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| author | Stephen Frost <sfrost@snowman.net> | 2018-12-06 11:05:39 -0500 |
|---|---|---|
| committer | Stephen Frost <sfrost@snowman.net> | 2018-12-06 11:05:39 -0500 |
| commit | a243c55326686b284fbe2c7b70f06d8a022b7dcc (patch) | |
| tree | 2879e146a52ea099b47e4b61b6b76cb162c9872f /src | |
| parent | 228b0485f471480cf825b7b30ffa3d66bd692c57 (diff) | |
| download | postgresql-a243c55326686b284fbe2c7b70f06d8a022b7dcc.tar.gz postgresql-a243c55326686b284fbe2c7b70f06d8a022b7dcc.zip | |
Cleanup comments in xlog compression
Skipping over the "hole" in full page images in the XLOG code was
described as being a form of compression, but this got a bit confusing
since we now have PGLZ-based compression happening, so adjust the
wording to discuss "removing" the "hole" and keeping the talk about
compression to where we're talking about using PGLZ-based compression of
the full page images.
Reviewed-By: Kyotaro Horiguchi
Discussion: https://postgr.es/m/20181127234341.GM3415@tamriel.snowman.net
Diffstat (limited to 'src')
| -rw-r--r-- | src/backend/access/transam/xloginsert.c | 2 | ||||
| -rw-r--r-- | src/include/access/xlogrecord.h | 15 |
2 files changed, 8 insertions, 9 deletions
diff --git a/src/backend/access/transam/xloginsert.c b/src/backend/access/transam/xloginsert.c index 34d4db42977..034d5b3b624 100644 --- a/src/backend/access/transam/xloginsert.c +++ b/src/backend/access/transam/xloginsert.c @@ -605,7 +605,7 @@ XLogRecordAssemble(RmgrId rmid, uint8 info, } else { - /* No "hole" to compress out */ + /* No "hole" to remove */ bimg.hole_offset = 0; cbimg.hole_length = 0; } diff --git a/src/include/access/xlogrecord.h b/src/include/access/xlogrecord.h index 863781937e4..f594d600ada 100644 --- a/src/include/access/xlogrecord.h +++ b/src/include/access/xlogrecord.h @@ -107,15 +107,14 @@ typedef struct XLogRecordBlockHeader * Additional header information when a full-page image is included * (i.e. when BKPBLOCK_HAS_IMAGE is set). * - * As a trivial form of data compression, the XLOG code is aware that - * PG data pages usually contain an unused "hole" in the middle, which - * contains only zero bytes. If the length of "hole" > 0 then we have removed - * such a "hole" from the stored data (and it's not counted in the - * XLOG record's CRC, either). Hence, the amount of block data actually - * present is BLCKSZ - the length of "hole" bytes. + * The XLOG code is aware that PG data pages usually contain an unused "hole" + * in the middle, which contains only zero bytes. Since we know that the + * "hole" is all zeros, we remove it from the stored data (and it's not counted + * in the XLOG record's CRC, either). Hence, the amount of block data actually + * present is (BLCKSZ - <length of "hole" bytes>). * - * When wal_compression is enabled, a full page image which "hole" was - * removed is additionally compressed using PGLZ compression algorithm. + * Additionally, when wal_compression is enabled, we will try to compress full + * page images using the PGLZ compression algorithm, after removing the "hole". * This can reduce the WAL volume, but at some extra cost of CPU spent * on the compression during WAL logging. In this case, since the "hole" * length cannot be calculated by subtracting the number of page image bytes |