Re: [sqlite] synchronization issue: no directory sync after unlink

[Florent Bayendrian]

The header is overwritten with some zeros only if I change the
journal_mode to PERSIST :
http://www.sqlite.org/pragma.html#pragma_journal_mode

By default, the journal_mode is set to DELETE so the journal file is
valid if the file is present after an unlink + power shutdown.
(I have checked with strace and with an exit just before the unlink)

BR,

Florent


On Feb 23, 5:34 pm, Pavel Ivanov  wrote:
> AFAIK, before deleting journal file SQLite writes some zeroed header
> into it. This kind of header means that transaction is finished. And
> even if after power shutdown SQLite finds this journal persisting on
> disk it will see the header and will understand that this journal just
> needs to be deleted. So what important here is sync after writing
> zeroed header, not after deletion. Directory sync is important only
> when journal is created, so that it doesn't suddenly disappear when
> power shuts off. You can check in strace whether what I said above is
> actually true.
>
> Pavel
>
>
>
>
>
>
>
>
>
> On Thu, Feb 23, 2012 at 6:02 AM,FlorentBayendrian  wrote:
> > Hi,
>
> > I have a synchronization issue on an embedded device running Linux : if a
> > power shutdown is done just after a commit, sqlite will restore the
> > database to the previous state using the journal file. At the end of a
> > transaction the unlink of the journal file is not enough, the journal could
> > physically persist on the file system several seconds after the call to
> > unlink and only a sync on the directory could guarantee that the journal
> > file has been physically deleted. You can strace sqlite3 tool to see that
> > the unlink of the journal file is not followed by any sync.
>
> > The fix is simple, sqlite3OsDelete should be called with the last parameter
> > (dirSync) set to 1. This fix is necessary to be compliant with the last
> > property of an ACID database.
>
> > Regards,
>
> >Florent
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>
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[sqlite] synchronization issue: no directory sync after unlink

[Florent Bayendrian]

Hi,

I have a synchronization issue on an embedded device running Linux : if a
power shutdown is done just after a commit, sqlite will restore the
database to the previous state using the journal file. At the end of a
transaction the unlink of the journal file is not enough, the journal could
physically persist on the file system several seconds after the call to
unlink and only a sync on the directory could guarantee that the journal
file has been physically deleted. You can strace sqlite3 tool to see that
the unlink of the journal file is not followed by any sync.

The fix is simple, sqlite3OsDelete should be called with the last parameter
(dirSync) set to 1. This fix is necessary to be compliant with the last
property of an ACID database.

Regards,

Florent
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