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Re: [WebSQLDatabase] Adding a vacuum() call

From: Shane Harrelson <shane@sqlite.org>
Date: Fri, 12 Mar 2010 10:55:49 -0500
Message-ID: <969b6a981003120755n60886d8cs19f386b45256a0f2@mail.gmail.com>
To: public-webapps <public-webapps@w3.org>
Some things to consider:

(1) VACUUM is never "necessary", though can sometimes reduce the size of the
database file and provide a performance improvement if the database is large
and fragmented.

(2) Use PRAGMA freelist_count to estimate how much VACUUM will shrink a
database.

(3) VACUUM requires O(N) time where N is the size of the database file, and
can require up to 2N temporary space while processing.

Our recommendation is that the use of VACUUM be discouraged.   The
performance improvement only comes up with very large databases that are
highly fragmented.  Where space reclamation is an issue, AUTO_VACUUM would
be more appropriate.

-Shane


On Thu, Mar 11, 2010 at 7:33 PM, Dimitri Glazkov <dglazkov@chromium.org>wrote:

> I like the completion callback idea.
>
> Also like the notion of some sort of protection from "over-eager
> vacuum-calling syndrome".
>
> :DG<
>
> On Thu, Mar 11, 2010 at 4:20 PM, Michael Nordman <michaeln@google.com>
> wrote:
> > Instead of calling back on success only, maybe call back on completion
> > regardless of success or failure. This way the caller would know when the
> > potentially lengthy operation was done, regardless of the outcome.
> >
> > 2010/3/11 Dumitru Daniliuc <dumi@chromium.org>
> >>
> >> joao,
> >>
> >> it looks like we mostly agree on this feature, so i was wondering if we
> >> could formally agree on a spec. here's what i propose:
> >>
> >> 1. name: vacuum. to the best of my knowledge, all current
> WebSQLDatabases
> >> implementations use SQLite, and in SQLite the command is called VACUUM..
> so
> >> it seems to me that we might as well call the new function vacuum().
> what do
> >> you think?
> >>
> >> 2. spec: no need for an error callback.
> >>
> >> interface Database {
> >>   // the methods and properties currently in the spec
> >>   void vacuum(in optional SQLVoidCallback successCallback);
> >> };
> >>
> >> 3. what the call should do: the purpose of this call is to allow apps to
> >> vacuum/compact/defragment/clean up their databases whenever they see
> fit.. a
> >> call to vacuum() could take a considerable amount of time (especially on
> >> larger or highly fragmented databases); therefore, it is not recommended
> for
> >> web apps to call this method during periods of high activity.
> >>
> >> how to process a vacuum() call:
> >>
> >> if the UA does not support this call (mobile browsers?), jump to step 3.
> >> queue up a task to vacuum/compact/defragment/clean up the database.
> >> if the task succeeds, and a success callback is provided, queue up a
> task
> >> to invoke the success callback; in all other cases (the task failed, or
> no
> >> success callback was provided), do nothing: proceed to the next task in
> the
> >> queue.
> >>
> >> does this seem acceptable? we (google engineers interested in this) feel
> >> that UAs should either not implement the vacuum() call, or they should
> >> respect it (rather than taking it as a hint). it is ok for UAs to keep
> track
> >> of things like system idleness or databases closing to do more vacuuming
> >> that the apps asked for, if they want to. however, we feel that a
> vacuum()
> >> request by an app should not be postponed, simply because sometimes apps
> >> know better than UAs when the best time to vacuum is (it might be nice
> to
> >> give apps more information on how fragmented their databases are, but
> that's
> >> a separate discussion).
> >>
> >> thanks,
> >> dumi
> >>
> >>
> >> 2010/3/9 Jeremy Orlow <jorlow@google.com>
> >>>
> >>> On Mon, Mar 8, 2010 at 8:47 PM, Dumitru Daniliuc <dumi@google.com>
> wrote:
> >>>>
> >>>>
> >>>> On Mon, Mar 8, 2010 at 3:39 AM, Joćo Eiras <joaoe@opera.com> wrote:
> >>>>>
> >>>>>>> I don't see how the callbacks are useful though. Vacuum works
> >>>>>>> transparently, its effects are not visible, and what should the
> page
> >>>>>>> do in
> >>>>>>> case of error ?
> >>>>>>>
> >>>>>>
> >>>>>> i was thinking of something like:
> >>>>>>
> >>>>>> db.defragment(errorCallback, successCallback);
> >>>>>> showAPrettyImageAndAskTheUserToWait();
> >>>>>>
> >>>>>> function errorCallback(error) {}
> >>>>>> function successCallback() {
> >>>>>>  getRidOfThePrettyImageAndRestartTheApp();
> >>>>>> }
> >>>>>>
> >>>>>> just like you, i'm not sure if the error callback is useful at all,
> >>>>>> but i
> >>>>>> thought i'd add it to make the defragment() call look more like a
> >>>>>> transaction. maybe we don't need it.
> >>>>>>
> >>>>>
> >>>>> True, but this is a kind of operation that could very well just run
> on
> >>>>> the background, with a single optional callback when it's done (the
> webpage
> >>>>> can't do anything if an error is detected anyway).
> >>>>
> >>>> ok, so let's drop the errorCallback: vacuum([optional]
> successCallback);
> >>>>
> >>>>>
> >>>>> The user agent would need to queue any subsequent transactions if a
> >>>>> vacuum is running. I would consider it as an hint, and after all
> webpages
> >>>>> that own references to the underlying data files are closed, would do
> a
> >>>>> vacuum. So, if you have many tabs on gmail, and that a single gmail
> instance
> >>>>> tries to do multiple vacuums, it would equiv to one single vacuum
> operation.
> >>>>
> >>>> what do we do if some databases are opened for the entire life of the
> >>>> browser? for example, i open my browser which has myfavoriteapp.comset as
> >>>> its homepage. myfavoriteapp.com immediately opens a DB, and i only
> close
> >>>> that app when i close the browser. when would the browser vacuum
> >>>> myfavoriteapp's DBs in this case?
> >>>>
> >>>> i think it's ok for the UA to vacuum some DBs automatically when it
> >>>> thinks it's a good time to do so; however, if a platform supports the
> >>>> vacuum/defrag call (i.e. if it doesn't treat it is a no-op), then i
> think a
> >>>> vacuum call coming from the app should be immediately scheduled (yes,
> the
> >>>> subsequent transactions would have to wait for the vacuuming to finish
> >>>> running). in some cases, the apps know better than the UA when to
> vacuum
> >>>> their DBs.
> >>>>
> >>>> by the way, we should probably agree on a name for this call. which
> one
> >>>> do you prefer? vacuum, defragment, defrag, something else? i don't
> have a
> >>>> strong opinion.
> >>>
> >>> I think vacuum is fine since the spec is already tied to the SQLite SQL
> >>> dialect.
> >>> collectGarbage() is another possibility
> >>> Go with whatever you think is most clear and accurate though.
> >>> J
> >
> >
>
>
>
Received on Friday, 12 March 2010 16:07:25 GMT

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