>>>>> "ocie" == ocie <ocie@paracel.com> writes:
> Bruce Momjian wrote:
>> > > Hi, > > perhaps a stupid question: > > What will happen,
>> if you start PostgreSQL with -F and write a script > which is a
>> loop that fsyncs every 2 seconds? Does this lead to a >
>> database which is "almost" correct? Does this seem like a good
>> > compromise?
>>
>> Well, you really have to sync the data pages BEFORE sync'ing
>> pg_log.
> Why should this be necessary? If the transaction is considered
> committed once the log has be written, and the system crashes
> before the data are written, then postgres can look at the data
> and logs when it is next started up and apply all the
> transactions that were logged but not committed to the data
> pages.
> Am I missing something? It seems to me if you sync the data
> pages first, then what good is the log? (other than being able
> to retrace your steps)
I do not think that pg_log is used like a normal 'log' device in other
databases. My quick look at the code looks like pg_log only has a
list of transactions and not the actual data blocks. Notice that
TransRecover is commented out in backent/access/transam/transam.c.
Most database log has the before images and after images of any page
that has been modified in a transaction followed by commit/abort
record. This allows for only this file to have to be synced. The
rest of the database can float (generally checkpoints are done every
so often to reduce recover time). The method of recovering from a
crash is to replay the log from the last checkpoint until the end of
the log by applying the before/after images (as needed based on
weather the transaction commited) to the actual database relations.
I would appreciate anyone correcting any mistakes in my understanding
of how postgres works.
> Ocie Mitchell
Kent S. Gordon
Architect
iNetSpace Co.
voice: (972)851-3494 fax:(972)702-0384 e-mail:kgor@inetspace.com
