Oracle内部视图:X$BH

X$BH Fixed Table Buffer Cache Diagram

Column     Type Description
~~~~~~     ~~~~~ ~~~~~~~~~~~
ADDR        RAW(4) Hex address of the Buffer Header.
INDX        NUMBER Buffer Header number
BUF#        NUMBER
HLADDR      RAW(4) Hash Chain Latch Address
See . ADDR

LRU_FLAG    NUMBER     8.1+ LRU flag
           KCBBHLDF   0x01     8.1  LRU Dump Flag used in debug print routine
           KCBBHLMT   0x02     8.1  moved to tail of lru (for extended stats)
           KCBBHLAL   0x04     8.1  on auxiliary list
           KCBBHLHB   0x08     8.1  hot buffer - not in cold portion of lru

FLAG        NUMBER
           KCBBHFBD   0x00001       buffer dirty
           KCBBHFAM   0x00002  7.3  about to modify; try not to start io
           KCBBHFAM   0x00002  8.0  about to modify; try not to start io
           KCBBHNAC   0x00002  8.1  notify dbwr after change
           KCBBHFMS   0x00004       modification started, no new writes
           KCBBHFBL   0x00008       block logged
           KCBBHFTD   0x00010       temporary data - no redo for changes
           KCBBHFBW   0x00020       being written; can't modify
           KCBBHFWW   0x00040       waiting for write to finish
           KCBBHFCK   0x00080  7.3  checkpoint asap
                      0x00080  8.0  not used
           KCBBHFMW   0x00080  8.1  multiple waiters when gc lock acquired
           KCBBHFRR   0x00100       recovery reading, do not reuse, being read
           KCBBHFUL   0x00200       unlink from lock element - make non-current
           KCBBHFDG   0x00400       write block & stop using for lock down grade
           KCBBHFCW   0x00800       write block for cross instance call
           KCBBHFCR   0x01000       reading from disk into KCBBHCR buffer
           KCBBHFGC   0x02000       has been gotten in current mode
           KCBBHFST   0x04000       stale - unused CR buf made from current
                      0x08000  7.3  Not used.
           KCBBHFDP   0x08000  8.0  deferred ping
           KCBBHFDP   0x08000  8.1  deferred ping
           KCBBHFDA   0x10000       Direct Access to buffer contents
           KCBBHFHD   0x20000       Hash chain Dump used in debug print routine
           KCBBHFIR   0x40000       Ignore Redo for instance recovery
           KCBBHFSQ   0x80000       sequential scan only flag
           KCBBHFNW  0x100000  7.3  Set to indicate a buffer that is NEW
                     0x100000  8.0  Not used
           KCBBHFBP  0x100000  8.1  Indicates that buffer was prefetched
           KCBBHFRW  0x200000  7.3  re-write if being written (sort)
                     0x200000  8.0  Not used
           KCBBHFFW  0x200000  8.1  Buffer has been written once
           KCBBHFFB  0x400000       buffer is "logically" flushed
           KCBBHFRS  0x800000       ReSilvered already - do not redirty
           KCBBHFKW 0x1000000  7.3  ckpt writing flag to avoid rescan */
                    0x1000000  8.0  Not used
           KCBBHDRC 0x1000000  8.1  buffer is nocache
                    0x2000000  7.3  Not used
           KCBBHFRG 0x2000000  8.0  Redo Generated since block read
           KCBBHFRG 0x2000000  8.1  Redo Generated since block read
           KCBBHFWS 0x10000000 8.0  Skipped write for checkpoint.
           KCBBHFDB 0x20000000 8.1  buffer is directly from a foreign DB
           KCBBHFAW 0x40000000 8.0  Flush after writing
           KCBBHFAW 0x40000000 8.1  Flush after writing

TS#         NUMBER 8.X Tablespace number
DBARFIL     NUMBER 8.X Relative file number of block
DBAFIL      NUMBER 7.3 File number of block
DBABLK      NUMBER Block number of block
CLASS       NUMBER See Note 33434.1
              1,'data block',
              2,'sort block',
              3,'save undo block',
              4,'segment header',
              5,'save undo header',
              6,'free list',
              7,'extent map',
              8,'1st level bmb',
              9,'2nd level bmb',
              10,'3rd level bmb',
              11,'bitmap block',
              12,'bitmap index block',
              13,'file header block',
              14,'unused',
              15,'system undo header',
              16,'system undo block',
              17,'undo header',
              18,'undo block'                -- since 10g

STATE       NUMBER
           KCBBHFREE         0       buffer free
           KCBBHEXLCUR       1       buffer current (and if DFS locked X)
           KCBBHSHRCUR       2       buffer current (and if DFS locked S)
           KCBBHCR           3       buffer consistant read
           KCBBHREADING      4       Being read
           KCBBHMRECOVERY    5       media recovery (current & special)
           KCBBHIRECOVERY    6       Instance recovery (somewhat special)

MODE_HELD   NUMBER    Mode buffer held in (MODE pre 7.3)
   0=KCBMNULL, KCBMSHARE, KCBMEXCL

CHANGES     NUMBER
CSTATE      NUMBER
X_TO_NULL   NUMBER Count of PINGS out (OPS)
DIRTY_QUEUE NUMBER You wont normally see buffers on the LRUW
LE_ADDR     RAW(4) Lock Element address (OPS)
SET_DS      RAW(4) Buffer cache set this buffer is under
    OBJ         NUMBER       Data object number
TCH     NUMBER 8.1 Touch Count
TIM     NUMBER 8.1 Touch Time
BA          RAW(4)
CR_SCN_BAS  NUMBER       Consistent Read SCN base
CR_SCN_WRP  NUMBER       Consistent Read SCN wrap
CR_XID_USN  NUMBER CR XID Undo segment no
CR_XID_SLT  NUMBER CR XID slot
CR_XID_SQN  NUMBER CR XID Sequence
CR_UBA_FIL  NUMBER CR UBA file
CR_UBA_BLK  NUMBER CR UBA Block
CR_UBA_SEQ  NUMBER CR UBA sequence
CR_UBA_REC  NUMBER CR UBA record
CR_SFL      NUMBER
LRBA_SEQ    NUMBER } Lowest RBA needed to recover block in cache
LRBA_BNO    NUMBER }
LRBA_BOF    NUMBER }

HRBA_SEQ    NUMBER } Redo RBA to be flushed BEFORE this block
HRBA_BNO    NUMBER } can be written out
HRBA_BOF    NUMBER       }

RRBA_SEQ    NUMBER } Block recovery RBA
RRBA_BNO    NUMBER }
RRBA_BOF    NUMBER }
NXT_HASH    NUMBER Next buffer on this hash chain
PRV_HASH    NUMBER Previous buffer on this hash chain
NXT_LRU     NUMBER Next buffer on the LRU
PRV_LRU     NUMBER Previous buffer on the LRU
US_NXT      RAW(4)
US_PRV      RAW(4)
WA_NXT      RAW(4)
WA_PRV      RAW(4)
ACC         RAW(4)
MOD         RAW(4)

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Comments

  1. X$BH.BA column nicely tells us the physical address of a buffer cache buffer in SGA:

    SQL> SELECT
    2 dbarfil, dbablk, obj, ba, state –<– dbarfil = relative file number, dbablk = block number, obj = data object id
    3 FROM
    4 x$bh –<– X$BH lists all initialized buffers in buffer cache
    5 WHERE
    6 to_number(rawtohex(ba),'XXXXXXXXXXXXXXXX') –<– BA is the "buffer address", actual buffer location in SGA memory
    7 BETWEEN
    8 to_number('00000000BCFF7FE0','XXXXXXXXXXXXXXXX')
    9 AND to_number('00000000BCFF7FE0','XXXXXXXXXXXXXXXX') + 32800 – 1
    10 /

    DBARFIL DBABLK OBJ BA STATE
    ———- ———- ———- —————- ———-
    4 34936 81252 00000000BCFFE000 1
    3 46222 4294967295 00000000BCFF8000 1
    3 34676 4294967295 00000000BCFFA000 0
    2 7074 5938 00000000BCFFC000 1

  2. 关于字段mode_held 没有描述清楚呢,
    0=KCBMNULL, KCBMSHARE, KCBMEXCL

    share 模式是1 还是2, 独占模式是多少?

    我实验了一会也没确定,似乎share 是2 独占是1 对吗?

  3. Hi Maclean,

    关于你查出的这个X$BH详细信息介绍,特别是每个column的详细description,这个信息是怎么查找的 :)?
    是通过“oradebug dump buffers …” 的方式?还是有什么好的内部视图查找,还是Oracle internal doc?

    Thanks
    Stone

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