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IDS Forum

Re: Changing the Logging Mode

Posted By: Art Kagel
Date: Friday, 27 June 2008, at 1:57 a.m.

In Response To: Re: Changing the Logging Mode (VIKAS HIVARKAR)

No, transaction rollback is performed on using the logical log. The
physical log is used ONLY at engine startup to restore the server to a known
state before logical log are processed. At the beginning of startup the
engine checks the contents of the physical log. If the server was shutdown
normally, then there will have been a checkpoint taken immediately after all
users were disconnected from the server which will have emptied the physical
log (see below). So, if the physical log is not empty, that means that the
engine crashed or the machine crashed out from under it, or for some other
reason a normal shutdown was not possible. If this happens the pre-image
pages contained in the physical log are slapped back onto disk to create a
clean-slate for the logical log rollforward and rollback operations that
constitute the remainder of fast recovery restoring the disk to its state as
of the last known checkpoint. So, yes the physical log is written back to
disk, but it really has nothing to do with the rollback of individual
transactions. That is done by/using the logical logs. The log records
since the last checkpoint are rolled forward (ie replayed) until the end of
the last logical log. When the end is reached any open transactions at the
time of the checkpoint (and those begun since the checkpoint) which were not
committed during the rollforward are then rolled back. Then a new
checkpoint record is written noting that the server is in a consistent state
and that all of these changes have been committed to disk, and the engine
moves to an operational state.

On the other comment, no, the physical log is ONLY needed until the
checkpoint guarantees that all logical log records are safely on disk along
with the checkpoint record. The physical log is cleared at the end of every
checkpoint. HOWEVER, the physical log cannot be allowed to fill up between
checkpoints. This means that if the physical log reaches 75% full the
engine will force a checkpoint immediately - even if one isn't scheduled
yet. If the physical log reaches 100%, known as a physical log overflow,
all activity on the server is blocked until the checkpoint completes. This
almost never happens, it would require an extremely small physical log and a
very active highly volatile server.

On Fri, Jun 27, 2008 at 1:42 AM, VIKAS HIVARKAR <vikas.hivarkar@tcs.com>
wrote:

> Oops! i got the point, in case of Roll back of the transaction it will read
> from the physdbs and bring the database to a point when the transaction was
> started.
>
> so does that means if one single transaction (with out commit) spans over
> the
> size of physdbs then the transaction will give error and roll back.
>
> Mr Kagel Pls correct me if i am worng here.
>
> Regards,
> vikas
>
>
>
> *******************************************************************************
> Forum Note: Use "Reply" to post a response in the discussion forum.
>
>

--
Art S. Kagel
Oninit (www.oninit.com)
IIUG Board of Directors (art@iiug.org)

Disclaimer: Please keep in mind that my own opinions are my own opinions and
do not reflect on my employer, Oninit, the IIUG, nor any other organization
with which I am associated either explicitly or implicitly. Neither do those
opinions reflect those of other individuals affiliated with any entity with
which I am affiliated nor those of the entities themselves.

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