diff options
Diffstat (limited to 'src/include/access/xlogrecord.h')
| -rw-r--r-- | src/include/access/xlogrecord.h | 15 |
1 files changed, 7 insertions, 8 deletions
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 |