![Gray & Reuter Log
10a: 10
Reading the Log: SQL is easier than CReading the Log: SQL is easier than C
long c_count( RMID rmid)/* count log records written by this rmid */
{ log_record_header header; /* structure to receive log record header */
LSN lsn; /* log sequence number of next log rec */
char buffer[1];/* null buffer to receive log record body. */
long rec_count = 0; /* count of records */
int n = 1; /* size of log body returned */
if (!log_open(READ)) panic(); /* open the log (authorization check)*/
lsn = log_max_lsn( ); /* get most recent lsn */
while (lsn != NullLSN) /* scan backward through the log */
{ n = log_read_lsn( lsn, /* lsn of record to be read */
header, /* log record header fields */
0L, &buffer, 1L );/* log rec body ignored. */
if (header.rmid == rmid) /* if record written by this RMID then */
rec_count = rec_count + 1; /* increment count */
lsn = header.prev_lsn; /* go to previous LSN. */
}; /* loop over LSNs */
logtable_close( ); /* close log table */
return rec_count; /* return the answer. */
}; /* */](https://image.slidesharecdn.com/10alog-150218104255-conversion-gate01/85/10a-log-10-320.jpg)
![Gray & Reuter Log
10a: 11
Writing The Log
Add a log record, Log manager fills in header.
LSN log_insert( char * buffer, long n);
/* log body is buffer[0..n-1] */
Force log up to a certain LSN to persistent storage:
LSN log_flush( LSN lsn, Boolean lazy); /**/
(lazy waits for a batch write or timeout == boxcar)
Note: many real interfaces allow some of:
empty buffer: to allow RM to fill it in (avoids data copies)
incremental copy: build the "buffer" in steps.
gather: take log data from many buffers.
Few offer SQL access to the log.](https://image.slidesharecdn.com/10alog-150218104255-conversion-gate01/85/10a-log-11-320.jpg)
![Gray & Reuter Log
10a: 13
Log Anchor Logging and Locking
Log records never updated: only inserted and read.
So no locks needed on log.
Semaphore (or something) needed on "end" of log
to manage space/growth/LSN for inserts
typedef struct {
filename tablename; /* name of log table */
struct log_files;/* A & B file prefix names & active file # */
xsemaphore lock; /* semaphore regulates log write */
LSN prev_lsn; /* LSN of most recent write */
LSN lsn; /* LSN of next record */
LSN durable_lsn; /* max lsn in durable storage */
LSN TM_anchor_lsn; /* lsn of trans mgr's last ckpt */
struct { /* array of open log parts */
long partno; /* partition number */
int os_fnum; /* operating system file # */
} part [MAXOPENS]; /* */
} log_anchor ; /* */](https://image.slidesharecdn.com/10alog-150218104255-conversion-gate01/85/10a-log-13-320.jpg)
![Gray & Reuter Log
10a: 15
Log InsertLog Insert
Log semaphore covers
Incrementing LSN
Finding the log end
filling in the page(s)
allocating space on a page, perhaps allocating new pages.
LSN log_insert( char * buffer, long n) /* insert a log record with body buffer[0..n]*/
/* Acquire the log lock (an exclusive semaphore on the log) */
Xsem_get(&log_anchor.lock); /* lock the log end in exclusive mode */
lsn = log_anchor.lsn; /* make a copy of the record’s lsn. */
/* find page and allocate space in it. */
/* fill in log record header & body */
/* update the anchors */
log_anchor.prev_lsn = lsn; /* log anchor lsn points past this record */
log_anchor.lsn.rba = log_anchor.lsn.rba + rec_len; /* */
Xsem_give(&log_anchor.lock); /* unlock the log end */
return lsn; }; /* return lsn of record just inserted */](https://image.slidesharecdn.com/10alog-150218104255-conversion-gate01/85/10a-log-15-320.jpg)
10a log
- 1. Gray & Reuter Log 10a: 1 Log ManagerLog Manager Jim GrayJim Gray Microsoft, Gray @ Microsoft.comMicrosoft, Gray @ Microsoft.com Andreas ReuterAndreas Reuter International University, Andreas.Reuter@i-u.deInternational University, Andreas.Reuter@i-u.de 9:00 11:00 1:30 3:30 7:00 Overview Faults Tolerance T Models Party TP mons Lock Theory Lock Techniq Queues Workflow Log ResMgr CICS & Inet Adv TM Cyberbrick Files &Buffers COM+ Corba Replication Party B-tree Access Paths Groupware Benchmark Mon Tue Wed Thur Fri
- 2. Gray & Reuter Log 10a: 2 Log ConceptLog Concept • Log is a history of all changes to the state.Log is a history of all changes to the state. • Log + old state gives new state • Log + new state gives old state (not in this picture) • Log is a sequential file. • Complete log is the complete history • Current state is just a "cache" of the log records. Archive Sunday Master Monday Master Monday Transactions Monday Night Batch Run Monday Master Tuesday Master Tuesday Transactions Tuesday Night Batch Run Tuesday Master Wednesday Transactions Wednesday Night Batch Run Wednesday Master Wednesday Master
- 3. Gray & Reuter Log 10a: 3 How Log is UsedHow Log is Used • Recovery from faults A redundant copy of the state and transitions • Security audits: Who did what to whom. Often too low-level for this. • Performance Monitor & Accounting: But only records changes (not reads). • ISSUES: Who should be allowed to read the log? It is a security hole. Must authorize access on a per-record basis.
- 4. Gray & Reuter Log 10a: 4 The Log Manager in the Scheme of Things Interesting thing is the cycle: Need log to recover archive to recover log. Break the cycle with a bootstrap file. Log Manager Transaction Manager Lock Manager Buffer Manager Media Manager SQL & Other Resource Managers Archive Manager Operating System File System File Manager
- 5. Gray & Reuter Log 10a: 5 Log Is a Sequential File.Log Is a Sequential File. Encapsulation of the log: it is a shared resource. Startup: Log manager holds startup info for all others. Careful writes: Log manager provides a • High performance. • Very reliable • Semi-infinite • Archived Sequential file. Some RMs keep private logs anyway. (Notably PORTABLE DB systems.) Then user or system has to manage multiple logs
- 6. Gray & Reuter Log 10a: 6 The Log Table Log table is a sequential set (relation). Log Records have standard part and then a log body. Often want to query table via one attribute or another: . RMID, TRID, timestamp, create domain LSN unsigned integer(64); -- log sequence number (file #, rba) create domain RMID unsigned integer; -- resource manager identifier create domain TRID char(12); -- transaction identifier create table log_table ( lsn LSN, -- the record’s log sequence number prev_lsn LSN, -- the lsn of the previous record in log timestamp TIMESTAMP, -- time log record was created resource_manager RMID, -- resource mgr that wrote this record trid TRID, -- id of transaction that wrote this record tran_prev_lsn LSN, -- prev log record of this transaction (or 0) body varchar, -- log data: rm understands it primary key (lsn) -- lsn is primary key foreign key (prev_lsn) -- previous log record in this table references a_log_table(lsn), -- foreign key (tran_prev_lsn) -- transaction's prev log rec also in table references a_log_table(lsn), -- ) entry sequenced; -- inserts go at end of file
- 7. Gray & Reuter Log 10a: 7 Log is complete historyLog is complete history Log anchor points at chain of each transaction. May maintain other chains. Log records map to sequence of N-plexed files Old files are archived. Eventually, archive files are discarded (weeks, months, never) A files B files Archive lsn prev_lsn resource_mgr trid tran_prev_lsn body Log Table Log Anchor trid, max_lsn, min_lsn...
- 8. Gray & Reuter Log 10a: 8 The Log LSN Each log record has a logical sequence number. This number (LSN for Log Sequence Number) plays a key role in many algorithms. Key property MONOTONICITY: If action A happened after action B then LSN(A) > LSN(B).
- 9. Gray & Reuter Log 10a: 9 Reading The Log long log_read_lsn( LSN lsn, /* lsn of record to be read */ log_record_header header, /* header fields of record to be read */ long offset, /* offset into body to start read */ pointer buffer, /* buffer to receive log data */ long n); /* length of buffer */ LSN log_max_lsn(void); /* returns the current maximum lsn of the log table.*/ Read with C (see next slide) or SQL: long sql_count( RMID rmid) /* count log records written by this rmid */ { long rec_count; /* count of records */ exec sql SELECT count (*) /* ask sql to scan log counting records */ INTO :rec_count /* written by the calling resource mgr and */ FROM log_table /* place count in the rec_count */ WHERE resource_manager = :rmid; /* */ return rec_count; /* return the answer. */ };
- 10. Gray & Reuter Log 10a: 10 Reading the Log: SQL is easier than CReading the Log: SQL is easier than C long c_count( RMID rmid)/* count log records written by this rmid */ { log_record_header header; /* structure to receive log record header */ LSN lsn; /* log sequence number of next log rec */ char buffer[1];/* null buffer to receive log record body. */ long rec_count = 0; /* count of records */ int n = 1; /* size of log body returned */ if (!log_open(READ)) panic(); /* open the log (authorization check)*/ lsn = log_max_lsn( ); /* get most recent lsn */ while (lsn != NullLSN) /* scan backward through the log */ { n = log_read_lsn( lsn, /* lsn of record to be read */ header, /* log record header fields */ 0L, &buffer, 1L );/* log rec body ignored. */ if (header.rmid == rmid) /* if record written by this RMID then */ rec_count = rec_count + 1; /* increment count */ lsn = header.prev_lsn; /* go to previous LSN. */ }; /* loop over LSNs */ logtable_close( ); /* close log table */ return rec_count; /* return the answer. */ }; /* */
- 11. Gray & Reuter Log 10a: 11 Writing The Log Add a log record, Log manager fills in header. LSN log_insert( char * buffer, long n); /* log body is buffer[0..n-1] */ Force log up to a certain LSN to persistent storage: LSN log_flush( LSN lsn, Boolean lazy); /**/ (lazy waits for a batch write or timeout == boxcar) Note: many real interfaces allow some of: empty buffer: to allow RM to fill it in (avoids data copies) incremental copy: build the "buffer" in steps. gather: take log data from many buffers. Few offer SQL access to the log.
- 12. Gray & Reuter Log 10a: 12 Summary Of Log Structure And Verbs Operations: Open/Close Read(LSN), Insert(body), Flush(LSN) SQL read operations. Log Table header body A file Log pages in buffer pool log page header end of durable log current end of log B file empty page in buffer pool durable storage Pages written in next write
- 13. Gray & Reuter Log 10a: 13 Log Anchor Logging and Locking Log records never updated: only inserted and read. So no locks needed on log. Semaphore (or something) needed on "end" of log to manage space/growth/LSN for inserts typedef struct { filename tablename; /* name of log table */ struct log_files;/* A & B file prefix names & active file # */ xsemaphore lock; /* semaphore regulates log write */ LSN prev_lsn; /* LSN of most recent write */ LSN lsn; /* LSN of next record */ LSN durable_lsn; /* max lsn in durable storage */ LSN TM_anchor_lsn; /* lsn of trans mgr's last ckpt */ struct { /* array of open log parts */ long partno; /* partition number */ int os_fnum; /* operating system file # */ } part [MAXOPENS]; /* */ } log_anchor ; /* */
- 14. Gray & Reuter Log 10a: 14 Making Optimistic Log Reads Work Log is duplexed. Log manager reads only one copy of the page. What if the "other" copy has more data? Trick: read BOTH copies of FIRST and LAST page in log. Other pages have "full" flag and a timestamp. IF not full or timestamp < prev_timestamp THEN read other page and take highest timestamp Torn log pages Log page consists of disk sectors (512B). Write may only write some sectors. How detect missing fragments? 1. Checksum? 2. Byte stuffing: stuff a “parity” byte on each page
- 15. Gray & Reuter Log 10a: 15 Log InsertLog Insert Log semaphore covers Incrementing LSN Finding the log end filling in the page(s) allocating space on a page, perhaps allocating new pages. LSN log_insert( char * buffer, long n) /* insert a log record with body buffer[0..n]*/ /* Acquire the log lock (an exclusive semaphore on the log) */ Xsem_get(&log_anchor.lock); /* lock the log end in exclusive mode */ lsn = log_anchor.lsn; /* make a copy of the record’s lsn. */ /* find page and allocate space in it. */ /* fill in log record header & body */ /* update the anchors */ log_anchor.prev_lsn = lsn; /* log anchor lsn points past this record */ log_anchor.lsn.rba = log_anchor.lsn.rba + rec_len; /* */ Xsem_give(&log_anchor.lock); /* unlock the log end */ return lsn; }; /* return lsn of record just inserted */
- 16. Gray & Reuter Log 10a: 16 Log Write Demon Log Semaphore can be a hotspot so: No IO under semaphore Allocation (OS requests), and Archiving is done in advance. Flush to persistent storage (disc) is done asynchronously. Demons driven by timers and by events (requests) Demons need not touch end-of-log semaphore log daemon to flush (carefully write) log pages as needed log data in shared memory and on disc log daemon to allocate new log files as needed application programs resource managers log code
- 17. Gray & Reuter Log 10a: 17 Careful Writes If partial pages may be written then subsequent write may invalidate previous write. Standard technique: Serial Writes: write one page then write the second page. Problem: ~ 1/2 disc bandwidth, 2x delay. Ping-Pong technique: Never overwrite good page: Ping-Pong between I and I+1 When complete, assure that page I has final data Never worse than serial write, generally 2x better. Also note the careful techniques for optimistic reads and torn pages. Disc Page Disc Page Disc Page i: i+1: Parallel Ping-Pong Writes New Log
- 18. Gray & Reuter Log 10a: 18 Group Commit (Boxcaring) Batch processing of log writes. If receive 1,000 log force requests/second why not just execute 50 of them? Response time will be the same (~20ms). IOs will be 20x fewer CPU will be ~ 10x smaller (10x fewer dispatches, 20x fewer OS IO). Without it, systems are limited to about 50tps no ping-pong 100tps ping-pong. With it, systems are limited to disc bandwidth >>10ktps. Group commit threshold can be set automatically.
- 19. Gray & Reuter Log 10a: 19 WADS- Giving the Log Disc Zero Latency Log disc is dedicated, so only has rotational latency. Reserve some cylinders on the disc as scratch. For each write: Write at current position on next track (zero latency). When have a full-track (or two) of log data consolidate the write in ram do a single LARGE write (100KB = 1 rotation) to the log. cost of this is seek + rotation ~ 20ms. This reserved area is called the Write Ahead Data Set (WADS). At restart: read cylinders gather recent log data rewrite end of log. RAID Write Cache makes this obsolete (if it works).
- 20. Gray & Reuter Log 10a: 20 Log: Normal Use Transaction UNDO During Normal Operation Transaction log anchor: needed during normal operation Points to most recent log rec of that transaction. Follow the transaction prev_lsn chain. EASY!
- 21. Gray & Reuter Log 10a: 21 The Log Anchor: Where It All StartsThe Log Anchor: Where It All Starts REDO/UNDO at System / RM Restart. Need to bootstrap the most recent log state. Log manager is the first to restart Helps Transaction Manager recover Transaction manager helps Resource mangers recover. Alternate design (each RM has its own log). All this depends on rebuilding the log anchor. Log Anchor Transaction Manager Checkpoint Record Resource Manager Checkpoint Records The Log Previous Transaction Manager Checpoint Record
- 22. Gray & Reuter Log 10a: 22 Preparing For Restart: Careful Write of Log Anchor Use the "standard" careful write techniques: Put the anchor in a special well-known place(s) Ping-Pong to 2 or more copies Timestamp each copy N-plex the copies on devices with independent failures. Align copies so that writes are "atomic" Accept most recent copy on pessimistic reads. Now TM and RMs can bootstrap: their anchors are in the log.
- 23. Gray & Reuter Log 10a: 23 Finding the End of the Log Find the anchor If using WADS, go to the WADS area and write log end. else Scan forward from the most log-anchor lsn Read optimistic all full pages. At 1/2 full page or bad page read pessimistic. Now have end-of log. Finish 1/2 finished record at end of log and give to TM Pages End of log Half-finished record Invalid Page Pages End of log
- 24. Gray & Reuter Log 10a: 24 Archiving The Log And "Old" Transactions What if transaction/RM low water mark is 1-month old? Abort? Copy aside: copy the undo/redo log records to a side file Copy forward: copy the undo/redo log records forward in the file. Dynamic log: copy undo records aside (so can online-undo if needed). All advance the low water mark.
- 25. Gray & Reuter Log 10a: 25 Archiving the Log Online Log 1 2 2 3 1 3 1 2 3 Archive Staggered Allocation of Log Tables on Secondary Storage
- 26. Gray & Reuter Log 10a: 26 The Safety Spectrum Just UNDO transactional storage (no durable log) Just Online Restart: keep simplexed durable log. Online plus Off-line Archive (no single point of failure): periodic copies of data duplex log Electronic vaulting: archive copies and duplexing is done to remote site. via fast communications links (or Federal Express).
- 27. Gray & Reuter Log 10a: 27 Multiple Logs? Transaction Manager has a log (DECdtm, MS-DTC,…) Transaction Monitor has a log (CICS, Tuxedo, ACMS,...) Each DB instance (3 Oracle, 2 Informix, 4 Rdb) has a log. Some have 3 logs: UNDO, REDO, SNAPSHOT. Cons Lots of tapes/files. Lots of IOs at commit Lots of things to break. Pros: Portable Performance (in the 1 RM case) You decide
- 28. Gray & Reuter Log 10a: 28 Client/Server Logging One server design (can be process pair) Well known log server in the net. Client sends a BATCH of log records to the server. Gets back a LSN Uses "local" LSNs for his objects. Log servers can be N-plexed processes. Multi-server design Client forms a quorum (majority of servers). Client sends log batch to all, gets back N-LSNs. If less than majority, client must poll ALL N servers Servers synchronize their "logical" logs as "sum" of physical logs (need a majority).
- 29. Gray & Reuter Log 10a: 29 Summary • Log is a sequential file • Contains entire history of DB • Many tricks to write it efficiently and carefully • Many tricks to archive and recover it