On Wed, Dec 24, 2014 at 9:03 PM, Michael Paquier
<michael.paqu...@gmail.com> wrote:
> Returning only a boolean is fine for me (that's what my first patch
> did), especially if we add at some point hooks for compression and
> decompression calls.
Here is a patch rebased on current HEAD (60838df) for the core feature
with the APIs of pglz using booleans as return values.
--
Michael
From c8891e6086682d4e5bc197ef3047068b3a3875c5 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@otacoo.com>
Date: Tue, 25 Nov 2014 14:24:26 +0900
Subject: [PATCH] Support compression for full-page writes in WAL
Compression is controlled with a new parameter called wal_compression.
This parameter can be changed at session level to control WAL compression.
---
contrib/pg_xlogdump/pg_xlogdump.c | 20 ++--
doc/src/sgml/config.sgml | 29 +++++
src/backend/access/transam/xlog.c | 1 +
src/backend/access/transam/xloginsert.c | 155 ++++++++++++++++++++++----
src/backend/access/transam/xlogreader.c | 77 ++++++++++---
src/backend/utils/misc/guc.c | 9 ++
src/backend/utils/misc/postgresql.conf.sample | 1 +
src/include/access/xlog.h | 1 +
src/include/access/xlogreader.h | 7 +-
src/include/access/xlogrecord.h | 36 ++++--
src/include/pg_config.h.in | 4 +-
11 files changed, 283 insertions(+), 57 deletions(-)
diff --git a/contrib/pg_xlogdump/pg_xlogdump.c b/contrib/pg_xlogdump/pg_xlogdump.c
index 9f05e25..3ba1d8f 100644
--- a/contrib/pg_xlogdump/pg_xlogdump.c
+++ b/contrib/pg_xlogdump/pg_xlogdump.c
@@ -363,15 +363,12 @@ XLogDumpCountRecord(XLogDumpConfig *config, XLogDumpStats *stats,
* takes up BLCKSZ bytes, minus the "hole" length.
*
* XXX: We peek into xlogreader's private decoded backup blocks for the
- * hole_length. It doesn't seem worth it to add an accessor macro for
+ * length of block. It doesn't seem worth it to add an accessor macro for
* this.
*/
fpi_len = 0;
for (block_id = 0; block_id <= record->max_block_id; block_id++)
- {
- if (XLogRecHasBlockImage(record, block_id))
- fpi_len += BLCKSZ - record->blocks[block_id].hole_length;
- }
+ fpi_len += record->blocks[block_id].bkp_len;
/* Update per-rmgr statistics */
@@ -465,9 +462,16 @@ XLogDumpDisplayRecord(XLogDumpConfig *config, XLogReaderState *record)
blk);
if (XLogRecHasBlockImage(record, block_id))
{
- printf(" (FPW); hole: offset: %u, length: %u\n",
- record->blocks[block_id].hole_offset,
- record->blocks[block_id].hole_length);
+ if (record->blocks[block_id].is_compressed)
+ printf(" (FPW compressed); hole offset: %u, "
+ "compressed length: %u, original length: %u\n",
+ record->blocks[block_id].hole_offset,
+ record->blocks[block_id].bkp_len,
+ record->blocks[block_id].bkp_uncompress_len);
+ else
+ printf(" (FPW); hole offset: %u, length: %u\n",
+ record->blocks[block_id].hole_offset,
+ record->blocks[block_id].bkp_len);
}
putchar('\n');
}
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 6bcb106..acbbd20 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -2282,6 +2282,35 @@ include_dir 'conf.d'
</listitem>
</varlistentry>
+ <varlistentry id="guc-wal-compression" xreflabel="wal_compression">
+ <term><varname>wal_compression</varname> (<type>boolean</type>)
+ <indexterm>
+ <primary><varname>wal_compression</> configuration parameter</primary>
+ </indexterm>
+ </term>
+ <listitem>
+ <para>
+ When this parameter is <literal>on</>, the <productname>PostgreSQL</>
+ server compresses the content of full-page writes when necessary and
+ inserts in WAL records with smaller sizes, reducing the amount of
+ WAL stored on disk.
+ </para>
+
+ <para>
+ Compression has the advantage of reducing the amount of disk I/O when
+ doing WAL-logging, at the cost of some extra CPU to perform the
+ compression of a block image. At WAL replay, compressed block images
+ need extra CPU cycles to perform the decompression of each block
+ image, but it can reduce as well replay time in I/O bounded
+ environments.
+ </para>
+
+ <para>
+ The default value is <literal>off</>.
+ </para>
+ </listitem>
+ </varlistentry>
+
<varlistentry id="guc-wal-buffers" xreflabel="wal_buffers">
<term><varname>wal_buffers</varname> (<type>integer</type>)
<indexterm>
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index e5dddd4..d68d9e3 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -88,6 +88,7 @@ char *XLogArchiveCommand = NULL;
bool EnableHotStandby = false;
bool fullPageWrites = true;
bool wal_log_hints = false;
+bool wal_compression = false;
bool log_checkpoints = false;
int sync_method = DEFAULT_SYNC_METHOD;
int wal_level = WAL_LEVEL_MINIMAL;
diff --git a/src/backend/access/transam/xloginsert.c b/src/backend/access/transam/xloginsert.c
index f3d610f..a1496aa 100644
--- a/src/backend/access/transam/xloginsert.c
+++ b/src/backend/access/transam/xloginsert.c
@@ -24,12 +24,16 @@
#include "access/xlog_internal.h"
#include "access/xloginsert.h"
#include "catalog/pg_control.h"
+#include "common/pg_lzcompress.h"
#include "miscadmin.h"
#include "storage/bufmgr.h"
#include "storage/proc.h"
#include "utils/memutils.h"
#include "pg_trace.h"
+/* maximum size for compression buffer of block image */
+#define PGLZ_MAX_BLCKSZ PGLZ_MAX_OUTPUT(BLCKSZ)
+
/*
* For each block reference registered with XLogRegisterBuffer, we fill in
* a registered_buffer struct.
@@ -50,6 +54,8 @@ typedef struct
XLogRecData bkp_rdatas[2]; /* temporary rdatas used to hold references to
* backup block data in XLogRecordAssemble() */
+ char compressed_page[PGLZ_MAX_BLCKSZ]; /* recipient for compressed
+ * page */
} registered_buffer;
static registered_buffer *registered_buffers;
@@ -81,6 +87,9 @@ static char *hdr_scratch = NULL;
MaxSizeOfXLogRecordBlockHeader * (XLR_MAX_BLOCK_ID + 1) + \
SizeOfXLogRecordDataHeaderLong)
+/* Scratch buffer holding block image data to be compressed */
+static char *compression_scratch = NULL;
+
/*
* An array of XLogRecData structs, to hold registered data.
*/
@@ -97,6 +106,9 @@ static XLogRecData *XLogRecordAssemble(RmgrId rmid, uint8 info,
XLogRecPtr RedoRecPtr, bool doPageWrites,
XLogRecPtr *fpw_lsn);
+static bool XLogCompressBackupBlock(char *page, uint32 orig_len,
+ char *dest, uint16 *len);
+
/*
* Begin constructing a WAL record. This must be called before the
* XLogRegister* functions and XLogInsert().
@@ -529,9 +541,14 @@ XLogRecordAssemble(RmgrId rmid, uint8 info,
if (needs_backup)
{
Page page = regbuf->page;
+ uint16 hole_length;
+ uint16 hole_offset;
+ uint16 compress_len = 0;
+ bool is_compressed = false;
/*
- * The page needs to be backed up, so set up *bimg
+ * The page needs to be backed up, so calculate its hole length
+ * and offset.
*/
if (regbuf->flags & REGBUF_STANDARD)
{
@@ -543,49 +560,107 @@ XLogRecordAssemble(RmgrId rmid, uint8 info,
upper > lower &&
upper <= BLCKSZ)
{
- bimg.hole_offset = lower;
- bimg.hole_length = upper - lower;
+ hole_offset = lower;
+ hole_length = upper - lower;
}
else
{
/* No "hole" to compress out */
- bimg.hole_offset = 0;
- bimg.hole_length = 0;
+ hole_offset = 0;
+ hole_length = 0;
}
}
else
{
/* Not a standard page header, don't try to eliminate "hole" */
- bimg.hole_offset = 0;
- bimg.hole_length = 0;
+ hole_offset = 0;
+ hole_length = 0;
}
- /* Fill in the remaining fields in the XLogRecordBlockData struct */
- bkpb.fork_flags |= BKPBLOCK_HAS_IMAGE;
+ /*
+ * First try to compress block without its hole to improve the
+ * compression of the whole. If the block is considered as
+ * not compressible, complete the block header information
+ * accordingly.
+ */
+ if (wal_compression)
+ {
+ int page_len = BLCKSZ - hole_length;
+ char *scratch_buf;
+
+ if (hole_length != 0)
+ {
+ scratch_buf = compression_scratch;
+ memcpy(scratch_buf, page, hole_offset);
+ memcpy(scratch_buf + hole_offset,
+ page + (hole_offset + hole_length),
+ BLCKSZ - (hole_length + hole_offset));
+ }
+ else
+ scratch_buf = page;
- total_len += BLCKSZ - bimg.hole_length;
+ /* Perform compression of block */
+ if (XLogCompressBackupBlock(scratch_buf,
+ page_len,
+ regbuf->compressed_page,
+ &compress_len))
+ {
+ /* compression is done, add record */
+ is_compressed = true;
+ }
+ }
/*
- * Construct XLogRecData entries for the page content.
+ * Fill in the remaining fields in the XLogRecordBlockImageHeader
+ * struct and add new entries in the record chain.
*/
- rdt_datas_last->next = ®buf->bkp_rdatas[0];
- rdt_datas_last = rdt_datas_last->next;
- if (bimg.hole_length == 0)
+ bkpb.fork_flags |= BKPBLOCK_HAS_IMAGE;
+
+ /* hole offset length should be 15-bit long */
+ Assert((hole_offset & 0x8000) == 0);
+
+ if (is_compressed)
{
- rdt_datas_last->data = page;
- rdt_datas_last->len = BLCKSZ;
+ /* compressed block information */
+ bimg.length = compress_len;
+ bimg.hole_offset = hole_offset;
+ bimg.is_compressed = 1;
+
+ /* record entry for compressed block */
+ rdt_datas_last->next = ®buf->bkp_rdatas[0];
+ rdt_datas_last = rdt_datas_last->next;
+ rdt_datas_last->data = regbuf->compressed_page;
+ rdt_datas_last->len = compress_len;
+ total_len += bimg.length;
}
else
{
- /* must skip the hole */
- rdt_datas_last->data = page;
- rdt_datas_last->len = bimg.hole_offset;
-
- rdt_datas_last->next = ®buf->bkp_rdatas[1];
+ /* uncompressed block information */
+ bimg.length = BLCKSZ - hole_length;
+ bimg.hole_offset = hole_offset;
+ bimg.is_compressed = 0;
+ total_len += bimg.length;
+
+ /* record entries for uncompressed block */
+ rdt_datas_last->next = ®buf->bkp_rdatas[0];
rdt_datas_last = rdt_datas_last->next;
+ if (hole_length == 0)
+ {
+ rdt_datas_last->data = page;
+ rdt_datas_last->len = BLCKSZ;
+ }
+ else
+ {
+ /* must skip the hole */
+ rdt_datas_last->data = page;
+ rdt_datas_last->len = hole_offset;
+
+ rdt_datas_last->next = ®buf->bkp_rdatas[1];
+ rdt_datas_last = rdt_datas_last->next;
- rdt_datas_last->data = page + (bimg.hole_offset + bimg.hole_length);
- rdt_datas_last->len = BLCKSZ - (bimg.hole_offset + bimg.hole_length);
+ rdt_datas_last->data = page + (hole_offset + hole_length);
+ rdt_datas_last->len = BLCKSZ - (hole_offset + hole_length);
+ }
}
}
@@ -681,6 +756,35 @@ XLogRecordAssemble(RmgrId rmid, uint8 info,
}
/*
+ * Create a compressed version of a backup block. If successful, return
+ * true and set 'len' to its length. If block cannot be compressed or if
+ * compression failed return false.
+ */
+static bool
+XLogCompressBackupBlock(char *page, uint32 orig_len, char *dest, uint16 *len)
+{
+ /* leave if data can not be compressed */
+ if (!pglz_compress(page, orig_len, (PGLZ_Header *) dest,
+ PGLZ_strategy_default))
+ return false;
+
+ /*
+ * We recheck the actual size even if pglz_compress() report success,
+ * because it might be satisfied with having saved as little as one byte
+ * in the compressed data --- which could turn into a net loss once you
+ * consider header and alignment padding. Worst case, the compressed
+ * format might require three padding bytes (plus header, which is
+ * included in VARSIZE(buf)), whereas the uncompressed format would take
+ * only one header byte and no padding if the value is short enough. So
+ * we insist on a savings of more than 2 bytes to ensure we have a gain.
+ */
+ *len = VARSIZE((struct varlena *) dest);
+ if (*len >= orig_len - 2)
+ return false;
+ return true;
+}
+
+/*
* Determine whether the buffer referenced has to be backed up.
*
* Since we don't yet have the insert lock, fullPageWrites and forcePageWrites
@@ -893,4 +997,9 @@ InitXLogInsert(void)
if (hdr_scratch == NULL)
hdr_scratch = MemoryContextAllocZero(xloginsert_cxt,
HEADER_SCRATCH_SIZE);
+
+ /* allocate scratch buffer used for compression of block images */
+ if (compression_scratch == NULL)
+ compression_scratch = MemoryContextAllocZero(xloginsert_cxt,
+ BLCKSZ);
}
diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c
index 67d6223..e483288 100644
--- a/src/backend/access/transam/xlogreader.c
+++ b/src/backend/access/transam/xlogreader.c
@@ -20,6 +20,7 @@
#include "access/xlog_internal.h"
#include "access/xlogreader.h"
#include "catalog/pg_control.h"
+#include "common/pg_lzcompress.h"
static bool allocate_recordbuf(XLogReaderState *state, uint32 reclength);
@@ -74,13 +75,15 @@ XLogReaderAllocate(XLogPageReadCB pagereadfunc, void *private_data)
state->max_block_id = -1;
/*
- * Permanently allocate readBuf. We do it this way, rather than just
- * making a static array, for two reasons: (1) no need to waste the
- * storage in most instantiations of the backend; (2) a static char array
- * isn't guaranteed to have any particular alignment, whereas palloc()
- * will provide MAXALIGN'd storage.
+ * Permanently allocate readBuf compressBuf. We do it this way,
+ * rather than just making a static array, for two reasons:
+ * (1) no need to waste the storage in most instantiations of the
+ * backend; (2) a static char array isn't guaranteed to have any
+ * particular alignment, whereas palloc() will provide MAXALIGN'd
+ * storage.
*/
state->readBuf = (char *) palloc(XLOG_BLCKSZ);
+ state->compressBuf = (char *) palloc(BLCKSZ);
state->read_page = pagereadfunc;
/* system_identifier initialized to zeroes above */
@@ -98,6 +101,7 @@ XLogReaderAllocate(XLogPageReadCB pagereadfunc, void *private_data)
{
pfree(state->errormsg_buf);
pfree(state->readBuf);
+ pfree(state->compressBuf);
pfree(state);
return NULL;
}
@@ -125,6 +129,7 @@ XLogReaderFree(XLogReaderState *state)
if (state->readRecordBuf)
pfree(state->readRecordBuf);
pfree(state->readBuf);
+ pfree(state->compressBuf);
pfree(state);
}
@@ -923,6 +928,7 @@ ResetDecoder(XLogReaderState *state)
state->blocks[block_id].in_use = false;
state->blocks[block_id].has_image = false;
state->blocks[block_id].has_data = false;
+ state->blocks[block_id].is_compressed = false;
}
state->max_block_id = -1;
}
@@ -1032,9 +1038,12 @@ DecodeXLogRecord(XLogReaderState *state, XLogRecord *record, char **errormsg)
if (blk->has_image)
{
- COPY_HEADER_FIELD(&blk->hole_offset, sizeof(uint16));
- COPY_HEADER_FIELD(&blk->hole_length, sizeof(uint16));
- datatotal += BLCKSZ - blk->hole_length;
+ XLogRecordBlockImageHeader bkp_info;
+ COPY_HEADER_FIELD(&bkp_info, sizeof(XLogRecordBlockImageHeader));
+ blk->is_compressed = bkp_info.is_compressed;
+ blk->bkp_len = bkp_info.length;
+ blk->hole_offset = bkp_info.hole_offset;
+ datatotal += blk->bkp_len;
}
if (!(fork_flags & BKPBLOCK_SAME_REL))
{
@@ -1088,8 +1097,17 @@ DecodeXLogRecord(XLogReaderState *state, XLogRecord *record, char **errormsg)
continue;
if (blk->has_image)
{
+ /*
+ * Peek into the block image to grab the original length of the
+ * compressed image.
+ */
+ if (blk->is_compressed)
+ blk->bkp_uncompress_len = PGLZ_RAW_SIZE((PGLZ_Header *) ptr);
+ else
+ blk->bkp_uncompress_len = blk->bkp_len;
+
blk->bkp_image = ptr;
- ptr += BLCKSZ - blk->hole_length;
+ ptr += blk->bkp_len;
}
if (blk->has_data)
{
@@ -1195,6 +1213,8 @@ bool
RestoreBlockImage(XLogReaderState *record, uint8 block_id, char *page)
{
DecodedBkpBlock *bkpb;
+ char *block_image;
+ int hole_length;
if (!record->blocks[block_id].in_use)
return false;
@@ -1202,19 +1222,44 @@ RestoreBlockImage(XLogReaderState *record, uint8 block_id, char *page)
return false;
bkpb = &record->blocks[block_id];
+ block_image = bkpb->bkp_image;
+
+ /*
+ * Fetch page data, with different processing depending on if the
+ * page is compressed or not.
+ */
+ if (bkpb->is_compressed)
+ {
+ PGLZ_Header *header = (PGLZ_Header *) block_image;
+
+ if (!pglz_decompress(header, record->compressBuf))
+ {
+ report_invalid_record(record, "invalid compressed image at %X/%X, block %d",
+ (uint32) (record->ReadRecPtr >> 32),
+ (uint32) record->ReadRecPtr,
+ block_id);
+ return false;
+ }
+
+ block_image = record->compressBuf;
+ hole_length = BLCKSZ - PGLZ_RAW_SIZE(header);
+ }
+ else
+ hole_length = BLCKSZ - bkpb->bkp_len;
- if (bkpb->hole_length == 0)
+ /* generate page, taking into account hole if necessary */
+ if (hole_length == 0)
{
- memcpy(page, bkpb->bkp_image, BLCKSZ);
+ memcpy(page, block_image, BLCKSZ);
}
else
{
- memcpy(page, bkpb->bkp_image, bkpb->hole_offset);
+ memcpy(page, block_image, bkpb->hole_offset);
/* must zero-fill the hole */
- MemSet(page + bkpb->hole_offset, 0, bkpb->hole_length);
- memcpy(page + (bkpb->hole_offset + bkpb->hole_length),
- bkpb->bkp_image + bkpb->hole_offset,
- BLCKSZ - (bkpb->hole_offset + bkpb->hole_length));
+ MemSet(page + bkpb->hole_offset, 0, hole_length);
+ memcpy(page + (bkpb->hole_offset + hole_length),
+ block_image + bkpb->hole_offset,
+ BLCKSZ - (bkpb->hole_offset + hole_length));
}
return true;
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 77c3494..de17e29 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -927,6 +927,15 @@ static struct config_bool ConfigureNamesBool[] =
false,
NULL, NULL, NULL
},
+ {
+ {"wal_compression", PGC_USERSET, WAL_SETTINGS,
+ gettext_noop("Compresses full-page writes written in WAL file."),
+ NULL
+ },
+ &wal_compression,
+ false,
+ NULL, NULL, NULL
+ },
{
{"log_checkpoints", PGC_SIGHUP, LOGGING_WHAT,
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index b053659..b367e2c 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -186,6 +186,7 @@
# fsync_writethrough
# open_sync
#full_page_writes = on # recover from partial page writes
+#wal_compression = off # enable compression of full-page writes
#wal_log_hints = off # also do full page writes of non-critical updates
# (change requires restart)
#wal_buffers = -1 # min 32kB, -1 sets based on shared_buffers
diff --git a/src/include/access/xlog.h b/src/include/access/xlog.h
index d06fbc0..6bdfa4a 100644
--- a/src/include/access/xlog.h
+++ b/src/include/access/xlog.h
@@ -98,6 +98,7 @@ extern char *XLogArchiveCommand;
extern bool EnableHotStandby;
extern bool fullPageWrites;
extern bool wal_log_hints;
+extern bool wal_compression;
extern bool log_checkpoints;
/* WAL levels */
diff --git a/src/include/access/xlogreader.h b/src/include/access/xlogreader.h
index eb6cc89..e13b796 100644
--- a/src/include/access/xlogreader.h
+++ b/src/include/access/xlogreader.h
@@ -52,9 +52,11 @@ typedef struct
/* Information on full-page image, if any */
bool has_image;
+ bool is_compressed;
char *bkp_image;
+ uint16 bkp_len;
+ uint16 bkp_uncompress_len;
uint16 hole_offset;
- uint16 hole_length;
/* Buffer holding the rmgr-specific data associated with this block */
bool has_data;
@@ -138,6 +140,9 @@ struct XLogReaderState
/* Buffer for currently read page (XLOG_BLCKSZ bytes) */
char *readBuf;
+ /* Scratch buffer used for uncompressed pages */
+ char *compressBuf;
+
/* last read segment, segment offset, read length, TLI */
XLogSegNo readSegNo;
uint32 readOff;
diff --git a/src/include/access/xlogrecord.h b/src/include/access/xlogrecord.h
index e5826ee..3ccd69d 100644
--- a/src/include/access/xlogrecord.h
+++ b/src/include/access/xlogrecord.h
@@ -98,17 +98,37 @@ 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 hole_length > 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 - hole_length bytes.
+ * Block images are able to do several types of compression:
+ * - When wal_compression is off, as a trivial form of compression, the
+ * XLOG code is aware that PG data pages usually contain an unused "hole"
+ * in the middle, which contains only zero bytes. If length < BLCKSZ
+ * then we have removed such a "hole" from the stored data (and it is
+ * not counted in the XLOG record's CRC, either). Hence, the amount
+ * of block data actually present is "length" bytes. The hole "offset"
+ * on page is defined using "hole_offset".
+ * - When wal_compression is enabled, block images are compressed
+ * using a compression algorithm without their hole to improve
+ * compression process of the page. "length" corresponds in this case
+ * to the length of the block compressed, the original length of the
+ * block without its page hole being deducible from the compressed data
+ * itself. "hole_offset" is the hole offset of the page.
+ *
+ * "is_compressed" is used to identofy if a given block image is compressed
+ * or not. Maximum page size allowed on the system being 32k, the hole
+ * offset cannot be more than 15-bit long so the last free bit is used to
+ * store the compression state of block image. If the maximum page size
+ * allowed is increased to a value higher than that, we should consider
+ * increasing this structure size as well, but this would increase the
+ * length of block header in WAL records with alignment.
*/
typedef struct XLogRecordBlockImageHeader
{
- uint16 hole_offset; /* number of bytes before "hole" */
- uint16 hole_length; /* number of bytes in "hole" */
+ uint16 length; /* length of block data in record. If compressed
+ * this is the length of compressed block. If not
+ * compressed, this is the length of page without
+ * its hole */
+ uint16 hole_offset:15, /* number of bytes in "hole" */
+ is_compressed:1; /* compression status of image */
} XLogRecordBlockImageHeader;
#define SizeOfXLogRecordBlockImageHeader sizeof(XLogRecordBlockImageHeader)
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 465281c..f86f028 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -35,7 +35,9 @@
to have large tuples, since fields can be spread across multiple tuples).
BLCKSZ must be a power of 2. The maximum possible value of BLCKSZ is
currently 2^15 (32768). This is determined by the 15-bit widths of the
- lp_off and lp_len fields in ItemIdData (see include/storage/itemid.h).
+ lp_off and lp_len fields in ItemIdData (see include/storage/itemid.h) and
+ XLogRecordBlockImageHeader where page hole offset is limited to 15-bit
+ length (see src/include/access/xlogrecord.h).
Changing BLCKSZ requires an initdb. */
#undef BLCKSZ
--
2.2.1
--
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