Re: [sqlite] light weight write barriers

[Nico Williams]

On Wed, Oct 10, 2012 at 12:48 PM, Richard Hipp <drh@xxxxxxxxxx> wrote:
>> Could you list the requirements of such a light weight barrier?
>> i.e. what would it need to do minimally, what's different from
>> fsync/fdatasync ?
>
> For SQLite, the write barrier needs to involve two separate inodes.  The
> requirement is this:

...

> Note also that when fsync() works as advertised, SQLite transactions are
> ACID.  But when fsync() is reduced to a write-barrier, we loss the D
> (durable) and transactions are only ACI.  In our experience, nobody really
> cares very much about durable across a power-loss.  People are mainly
> interested in Atomic, Consistent, and Isolated.  If you take a power loss
> and then after reboot you find the 10 seconds of work prior to the power
> loss is missing, nobody much cares about that as long as all of the prior
> work is still present and consistent.

There is something you can do: use a combination of COW on-disk
formats in such a way that it's possible to detect partially-committed
transactions and rollback to the last good known root, and
backgrounded fsync()s (i.e., in a separate thread, without waiting for
the fsync() to complete).

Nico
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