Re: including backend ID in relpath of temp rels - updated patch

On Thu, Jun 10, 2010 at 4:39 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> For previous discussion of this topic, see:
>
> http://archives.postgresql.org/pgsql-hackers/2010-04/msg01181.php
> http://archives.postgresql.org/pgsql-hackers/2010-05/msg00352.php
> http://archives.postgresql.org/pgsql-hackers/2010-06/msg00302.php
>
> As in the original version of the patch, I have not simply added
> backend ID to RelFileNode, because there are too many places using
> RelFileNode in contexts where the backend ID can be determined from
> context, such as the shared and local buffer managers and the xlog
> code.  Instead, I have introduced BackendRelFileNode for contexts
> where we need both the RelFileNode and the backend ID.  The smgr layer
> has to use BackendRelFileNode across the board, since it operates on
> both permanent and temporary relation, including - potentially -
> temporary relations of other backends.  smgr invalidations must also
> include the backend ID, as must communication between regular backends
> and the bgwriter.  The relcache now stores rd_backend instead of
> rd_islocaltemp so that  it remains straightforward to call smgropen()
> based on a relcache entry. Some smgr functions no longer require an
> isTemp argument, because they can infer the necessary information from
> their BackendRelFileNode.  smgrwrite() and smgrextend() now take a
> skipFsync argument rather than an isTemp argument.
>
> In this version of the patch, I've improved the temporary file cleanup
> mechanism.  In CVS HEAD, when a transaction commits or aborts, we
> write an XLOG record with all relations that must be unlinked,
> including temporary relations.  With this patch, we no longer include
> temporary relations in the XLOG records (which may be a tiny
> performance benefit for some people); instead, on every startup of the
> database cluster, we just nuke all temporary relation files (which is
> now easy to do, since they are named differently than files for
> non-temporary relations) at the same time that we nuke other temp
> files.  This produces slightly different behavior.  In CVS HEAD,
> temporary files get removed whenever an xlog redo happens, so either
> at cluster start or after a backend crash, but only to the extent that
> they appear in WAL.  With this patch, we can be sure of removing ALL
> stray files, which is maybe ever-so-slightly leaky in CVS HEAD.  But
> on the other hand, because it hooks into the existing temporary file
> cleanup code, it only happens at cluster startup, NOT after a backend
> crash.  The existing coding leaves temporary sort files and similar
> around after a backend crash for forensics purposes.  That might or
> might not be worth rethinking for non-debug builds, but I don't think
> there's any very good reason to think that temporary relation files
> need to be handled differently than temporary work files.
>
> I believe that this patch will clear away one major obstacle to
> implementing global temporary and unlogged tables: it enables us to be
> sure of cleaning up properly after a crash without relying on catalog
> entries or XLOG.  Based on previous discussions, however, I believe
> there is support for making this change independently of what becomes
> of that project, just for the benefit of having a more robust cleanup
> mechanism.

Updated patch to remove minor bitrot.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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