Re: [Firebird-devel] Firebird Transaction ID limit solution

[Ann H. <aha...@ib...>]

On Mon, Jan 2, 2012 at 2:32 PM, Yi Lu <yl...@ra...> wrote:
> Approach 1 seems to be least risky. Disk space should not be a big issue with
> today's hardware.
>

The problem is not disk space, but locality of reference.  With small
records, adding
four bytes could decrease the number of rows per page by 10-15%, leading to more
disk I/O.  That's a significant cost to every database which can be
avoided by using
a slightly more complicated variable length transaction id or a flag
that indicates which
size is used for a particular record.  Firebird already checks the
flags to determine
whether a record is fragmented, so the extra check adds negligible overhead

And, as an aside, sweeping is not read-only now.  It's purpose is to
remove unneeded
old versions of records from the database.  The actual work may be
done by the garbage
collect thread, but the I/O is there.

Good luck,

Ann