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Logging

slides online at http://web.mit.edu/6.033/www/assignments/lec17.[ppt|pdf]. Logging. 6.033 Spring 2010 Lecture 17 Sam Madden. Key concepts: Logging for all-or-nothing atomicity Write ahead logging Checkpoints. Shadow Copy Commit. //in memory copy of DB, some records dirty commit(DB):

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Logging

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  1. slides online at http://web.mit.edu/6.033/www/assignments/lec17.[ppt|pdf] Logging 6.033 Spring 2010 Lecture 17 Sam Madden Key concepts: Logging for all-or-nothing atomicity Write ahead logging Checkpoints

  2. Shadow Copy Commit //in memory copy of DB, some records dirty commit(DB): copy(DB, DBnew) for each record r in memory: if r is dirty: write r into DBnew rename(DBnew, DB) //commit point! // rename must be atomic!

  3. Read with just a log // log is array of records with (type,TID,var,before,after) read(var, log): cset = { } for r in log[len-1] ... log[0]: if (r.type == commit): cset = cset U r.TID if (r.type == update): if (r.TID in cset and r.var == var): return r.after

  4. Read & write with cell storage read(var): return read(cell-storage, var) write(TID,var,newval): append(log,TID,var,read(var),newval) write(cell-storage,var,newval)

  5. Recover with cell-storage and log recover(log): doneset = { } for each record r in log[len-1] ... log[0]: //UNDO if r.type == commit or r.type == abort doneset = doneset U r.TID if r.type == update and r.TID not in doneset: write(cell-storage, r.var, r.before)

  6. Read with cache & cell-storage read(var): if var in cache: return cache[var] else: val = read(cell-storage, var) add (var,val) to cache // possibly evict other // cached values return val write(var,val): append(log,TID, var, cell-storage(var), newval) add (var,val) to cache

  7. Recover with cell-storage, log, and cache recover(log): doneset = { } for each record r in log[len-1] ... log[0]: //UNDO if r.type == commit or r.type == abort doneset = doneset U r.TID if r.type == update and r.TID not in doneset: write(cell-storage, r.var, r.before) for each record r in log[0]...log[len-1]: //REDO if r.type == update and r.TID in doneset: write(cell-storage, r.var, r.after)

  8. REDO Optimization for each record r in log[0]...log[len-1]: //REDO if r.type == update and r.TID in doneset: if (cell-storage does not reflect r) write(cell-storage, r.var, r.after) Log Sequence Number (LSN) is a unique increasing identifier for log records Cell Storage Page LSN | TID | Op | Before | After LSN Log Record Data Do write only if Page.LSN < Record.LSN On write, update LSN

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