HOLD ON BETA2 - Mailing list pgsql-hackers

[ Subject changed.]

Marc, please hold on announcing beta2 until we get this resolved.  Thanks.

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Tom Lane wrote:
> I think I've identified a primary cause for the "no such pg_clog file"
> problem that we've seen reported several times.
> 
> A look at htup.h shows that the WAL only stores the low 8 bits of a
> tuple's t_infomask (see xl_heap_header struct).  There is some fooling
> around in heapam.c's WAL redo routines to try to reconstitute some of
> the high-order bits, for example this:
> 
>             htup->t_infomask = HEAP_XMAX_INVALID | xlhdr.mask;
> 
> But this is implicitly assuming that we can reconstruct the
> XMIN_COMMITTED bit at will.  That was true when the WAL code was
> written, but with 7.2's ability to recycle allegedly-no-longer-needed
> pg_clog data, we cannot simply drop commit status bits.
> 
> The only scenario I've been able to identify in which this actually
> causes a failure is when VACUUM FULL moves an old tuple and then shortly
> afterwards (before the next checkpoint) there is a crash.  Post-crash,
> the tuple move will be redone from WAL, and the moved tuple will be
> inserted with zeroed-out commit status bits.  When we next examine the
> tuple, we have to try to retrieve its commit status from pg_clog ... but
> it's not there anymore.
> 
> As far as I can see, the only realistic solution is to store the full 16
> bits of t_infomask in the WAL.  We could do this without increasing WAL
> size by dropping the t_hoff field from xl_heap_header --- t_hoff is
> computable given the number of attributes and the HASNULL/HASOID bits,
> both of which are available.  (Actually, we could save some space now
> by getting rid of t_oid in xl_heap_header; it's not necessary given that
> OID isn't in the fixed tuple header anymore.)
> 
> This will require a WAL format change of course.  Fortunately we can do
> that without forcing a complete initdb (people will have to run
> pg_resetxlog if they want to update a 7.3beta2 database without initdb).
> 
> I see no way to fix the problem in the context of 7.2.  Perhaps we
> should put out a bulletin warning people to avoid VACUUM FULL in 7.2,
> or at least to do CHECKPOINT as soon as possible after one.
> 
>             regards, tom lane
> 
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