There's a project policy against using plain "char buf[BLCKSZ]" local
or static variables as page buffers; preferred style is to palloc or
malloc each buffer to ensure it is MAXALIGN'd. However, that policy's
been ignored in an increasing number of places. We've apparently got
away with it so far, probably because (a) relatively few people use
platforms on which misalignment causes core dumps and/or (b) the
variables chance to be sufficiently aligned anyway. But this is not
something to rely on. Moreover, even if we don't get a core dump,
we might be paying a lot of cycles for misaligned accesses.
To fix, invent new union types PGAlignedBlock and PGAlignedXLogBlock
that the compiler must allocate with sufficient alignment, and use
those in place of plain char arrays.
I used these types even for variables where there's no risk of a
misaligned access, since ensuring proper alignment should make
kernel data transfers faster. I also changed some places where
we had been palloc'ing short-lived buffers, for coding style
uniformity and to save palloc/pfree overhead.
Since this seems to be a live portability hazard (despite the lack
of field reports), back-patch to all supported versions.
Patch by me; thanks to Michael Paquier for review.
Discussion: https://postgr.es/m/1535618100.1286.3.camel@credativ.de
When the new GUC wal_consistency_checking is set to a non-empty value,
it triggers recording of additional full-page images, which are
compared on the standby against the results of applying the WAL record
(without regard to those full-page images). Allowable differences
such as hints are masked out, and the resulting pages are compared;
any difference results in a FATAL error on the standby.
Kuntal Ghosh, based on earlier patches by Michael Paquier and Heikki
Linnakangas. Extensively reviewed and revised by Michael Paquier and
by me, with additional reviews and comments from Amit Kapila, Álvaro
Herrera, Simon Riggs, and Peter Eisentraut.
Mostly these are just comments but there are a few in documentation
and a handful in code and tests. Hopefully this doesn't cause too much
unnecessary pain for backpatching. I relented from some of the most
common like "thru" for that reason. The rest don't seem numerous
enough to cause problems.
Thanks to Kevin Lyda's tool https://pypi.python.org/pypi/misspellings
Page image should be MAXALIGN'ed because existing code could directly align
pointers in page instead of align offset from beginning of page.
Found during play with indexes as extenstion, Alexander Korotkov and me
The reverted changes were intended to force a choice of whether any
newly-added BufferGetPage() calls needed to be accompanied by a
test of the snapshot age, to support the "snapshot too old"
feature. Such an accompanying test is needed in about 7% of the
cases, where the page is being used as part of a scan rather than
positioning for other purposes (such as DML or vacuuming). The
additional effort required for back-patching, and the doubt whether
the intended benefit would really be there, have indicated it is
best just to rely on developers to do the right thing based on
comments and existing usage, as we do with many other conventions.
This change should have little or no effect on generated executable
code.
Motivated by the back-patching pain of Tom Lane and Robert Haas
Rename this function to GenericXLogRegisterBuffer() to make it clearer
what it does, and leave room for other sorts of "register" actions in
future. Also, replace its "bool isNew" argument with an integer flags
argument, so as to allow adding more flags in future without an API
break.
Alexander Korotkov, adjusted slightly by me
The previous coding could allow the contents of the "hole" between pd_lower
and pd_upper to diverge during replay from what it had been when the update
was originally applied. This would pose a problem if checksums were in
use, and in any case would complicate forensic comparisons between master
and slave servers. So force the "hole" to contain zeroes, both at initial
application of a generically-logged action, and at replay.
Alexander Korotkov, adjusted slightly by me
Since we're requiring pages handled by generic_xlog.c to be standard
format, specify REGBUF_STANDARD when doing a full-page image, so that
xloginsert.c can compress out the "hole" between pd_lower and pd_upper.
Given the current API in which this path will be taken only for a newly
initialized page, the hole is likely to be particularly large in such
cases, so that this oversight could easily be performance-significant.
I don't notice any particular change in the runtime of contrib/bloom's
regression test, though.
Make the inner comparison loops of computeDelta() as tight as possible by
pulling considerations of valid and invalid ranges out of the inner loops,
and extending a match or non-match detection as far as possible before
deciding what to do next. To keep this tractable, give up the possibility
of merging fragments across the pd_lower to pd_upper gap. The fraction of
pages where that could happen (ie, there are 4 or fewer bytes in the gap,
*and* data changes immediately adjacent to it on both sides) is too small
to be worth spending cycles on.
Also, avoid two BLCKSZ-length memcpy()s by computing the delta before
moving data into the target buffer, instead of after. This doesn't save
nearly as many cycles as being tenser about computeDelta(), but it still
seems worth doing.
On my machine, this patch cuts a full 40% off the runtime of
contrib/bloom's regression test.
This routine is unsafe as implemented, because it invalidates the page
image pointers returned by previous GenericXLogRegister() calls.
Rather than complicate the API or the implementation to avoid that,
let's just get rid of it; the use-case for having it seems much
too thin to justify a lot of work here.
While at it, do some wordsmithing on the SGML docs for generic WAL.
Improve commentary, use more specific names for the delta fields,
const-ify pointer arguments where possible, avoid assuming that
initializing only the first element of a local array will guarantee
that the remaining elements end up as we need them. (I think that
code in generic_redo actually worked, but only because InvalidBuffer
is zero; this is a particularly ugly way of depending on that ...)
This patch is a no-op patch which is intended to reduce the chances
of failures of omission once the functional part of the "snapshot
too old" patch goes in. It adds parameters for snapshot, relation,
and an enum to specify whether the snapshot age check needs to be
done for the page at this point. This initial patch passes NULL
for the first two new parameters and BGP_NO_SNAPSHOT_TEST for the
third. The follow-on patch will change the places where the test
needs to be made.
This interface is designed to give an access to WAL for extensions which
could implement new access method, for example. Previously it was
impossible because restoring from custom WAL would need to access system
catalog to find a redo custom function. This patch suggests generic way
to describe changes on page with standart layout.
Bump XLOG_PAGE_MAGIC because of new record type.
Author: Alexander Korotkov with a help of Petr Jelinek, Markus Nullmeier and
minor editorization by my
Reviewers: Petr Jelinek, Alvaro Herrera, Teodor Sigaev, Jim Nasby,
Michael Paquier
