On Wed, Jan 15, 2014 at 10:35:44AM +0100, Jan Kara wrote:
> Filesystems could in theory provide facility like atomic write (at least up
> to a certain size say in MB range) but it's not so easy and when there are
> no strong usecases fs people are reluctant to make their code more complex
> unnecessarily. OTOH without widespread atomic write support I understand
> application developers have similar stance. So it's kind of chicken and egg
> problem. BTW, e.g. ext3/4 has quite a bit of the infrastructure in place
> due to its data=journal mode so if someone on the PostgreSQL side wanted to
> research on this, knitting some experimental ext4 patches should be doable.
For the record, a researcher (plus is PhD student) at HP Labs actually
implemented a prototype based on ext3 which created an atomic write
facility. It was good up to about 25% of the ext4 journal size (so, a
couple of MB), and it was use to research using persistent memory by
creating a persistent heap using standard in-memory data structures as
a replacement for using a database.
The results of their research work was that showed that ext3 plus
atomic write plus standard Java associative arrays beat using Sqllite.
It was a research prototype, so they didn't handle OOM kill
conditions, and they also didn't try benchmarking against a real
database instead of a toy database such as SqlLite, but if someone
wants to experiment with Atomic write, there are patches against ext3
that we can probably get from HP Labs.
- Ted
