[cm3]

https://www.backblaze.com/pod.html says ext4.

https://www.backblaze.com/blog/vault-cloud-storage-architect...

    Each of the drives in a Vault has a standard Linux file system,
    ext4, on it. This is where the shards are stored. There are
    fancier file systems out there, but we don’t need them for
    Vaults. All that is needed is a way to write files to disk, and
    read them back. Ext4 is good at handling power failure on a
    single drive cleanly, without losing any files. It’s also good at
    storing lots of files on a single drive, and providing efficient
    access to them.

They have since migrated to ext4, but judging by their job description they might still have JFS in use. https://www.backblaze.com/blog/seeking-data-protectors/

    Diagnose and repair file system problems with JFS and ext4 and debug
    other system problems

https://www.backblaze.com/blog/petabytes-on-a-budget-v2-0rev...

    We upgraded the Linux 64-bit OS from Debian 4 to Debian 5, but we
    no longer use JFS as the file system. We selected JFS years ago
    for its ability to accommodate large volumes and low CPU usage,
    and it worked well. However, ext4 has since matured in both
    reliability and performance, and we realized that with a little
    additional effort we could get all the benefits and live within
    the unfortunate 16 terabyte volume limitation of ext4. One of the
    required changes to work around ext4’s constraints was to add
    LVM (Logical Volume Manager) above the RAID 6 but below the file
    system. In our particular application (which features more writes
    than reads), ext4’s performance was a clear winner over ext3,
    JFS, and XFS.