shmem limits and redolog

Hi,

I'm currently hacking around on a solution for logging all
database operations at query level that can recover a crashed
database from the last successful backup by redoing all the
commands.

Well, I wanted it to be as flexible as can. So I decided to
make it per database configurable. One could say which
databases are logged and if a database is, if it is logged
sync or async (in sync mode, every COMMIT forces an fsync of
the actual logfile and controlfiles).

To make async mode as fast as can, I'm using a shared memory
of 32K per database (not per backend) that is used as a wrap
around buffer from the backends to place their query
information. So the log writer can fall a little behind if
there are many backends doing different things that don't
lock each other.

Now I'm a little in doubt about the shared memory limits
reported. Was it a good decision to use shared memory? Am I
better off using socket's?

The bad thing in what I have up to now (it's far from
complete) is, that even if a database isn't currently logged,
a redolog writer is started and creates the 32K shmem segment
(plus a semaphore set with 5 semaphores). This is because I
plan to create commands like

ALTER DATABASE LOG MODE=ASYNC LOGDIR='/somewhere/dbname';

and the like that can be used at runtime (while more than one
backend is connected to the database) to turn logging on/off,
switch to/from backup mode (all other activity is stopped)
etc.

So every 32 databases will require another megabyte of shared
memory. The logging master controls which databases have
activity and kills redolog writers after some time of
inactivity, and the shmem is freed then. But it can hurt if
someone really has many many databases that are all used at
the same time.

What do the others say?

Jan

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