Some of the caveats to consider when choosing Reiser4. It does not look like fun. Whatever increase in efficiency is claimed, it better be VERY good to overcome the potential issues.
To avoid having issues on laptops using TLP for power saving, it is recommended to disable the options for SATA Link power saving in /etc/default/tlp
LILO is the only boot loader officially supporting Reiser4, but it seems to have issues when /boot is formatted as Reiser4
Access Control Lists is not implemented and requires that journald either logs to a separate logging daemon or to tmpfs. Another workaround is to compile systemd by source without ACL support, but is not recommended.
You will need a Reiser4 patched kernel.
Backing up your boot loader configuration file should be considered if one is going to have /boot residing on a Reiser4 volume.
It is recommended that you make a small (20-200mb) partition for /boot with a filesystem other than Reiser4, and then copy your /boot folder to the partition. Update your boot loader configuration accordingly.
If you do not use EFI and wish to put everything including /boot on a Reiser4 partition (not recommended) you will need to use LILO. This is not advised, as you will probably encounter an error when trying to update lilo.conf.
Since Reiser4 supports different transaction models optimized for different types of storage media (SSDs, HDDs), the options used while formatting and mounting will differ.
Depending on what transaction model one wish to use which are optimized for different types of storage media, the mount option txmod=wa (for SSDs), txmod=journal (for HDDs) must be defined when mounting the partitions through the -o switch. The default is txmod=hybrid which heuristically alternates between the “wa” (write-anywhere) and “journal” models for optimized performance on rotating disks while trying to avoid excess fragmentation at the same time.