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Commita243c55

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Cleanup comments in xlog compression
Skipping over the "hole" in full page images in the XLOG code wasdescribed as being a form of compression, but this got a bit confusingsince we now have PGLZ-based compression happening, so adjust thewording to discuss "removing" the "hole" and keeping the talk aboutcompression to where we're talking about using PGLZ-based compression ofthe full page images.Reviewed-By: Kyotaro HoriguchiDiscussion:https://postgr.es/m/20181127234341.GM3415@tamriel.snowman.net
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2 files changed

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‎src/backend/access/transam/xloginsert.c

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -605,7 +605,7 @@ XLogRecordAssemble(RmgrId rmid, uint8 info,
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}
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else
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{
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/* No "hole" tocompress out */
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/* No "hole" toremove */
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bimg.hole_offset=0;
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cbimg.hole_length=0;
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}

‎src/include/access/xlogrecord.h

Lines changed: 7 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -107,15 +107,14 @@ typedef struct XLogRecordBlockHeader
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* Additional header information when a full-page image is included
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* (i.e. when BKPBLOCK_HAS_IMAGE is set).
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*
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* As a trivial form of data compression, the XLOG code is aware that
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* PG data pages usually contain an unused "hole" in the middle, which
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* contains only zero bytes. If the length of "hole" > 0 then we have removed
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* such a "hole" from the stored data (and it's not counted in the
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* XLOG record's CRC, either). Hence, the amount of block data actually
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* present is BLCKSZ - the length of "hole" bytes.
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* The XLOG code is aware that PG data pages usually contain an unused "hole"
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* in the middle, which contains only zero bytes. Since we know that the
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* "hole" is all zeros, we remove it from the stored data (and it's not counted
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* in the XLOG record's CRC, either). Hence, the amount of block data actually
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* present is (BLCKSZ - <length of "hole" bytes>).
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*
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*Whenwal_compression is enabled,a full page image which "hole" was
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*removed is additionally compressedusing PGLZ compression algorithm.
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*Additionally, whenwal_compression is enabled,we will try to compress full
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*page imagesusingthePGLZ compression algorithm, after removing the "hole".
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* This can reduce the WAL volume, but at some extra cost of CPU spent
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* on the compression during WAL logging. In this case, since the "hole"
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* length cannot be calculated by subtracting the number of page image bytes

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