S3QL does not verify TLS/SSL server certificates, so a man-in-the-middle attack is principally possible. See issue 267 for more details.
S3QL is rather slow when an application tries to write data in unreasonably small chunks. If a 1 MB file is copied in chunks of 1 KB, this will take more than 10 times as long as when it’s copied with the (recommended) chunk size of 128 KB.
This is a limitation of the FUSE library (which does not yet support write caching) which will hopefully be addressed in some future FUSE version.
Most applications, including e.g. GNU cp and rsync, use reasonably large buffers and are therefore not affected by this problem and perform very efficient on S3QL file systems.
However, if you encounter unexpectedly slow performance with a specific program, this might be due to the program using very small write buffers. Although this is not really a bug in the program, it might be worth to ask the program’s authors for help.
S3QL always updates file and directory access times as if the relatime mount option has been specified: the access time (“atime”) is only updated if it is currently earlier than either the status change time (“ctime”) or modification time (“mtime”).
S3QL directories always have an st_nlink value of 1. This may confuse programs that rely on directories having st_nlink values of (2 + number of sub directories).
Note that this is not a bug in S3QL. Including sub directories in the st_nlink value is a Unix convention, but by no means a requirement. If an application blindly relies on this convention being followed, then this is a bug in the application.
A prominent example are early versions of GNU find, which required the --noleaf option to work correctly on S3QL file systems. This bug has already been fixed in recent find versions.
The umount and fusermount -u commands will not block until all data has been uploaded to the backend. (this is a FUSE limitation that will hopefully be removed in the future, see issue 159). If you use either command to unmount an S3QL file system, you have to take care to explicitly wait for the mount.s3ql process to terminate before you shut down or restart the system. Therefore it is generally not a good idea to mount an S3QL file system in /etc/fstab (you should use a dedicated init script instead).
S3QL relies on the backends not to run out of space. This is a given for big storage providers like Amazon S3, but you may stumble upon this if you store buckets e.g. on smaller servers or servies.
If there is no space left in the backend, attempts to write more data into the S3QL file system will fail and the file system will be in an inconsistent state and require a file system check (and you should make sure to make space available in the backend before running the check).
Unfortunately, there is no way to handle insufficient space in the backend without leaving the file system inconsistent. Since S3QL first writes data into the cache, it can no longer return an error when it later turns out that the cache can not be committed to the backend.