android barcode scanner javascript Exploring the Database Architecture in Software

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The only moment when a checkpoint is absolutely necessary is as the database is closed and the instance is shut down a full description of this sequence is given in 5 A checkpoint writes all dirty buffers to disk: this synchronizes the buffer cache with the datafiles, the instance with the database In normal running, the datafiles are always out of date: they may be missing changes (committed and uncommitted) This does not matter, because the copies of blocks in the buffer cache are up to date, and it is these that the sessions work on But on shutdown, it is necessary to write everything to disk Automatic checkpoints only occur on shutdown, but a checkpoint can be forced at any time with this statement:
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alter system checkpoint;
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Note that from release 8i onward, checkpoints do not occur on log switch (log switches are discussed in 15) The checkpoint described so far is a full checkpoint Partial checkpoints that force DBWn to write all the dirty buffers containing blocks from just one or more datafiles rather than the whole database, occur more frequently: when a datafile or tablespace is taken offline; when a tablespace is put into backup mode; when a tablespace is made read only These are less drastic than full checkpoints, and occur automatically whenever the relevant event happens To conclude, the DBWn writes on a very lazy algorithm: as little as possible, as rarely as possible except when a checkpoint occurs, when all dirty buffers are written to disk, as fast as possible
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LGWR writes the contents of the log buffer to the online log files on disk A write of the log buffer to the online redo log files is often referred to as flushing the log buffer When a session makes any change (by executing INSERT, UPDATE, or DELETE commands) to blocks in the database buffer cache, before it applies the change to the block it writes out the change vector that it is about to apply to the log buffer In order that no work can be lost, these change vectors must be written to disk with only minimal delay To this end, the LGWR streams the contents of the log buffer to the online redo log files on disk in very nearly real time And when a session issues a COMMIT, the LGWR writes in real time: the session hangs, while LGWR writes the buffer to disk Only then is the transaction recorded as committed, and therefore non-reversible LGWR is one of the ultimate bottlenecks in the Oracle architecture It is impossible to do DML faster than LGWR can write the change vectors to disk There are three
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circumstances that will cause LGWR to flush the log buffer: if a session issues a COMMIT; if the log buffer is one-third full; if DBWn is about to write dirty buffers First, the write-on-commit To process a COMMIT, the server process inserts a commit record into the log buffer It will then hang, while LGWR flushes the log buffer to disk Only when this write has completed is a commit-complete message returned to the session, and the server process can then continue working This is the guarantee that transactions will never be lost: every change vector for a committed transaction will be available in the redo log on disk and can therefore be applied to datafile backups Thus, if the database is ever damaged, it can be restored from backup and all work done since the backup was made can be redone It is in fact possible to prevent the LGWR write-on-commit If this is done, sessions will not have to wait for LGWR when they commit: they issue the command and then carry on workingThis will improve performance but can also mean that work can be lost It becomes possible for a session to COMMIT, then for the instance to crash before LGWR has saved the change vectors Enable this with caution! It is dangerous, and hardly ever necessary There are only a few applications where performance is more important than data loss Second, when the log buffer is one-third full, LGWR will flush it to disk This is about performance If the log buffer is small (as it usually should be) this one-thirdfull trigger will force LGWR to write the buffer to disk in very nearly real time even if no one is committing transactions The log buffer for many applications will be optimally sized at only a few megabytes The application will generate enough redo to fill one third of this in a fraction of a second, so LGWR will be forced to stream the change vectors to disk continuously, in very nearly real time Then, when a session does COMMIT, there will be hardly anything to write: so the COMMIT will complete almost instantaneously Third, when DBWn needs to write dirty buffers from the database buffer cache to the datafiles, before it does so it will signal LGWR to flush the log buffer to the online redo log files This is to ensure that it will always be possible to reverse an uncommitted transaction The mechanism of transaction rollback is fully explained in 11 For now, it is necessary to know that it is perfectly possible for DBWn to write an uncommitted transaction to the datafiles This is fine, so long as the undo data needed to reverse the transaction is guaranteed to be available Generating undo data also generates change vectors: as these will be in the redo log files before the datafiles are updated, then the undo data needed to roll back a transaction (should this be necessary) can be reconstructed if necessary
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