one with the most space), works across multiple disks and filesystems, and if you lose a drive, you only lose the data on it.

The moment you have volumes that big though... you have bigger issues.

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