Isn't that kind limited? How exactly are two different clients going to access the metadata at the same time, if SQLite3 is used?!

I don't think that's the aim of the project. They do state that it's "especially suitable for online backup and archival". We typically have one network location where we store data for backup and archival, so only one machine would be having a connection anyway.

SQLite supports multiple readers when Write Ahead Logging (WAL) is enabled. This mode is so good and transforms the whole operation of SQLite that it is shame that 1) it isn't the default, 2) more people don't know about it.

There is also `PRAGMA synchronous = NORMAL` that helps a lot. Your database will still be consistent and cannot become corrupted, although you do have to give up transaction durabibility after a power loss:

https://www.sqlite.org/pragma.html#pragma_synchronous

The way s3ql works is that it copies the db from s3 to your local machine. You make changes and then write the new version back to s3 later with the data. So it won't help in this case.

I see. Out of interest how does it write back the new version of the SQLite database file with only the differences? Does S3 support binary patching?

I believe it uploads the whole thing each time. I think it may even upload it with a new index counter in the name to version it (but I can't find that in the docs now).

http://www.rath.org/s3ql-docs/impl_details.html#metadata-sto...

I think you found the Achilles heel. This won't scale, so it is only suitable for small file systems.

Indeed, I'm good at that ;)

It could probably be improved (subject to the nuances of S3 which I'm not fully familiar with). One way to fix it would be to copy the concept of SQLite's WAL mode. Use an appended write operation on S3 (if it supports it) to append to an existing file that contains the transaction log. Then at certain intervals (say every few thousand transactions) one can finally flush that log to be stored in the main database file.

This would substantially reduce the number of times the database would need to be re-uploaded in full.

They won't: https://bitbucket.org/nikratio/s3ql/wiki/FAQ#!can-i-access-a...

"""In principle, both VPN and NFS/CIFS-alike functionality could be integrated into S3QL to allow simultaneous mounting using mount.s3ql directly. However, consensus among the S3QL developers is that this is not worth the increased complexity."""