Known Issues — S3QL 5.2.3 documentation

# Known Issues

- S3QL de-duplicates data blocks based solely only on SHA256 checksums, without doing a byte-by-byte comparison of the data. Since it is possible for two data blocks to have the same checksum despite having different contents, this can lead to problems. If two such blocks are stored in an S3QL file system, the data in one block will be lost and replaced by the data in the other block. However, the chances of this occurring for any two blocks are about 1 in 10^77 (2^256). For a file system that holds a total of 10^34 blocks, the chances of a collision increase to about 1 in 10^9. Storing more than 10^34 blocks (or about 10^25 TB with an (extremely small) block size of 4 kB) is therefore not recommended. Being exceptionally unlucky may also be a disadvantage.

- S3QL does not support Access Control Lists (ACLs). There is nothing fundamentally that prevents this, someone just has to write the necessary code.

- As of Linux kernel 3.5 S3QL file systems do not implement the “write protect” bit on directories. In other words, even if a directory has the write protect bit set, the owner of the directory can delete any files and (empty) subdirectories inside it. This is a bug in the FUSE kernel module (cf. https://github.com/libfuse/libfuse/issues/23) and needs to be fixed in the kernel. Unfortunately it does not look as if this is going to be fixed anytime soon (as of 2016/2/28).

- 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, cf. https://github.com/libfuse/libfuse/issues/1). 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 or Google Storage, but you may stumble upon this if you use your own server or smaller providers.

  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.