On Fri, 2015-12-11 at 16:23 +0000, Ian Jackson wrote: > The purpose of these locks is partly to prevent transactions being > aborted (which I'm not sure the existing code would in practice cope > with, although this is a bug) and also to avoid bugs due to the fact > that > SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; > not mean that the transactions are necessarily serialisable!
^does not? > http://www.postgresql.org/docs/8.3/static/transaction-iso.html > > In SQL in general it is possible for read-only transactions to > conflict with writing transactions. > > However, in PostgreSQL this is not a problem because Postgres uses > multi-version concurrency control: it retains the old version of the > data while the read transaction is open: > http://www.postgresql.org/docs/8.3/static/transaction-iso.html > > So a read transaction cannot cause a write transaction to abort, nor > vice versa. So there is no need to have the database explicit table > locks prevent concurrent read access. > > Preventing concurrent read access means that simple and urgent updates > can be unnecessarily delayed by long-running reader transactions in > the history reporters and archaelogists. archaeologists > So, reduce the lock mode from ACCESS EXCLUSIVE to ACCESS. This still > conflicts with all kinds of updates and prospective updates, but no > longer with SELECT: > http://www.postgresql.org/docs/8.3/static/explicit-locking.html > > Signed-off-by: Ian Jackson <ian.jack...@eu.citrix.com> Acked-by: Ian Campbell <ian.campb...@citrix.com> > --- > Osstest/JobDB/Executive.pm | 2 +- > tcl/JobDB-Executive.tcl | 2 +- > 2 files changed, 2 insertions(+), 2 deletions(-) > > diff --git a/Osstest/JobDB/Executive.pm b/Osstest/JobDB/Executive.pm > index 69cb276..124e7c0 100644 > --- a/Osstest/JobDB/Executive.pm > +++ b/Osstest/JobDB/Executive.pm > @@ -43,7 +43,7 @@ sub begin_work ($$$) { #method > > return if $ENV{'OSSTEST_DEBUG_NOSQLLOCK'}; > foreach my $tab (@$tables) { > - $dbh->do("LOCK TABLE $tab IN ACCESS EXCLUSIVE MODE"); > + $dbh->do("LOCK TABLE $tab IN EXCLUSIVE MODE"); > } > } > > diff --git a/tcl/JobDB-Executive.tcl b/tcl/JobDB-Executive.tcl > index f37bbaf..a3dbb9e 100644 > --- a/tcl/JobDB-Executive.tcl > +++ b/tcl/JobDB-Executive.tcl > @@ -133,7 +133,7 @@ proc lock-tables {tables} { > # must be inside transaction > foreach tab $tables { > db-execute " > - LOCK TABLE $tab IN ACCESS EXCLUSIVE MODE > + LOCK TABLE $tab IN EXCLUSIVE MODE > " > } > } _______________________________________________ Xen-devel mailing list Xen-devel@lists.xen.org http://lists.xen.org/xen-devel
