本节简单介绍了WAL相关的数据结构，包括XLogLongPageHeaderData、XLogPageHeaderData和XLogRecord。
一、数据结构
XLogPageHeaderData
每一个事务日志文件(WAL segment file)的page(大小默认为8K)都有头部数据.
注:每个文件第一个page的头部数据是XLogLongPageHeaderData(详见后续描述),而不是XLogPageHeaderData
/*
* Each page of XLOG file has a header like this:
* 每一个事务日志文件的page都有头部信息,结构如下:
*/
//可作为WAL版本信息
#define XLOG_PAGE_MAGIC 0xD098 /* can be used as WAL version indicator */
typedef struct XLogPageHeaderData
{
//WAL版本信息,PG V11.1 --> 0xD98
uint16 xlp_magic; /* magic value for correctness checks */
//标记位(详见下面说明)
uint16 xlp_info; /* flag bits, see below */
//page中第一个XLOG Record的TimeLineID,类型为uint32
TimeLineID xlp_tli; /* TimeLineID of first record on page */
//page的XLOG地址(在事务日志中的偏移),类型为uint64
XLogRecPtr xlp_pageaddr; /* XLOG address of this page */
/*
* When there is not enough space on current page for whole record, we
* continue on the next page. xlp_rem_len is the number of bytes
* remaining from a previous page.
* 如果当前页的空间不足以存储整个XLOG Record,在下一个页面中存储余下的数据
* xlp_rem_len表示上一页XLOG Record剩余部分的大小
*
* Note that xl_rem_len includes backup-block data; that is, it tracks
* xl_tot_len not xl_len in the initial header. Also note that the
* continuation data isn't necessarily aligned.
* 注意xl_rem_len包含backup-block data(full-page-write);
* 也就是说在初始的头部信息中跟踪的是xl_tot_len而不是xl_len.
* 另外要注意的是剩余的数据不需要对齐.
*/
//上一页空间不够存储XLOG Record,该Record在本页继续存储占用的空间大小
uint32 xlp_rem_len; /* total len of remaining data for record */
} XLogPageHeaderData;
#define SizeOfXLogShortPHD MAXALIGN(sizeof(XLogPageHeaderData))
typedef XLogPageHeaderData *XLogPageHeader;
XLogLongPageHeaderData
如设置了XLP_LONG_HEADER标记,在page header中存储额外的字段.
(通常在每个事务日志文件也就是segment file的的第一个page中存在).
这些附加的字段用于准确的识别文件。
/*
* When the XLP_LONG_HEADER flag is set, we store additional fields in the
* page header. (This is ordinarily done just in the first page of an
* XLOG file.) The additional fields serve to identify the file accurately.
* 如设置了XLP_LONG_HEADER标记,在page header中存储额外的字段.
* (通常在每个事务日志文件也就是segment file的的第一个page中存在).
* 附加字段用于准确识别文件。
*/
typedef struct XLogLongPageHeaderData
{
//标准的头部域字段
XLogPageHeaderData std; /* standard header fields */
//pg_control中的系统标识码
uint64 xlp_sysid; /* system identifier from pg_control */
//交叉检查
uint32 xlp_seg_size; /* just as a cross-check */
//交叉检查
uint32 xlp_xlog_blcksz; /* just as a cross-check */
} XLogLongPageHeaderData;
#define SizeOfXLogLongPHD MAXALIGN(sizeof(XLogLongPageHeaderData))
//指针
typedef XLogLongPageHeaderData *XLogLongPageHeader;
/* When record crosses page boundary, set this flag in new page's header */
//如果XLOG Record跨越page边界,在新page header中设置该标志位
#define XLP_FIRST_IS_CONTRECORD 0x0001
//该标志位标明是"long"页头
/* This flag indicates a "long" page header */
#define XLP_LONG_HEADER 0x0002
/* This flag indicates backup blocks starting in this page are optional */
//该标志位标明从该页起始的backup blocks是可选的(不一定存在)
#define XLP_BKP_REMOVABLE 0x0004
//xlp_info中所有定义的标志位(用于page header的有效性检查)
/* All defined flag bits in xlp_info (used for validity checking of header) */
#define XLP_ALL_FLAGS 0x0007
#define XLogPageHeaderSize(hdr) \
(((hdr)->xlp_info & XLP_LONG_HEADER) ? SizeOfXLogLongPHD : SizeOfXLogShortPHD)
XLogRecord
事务日志文件由N个的XLog Record组成,逻辑上对应XLOG Record这一概念的数据结构是XLogRecord.
XLOG Record的整体布局如下:
头部数据(固定大小的XLogRecord结构体)
XLogRecordBlockHeader 结构体
XLogRecordBlockHeader 结构体
...
XLogRecordDataHeader[Short|Long] 结构体
block data
block data
...
main data
XLOG Record按存储的数据内容来划分,大体可以分为三类:
1.Record for backup block:存储full-write-page的block,这种类型Record的目的是为了解决page部分写的问题;
2.Record for (tuple)data block:在full-write-page后,相应的page中的tuple变更,使用这种类型的Record记录;
3.Record for Checkpoint:在checkpoint发生时,在事务日志文件中记录checkpoint信息(其中包括Redo point).
XLOG Record的详细解析后续会解析,这里暂且不提
/*
* The overall layout of an XLOG record is:
* Fixed-size header (XLogRecord struct)
* XLogRecordBlockHeader struct
* XLogRecordBlockHeader struct
* ...
* XLogRecordDataHeader[Short|Long] struct
* block data
* block data
* ...
* main data
* XLOG record的整体布局如下:
* 固定大小的头部(XLogRecord 结构体)
* XLogRecordBlockHeader 结构体
* XLogRecordBlockHeader 结构体
* ...
* XLogRecordDataHeader[Short|Long] 结构体
* block data
* block data
* ...
* main data
*
* There can be zero or more XLogRecordBlockHeaders, and 0 or more bytes of
* rmgr-specific data not associated with a block. XLogRecord structs
* always start on MAXALIGN boundaries in the WAL files, but the rest of
* the fields are not aligned.
* 其中,XLogRecordBlockHeaders可能有0或者多个,与block无关的0或多个字节的rmgr-specific数据
* XLogRecord通常在WAL文件的MAXALIGN边界起写入,但后续的字段并没有对齐
*
* The XLogRecordBlockHeader, XLogRecordDataHeaderShort and
* XLogRecordDataHeaderLong structs all begin with a single 'id' byte. It's
* used to distinguish between block references, and the main data structs.
* XLogRecordBlockHeader/XLogRecordDataHeaderShort/XLogRecordDataHeaderLong开头是占用1个字节的"id".
* 用于区分block引用和main data结构体.
*/
typedef struct XLogRecord
{
//record的大小
uint32 xl_tot_len; /* total len of entire record */
//xact id
TransactionId xl_xid; /* xact id */
//指向log中的前一条记录
XLogRecPtr xl_prev; /* ptr to previous record in log */
//标识位,详见下面的说明
uint8 xl_info; /* flag bits, see below */
//该记录的资源管理器
RmgrId xl_rmid; /* resource manager for this record */
/* 2 bytes of padding here, initialize to zero */
//2个字节的crc校验位,初始化为0
pg_crc32c xl_crc; /* CRC for this record */
/* XLogRecordBlockHeaders and XLogRecordDataHeader follow, no padding */
//接下来是XLogRecordBlockHeaders和XLogRecordDataHeader
} XLogRecord;
//宏定义:XLogRecord大小
#define SizeOfXLogRecord (offsetof(XLogRecord, xl_crc) + sizeof(pg_crc32c))
/*
* The high 4 bits in xl_info may be used freely by rmgr. The
* XLR_SPECIAL_REL_UPDATE and XLR_CHECK_CONSISTENCY bits can be passed by
* XLogInsert caller. The rest are set internally by XLogInsert.
* xl_info的高4位由rmgr自由使用.
* XLR_SPECIAL_REL_UPDATE和XLR_CHECK_CONSISTENCY由XLogInsert函数的调用者传入.
* 其余由XLogInsert内部使用.
*/
#define XLR_INFO_MASK 0x0F
#define XLR_RMGR_INFO_MASK 0xF0
/*
* If a WAL record modifies any relation files, in ways not covered by the
* usual block references, this flag is set. This is not used for anything
* by PostgreSQL itself, but it allows external tools that read WAL and keep
* track of modified blocks to recognize such special record types.
* 如果WAL记录使用特殊的方式(不涉及通常块引用)更新了关系的存储文件,设置此标记.
* PostgreSQL本身并不使用这种方法，但它允许外部工具读取WAL并跟踪修改后的块，
* 以识别这种特殊的记录类型。
*/
#define XLR_SPECIAL_REL_UPDATE 0x01
/*
* Enforces consistency checks of replayed WAL at recovery. If enabled,
* each record will log a full-page write for each block modified by the
* record and will reuse it afterwards for consistency checks. The caller
* of XLogInsert can use this value if necessary, but if
* wal_consistency_checking is enabled for a rmgr this is set unconditionally.
* 在恢复时强制执行一致性检查.
* 如启用此功能,每个记录将为记录修改的每个块记录一个完整的页面写操作，并在以后重用它进行一致性检查。
* 在需要时,XLogInsert的调用者可使用此标记,但如果rmgr启用了wal_consistency_checking,
* 则会无条件执行一致性检查.
*/
#define XLR_CHECK_CONSISTENCY 0x02
/*
* Header info for block data appended to an XLOG record.
* 追加到XLOG record中block data的头部信息
*
* 'data_length' is the length of the rmgr-specific payload data associated
* with this block. It does not include the possible full page image, nor
* XLogRecordBlockHeader struct itself.
* 'data_length'是与此块关联的rmgr特定payload data的长度。
* 它不包括可能的full page image，也不包括XLogRecordBlockHeader结构体本身。
*
* Note that we don't attempt to align the XLogRecordBlockHeader struct!
* So, the struct must be copied to aligned local storage before use.
* 注意:我们不打算尝试对齐XLogRecordBlockHeader结构体!
* 因此,在使用前,XLogRecordBlockHeader必须拷贝到一队齐的本地存储中.
*/
typedef struct XLogRecordBlockHeader
{
//块引用ID
uint8 id; /* block reference ID */
//在关系中使用的fork和flags
uint8 fork_flags; /* fork within the relation, and flags */
//payload字节大小
uint16 data_length; /* number of payload bytes (not including page
* image) */
/* If BKPBLOCK_HAS_IMAGE, an XLogRecordBlockImageHeader struct follows */
//如BKPBLOCK_HAS_IMAGE,后续为XLogRecordBlockImageHeader结构体
/* If BKPBLOCK_SAME_REL is not set, a RelFileNode follows */
//如BKPBLOCK_SAME_REL没有设置,则为RelFileNode
/* BlockNumber follows */
//后续为BlockNumber
} XLogRecordBlockHeader;
#define SizeOfXLogRecordBlockHeader (offsetof(XLogRecordBlockHeader, data_length) + sizeof(uint16))
/*
* Additional header information when a full-page image is included
* (i.e. when BKPBLOCK_HAS_IMAGE is set).
* 当包含完整页图像时(即当设置BKPBLOCK_HAS_IMAGE时)，附加的头部信息。
*
* 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>).
* XLOG代码知道PG数据页通常在中间包含一个未使用的“hole”(空闲空间)，
* 大小为零字节。
* 因为我们知道“hole”都是零，
* 以我们从存储的数据中删除它(而且它也没有被计入XLOG记录的CRC中)。
* 因此，实际呈现的块数据量为(BLCKSZ - <“hole”的大小>)。
*
* 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
* from BLCKSZ, basically it needs to be stored as an extra information.
* But when no "hole" exists, we can assume that the "hole" length is zero
* and no such an extra information needs to be stored. Note that
* the original version of page image is stored in WAL instead of the
* compressed one if the number of bytes saved by compression is less than
* the length of extra information. Hence, when a page image is successfully
* compressed, the amount of block data actually present is less than
* BLCKSZ - the length of "hole" bytes - the length of extra information.
* 另外，在启用wal_compression时，会在去掉“hole”后，尝试使用PGLZ压缩算法压缩full page image。
* 这可以简化WAL大小,但会增加额外的解压缩CPU时间.
* 在这种情况下，由于“hole”的长度不能通过从BLCKSZ中减去page image字节数来计算，
* 所以它基本上需要作为额外的信息来存储。
* 但如果"hole"不存在,我们可以假设"hole"的大小为0,不需要存储额外的信息.
* 请注意，如果压缩节省的字节数小于额外信息的长度，
* 那么page image的原始版本存储在WAL中，而不是压缩后的版本。
* 因此，当一个page image被成功压缩时，
* 实际的块数据量小于BLCKSZ - “hole”的大小 - 额外信息的大小。
*/
typedef struct XLogRecordBlockImageHeader
{
uint16 length; /* number of page image bytes */
uint16 hole_offset; /* number of bytes before "hole" */
uint8 bimg_info; /* flag bits, see below */
/*
* If BKPIMAGE_HAS_HOLE and BKPIMAGE_IS_COMPRESSED, an
* XLogRecordBlockCompressHeader struct follows.
* 如标记BKPIMAGE_HAS_HOLE和BKPIMAGE_IS_COMPRESSED设置,则后跟XLogRecordBlockCompressHeader
*/
} XLogRecordBlockImageHeader;
#define SizeOfXLogRecordBlockImageHeader \
(offsetof(XLogRecordBlockImageHeader, bimg_info) + sizeof(uint8))
/* Information stored in bimg_info */
//------------ bimg_info标记位
//存在"hole"
#define BKPIMAGE_HAS_HOLE 0x01 /* page image has "hole" */
//压缩存储
#define BKPIMAGE_IS_COMPRESSED 0x02 /* page image is compressed */
//在回放时,page image需要恢复
#define BKPIMAGE_APPLY 0x04 /* page image should be restored during
* replay */
/*
* Extra header information used when page image has "hole" and
* is compressed.
* page image存在"hole"和压缩存储时,额外的头部信息
*/
typedef struct XLogRecordBlockCompressHeader
{
//"hole"的大小
uint16 hole_length; /* number of bytes in "hole" */
} XLogRecordBlockCompressHeader;
#define SizeOfXLogRecordBlockCompressHeader \
sizeof(XLogRecordBlockCompressHeader)
/*
* Maximum size of the header for a block reference. This is used to size a
* temporary buffer for constructing the header.
* 块引用的header的最大大小。
* 它用于设置用于构造头部临时缓冲区的大小。
*/
#define MaxSizeOfXLogRecordBlockHeader \
(SizeOfXLogRecordBlockHeader + \
SizeOfXLogRecordBlockImageHeader + \
SizeOfXLogRecordBlockCompressHeader + \
sizeof(RelFileNode) + \
sizeof(BlockNumber))
/*
* The fork number fits in the lower 4 bits in the fork_flags field. The upper
* bits are used for flags.
* fork号适合于fork_flags字段的低4位。
* 高4位用于标记。
*/
#define BKPBLOCK_FORK_MASK 0x0F
#define BKPBLOCK_FLAG_MASK 0xF0
//块数据是XLogRecordBlockImage
#define BKPBLOCK_HAS_IMAGE 0x10 /* block data is an XLogRecordBlockImage */
#define BKPBLOCK_HAS_DATA 0x20
//重做时重新初始化page
#define BKPBLOCK_WILL_INIT 0x40 /* redo will re-init the page */
//重做时重新初始化page,但会省略RelFileNode
#define BKPBLOCK_SAME_REL 0x80 /* RelFileNode omitted, same as previous */
/*
* XLogRecordDataHeaderShort/Long are used for the "main data" portion of
* the record. If the length of the data is less than 256 bytes, the short
* form is used, with a single byte to hold the length. Otherwise the long
* form is used.
* XLogRecordDataHeaderShort/Long用于记录的“main data”部分。
* 如果数据的长度小于256字节，则使用短格式，用一个字节保存长度。
* 否则使用长形式。
*
* (These structs are currently not used in the code, they are here just for
* documentation purposes).
* (这些结构体不会再代码中使用,在这里是为了文档记录的目的)
*/
typedef struct XLogRecordDataHeaderShort
{
uint8 id; /* XLR_BLOCK_ID_DATA_SHORT */
uint8 data_length; /* number of payload bytes */
} XLogRecordDataHeaderShort;
#define SizeOfXLogRecordDataHeaderShort (sizeof(uint8) * 2)
typedef struct XLogRecordDataHeaderLong
{
uint8 id; /* XLR_BLOCK_ID_DATA_LONG */
/* followed by uint32 data_length, unaligned */
//接下来是无符号32位整型的data_length(未对齐)
} XLogRecordDataHeaderLong;
#define SizeOfXLogRecordDataHeaderLong (sizeof(uint8) + sizeof(uint32))
/*
* Block IDs used to distinguish different kinds of record fragments. Block
* references are numbered from 0 to XLR_MAX_BLOCK_ID. A rmgr is free to use
* any ID number in that range (although you should stick to small numbers,
* because the WAL machinery is optimized for that case). A couple of ID
* numbers are reserved to denote the "main" data portion of the record.
* 块id用于区分不同类型的记录片段。
* 块引用编号从0到XLR_MAX_BLOCK_ID。
* rmgr可以自由使用该范围内的任何ID号
* (尽管您应该坚持使用较小的数字，因为WAL机制针对这种情况进行了优化)。
* 保留两个ID号来表示记录的“main”数据部分。
*
* The maximum is currently set at 32, quite arbitrarily. Most records only
* need a handful of block references, but there are a few exceptions that
* need more.
* 目前的最大值是32，非常随意。
* 大多数记录只需要少数块引用，但也有少数例外需要更多。
*/
#define XLR_MAX_BLOCK_ID 32
#define XLR_BLOCK_ID_DATA_SHORT 255
#define XLR_BLOCK_ID_DATA_LONG 254
#define XLR_BLOCK_ID_ORIGIN 253
#endif /* XLOGRECORD_H */
这些数据结构在WAL segment file文件中如何布局,请参见后续的章节
二、参考资料
Write Ahead Logging — WAL
PostgreSQL 源码解读（4）- 插入数据#3（heap_insert）
PG Source Code
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