Re: implementing a database log

On Apr 24, 10:11 pm, "Brian Selzer" <br..._at_selzer-software.com> wrote:
> "David BL" <davi..._at_iinet.net.au> wrote in message
>
> news:fd63f466-18f7-4986-b378-f5e9f512bbd8_at_r66g2000hsg.googlegroups.com...
>
>
>
>
>
> > On Apr 22, 11:39 pm, "Brian Selzer" <br..._at_selzer-software.com> wrote:
> >> "David BL" <davi..._at_iinet.net.au> wrote in message
>
> >>news:265621e9-25fd-46d5-9e2d-7a4f63fa84b4_at_m3g2000hsc.googlegroups.com...
>
> >> > On Apr 22, 6:58 am, "Brian Selzer" <br..._at_selzer-software.com> wrote:
> >> >> "Christoph Rupp" <cruppst..._at_gmail.com> wrote in message
>
> >> >>news:91140c56-1f05-4b5d-b45f-b34920db2051_at_x41g2000hsb.googlegroups.com...
>
> >> >> > Brian,
>
> >> >> > On Apr 21, 11:00 pm, "Brian Selzer" <br..._at_selzer-software.com>
> >> >> > wrote:
> >> >> >> Why not go with #4:
>
> >> >> >> 4. a physical log based on modified rows. Whenever a row is
> >> >> >> modified,
> >> >> >> added
> >> >> >> or removed, it is logged. Then you could also implement row
> >> >> >> versioning--just add a row version field to the physical rows. I
> >> >> >> believe
> >> >> >> that this what snapshot isolation is built on.
>
> >> >> > It's not an SQL database, i don't even have the notion of "rows",
> >> >> > but
> >> >> > basically i think your #4 is the same as my #1 or #2.
>
> >> >> No, it isn't. #1 requires the logging of additional records that may
> >> >> not
> >> >> have been affected by an update. #2 doesn't log the entire changed
> >> >> record,
> >> >> but only bits and pieces. I would think that limiting the units of
> >> >> change
> >> >> to individual records--entire records--would simplify the process of
> >> >> marking
> >> >> and isolating units of work while at the same time guaranteeing
> >> >> consistency.
>
> >> > I don't think an atomic unit of work is always associated with a
> >> > change to an individual record. Are you suggesting transactions to
> >> > define arbitrarily large units of work aren't needed?
>
> >> No, that's not what I'm suggesting. What I'm suggesting is that the
> >> atomic
> >> unit of work should be a /set/ of /records/--either the old records in
> >> the
> >> case of a before image or the new records in the case of an after image.
>
> > Ok but that sounds like the system snapshots an entire table in the
> > before/after images.
>
> > For efficiency one would expect to only store the set of records that
> > have been added and the set that have been removed by a given
> > transaction. It is easy to see how we get the inverse which is
> > required for roll back of an uncommitted transaction during recovery.
> > However these logical operations aren't idempotent (at least for bags
> > of records). How does recovery deal with non-idempotent redo()/
> > undo() changes in the log?
>
> Why would there be bags of records? At the physical level, each record has
> a specific offset in some file, and that offset would uniquely identify it.
> Why would you strip off that identification? Consequently, there wouldn't
> be bags of records, only sets of records.

One could say the log records are representing physical changes if they record the physical locations.

> If you start out with a set of records and you know what is to be inserted,
> updated, and deleted, you can compute the resulting set of records. If you
> start out with the resulting set of records, and you know what was inserted,
> updated and deleted in order to arrive at that result, then you can compute
> the original set of records. For simplicity, even if it isn't necessarily
> the most efficient, what is updated could be implemented as a set of ordered
> pairs of records, tying each original record to its replacement. So the log
> would consist of a sequence of triples (D, U, I) separated by transaction
> markers where D is a set of records that were deleted, U is a set of pairs
> of records that were updated, and I is a set of records that were inserted.
>
> Now, provided that the log is written before the database--that is, (1)
> write the triple to the log, (2) write the database, (3) write the
> transaction marker in the log--, it should be possible to determine whether
> or not what was written to the log actually made it into the database, and
> thus it should be possible to roll back any uncommitted transaction.

A modern DBMS using WAL will tend to use a "lazy writer" that writes dirty pages to disk in the background. For performance this will tend to write dirty pages in a physical order so the disk head moves uniformly over the platter. This assumes the only constraint between the writing to the log and the dirty pages is the WAL constraint - ie dirty pages must be written to disk strictly after the associated log records have been written to disk. To meet this constraint the lazy writer simply needs to ignore dirty pages that haven't (yet) been logged.

Your suggestion brings far more onerous constraints on the disk writing policies. In fact to make a transaction durable it seems necessary to write all the dirty pages to disk as part of the transaction. This is certainly not normally the case in a modern DBMS. A database that avoids a "force" policy requires REDO during recovery.

Often a DBMS that supports long running transactions will allow dirty pages to be written to disk even though the associated transaction hasn't yet committed. This is the so called "steal" policy and leads to the need for UNDO during recovery.

AFAIK most databases have very flexible disk writing policies (steal/ no-force) and require both UNDO and REDO during recovery. See the ARIES algorithm. Received on Mon Apr 28 2008 - 04:50:41 CEST