Log Manager

1. Log Manager Mon Tue Wed Thur Fri 9:00 Overview TP mons Log Files &Buffers B-tree Jim Gray Microsoft, Gray @ Microsoft.com Andreas Reuter International University, Andreas.Reuter@i-u.de 11:00 Faults Lock Theory ResMgr COM+ Access Paths 1:30 Tolerance Lock Techniq CICS & Inet Corba Groupware 3:30 T Models Queues Adv TM Replication Benchmark 7:00 Party Workflow Cyberbrick Party

2. Tuesday Master Wednesday Master Sunday Master Monday Master Wednesday Tuesday Monday Night Night Night Batch Batch Batch Run Run Run Tuesday Monday Wednesday Transactions Transactions Transactions Tuesday Master Monday Master Wednesday Master Log Concept • 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

3. How 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. The Log Manager in the Scheme of Things Archive Manager Interesting thing is the cycle: Need log to recover archive to recover log.Break the cycle with a bootstrap file. SQL & Other Transaction Manager Resource Managers Lock Manager Log Manager Buffer Manager File Manager Operating System File Media Manager System

5. 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. 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. max_lsn, trid, min_lsn... Log is complete history B files Log Table A files 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) Log Anchor lsn prev_lsn resource_mgr trid tran_prev_lsn Archive body

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. 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. Reading 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. 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. Summary Of Log Structure And Verbs B file A file Operations: Open/Close Read(LSN), Insert(body), Flush(LSN) SQL read operations. durable storage log page header Log pages in buffer pool empty page in Pages written in next write buffer pool current end of log end of header durable Log Table body log

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. 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. Log 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. application programs resource managers log code 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 log daemon to flush to allocate (carefully write) new log files log pages as needed as needed log data in shared memory and on disc

17. Disc Page Disc Page Disc Page Parallel i: Ping-Pong i+1: Writes 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+1When 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. New Log

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. 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. 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. The 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 The Log Checkpoint Record Previous Transaction Resource Manager Checkpoint Records Manager Checpoint Record

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. 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 Half-finished record Pages Invalid Page End of log Pages End of log

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. Archiving the Log Online Archive Staggered 1 2 3 Allocation of Log Tables on 2 1 3 Secondary Storage Log 2 3 1

26. The Safety Spectrum Just UNDOtransactional storage (no durable log) Just Online Restart: keep simplexed durable log. Online plus Off-line Archive (no single point of failure):periodic copies of dataduplex log Electronic vaulting:archive copies and duplexing is done to remote site.via fast communications links (or Federal Express).

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. ConsLots of tapes/files.Lots of IOs at commitLots of things to break. Pros:PortablePerformance (in the 1 RM case) You decide

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 LSNUses "local" LSNs for his objects.Log servers can be N-plexed processes. Multi-server designClient 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 serversServers synchronize their "logical" logs as "sum" of physical logs (need a majority).

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