Postgre vs My SQL

Karel Zak zakkr na
Úterý Únor 5 14:56:56 CET 2002

On Tue, Feb 05, 2002 at 02:36:31PM +0100, Jan ' Kozo ' Vajda wrote:
> :> > Postgresql robusni a vcelku stabilni. Jediny problem, ktery jsem
> :> > zaznamenal - kdyz na databazovem serveru nejaky proces "sezere" 100%
> :> > procesoru, postgresql pada - overeno od verze 6.5 az po 7.3. Takze pokud
> :> 
> :>  Pada znamena co? Co rika log? Jak bych si to mohl nasimulovat?
> :> 
> :>  BTW, verze 7.3 jeste neexistuje a to ani v CVS :-)
> jedna otazka k PostgreSQL: je uz spravene seriozne Crash-recovery-howto a aj
> nastroje k tomu ? kedysi sa nam podarilo nejako ( ako si uz nespomeniem )
> crashnut databazu a ked sme hladali ako z toho von, tak v docs bol text
> zhruba v zmysle "crash recovery neexistuje a bolo by dobre, keby to niekto
> napisal"


   Write-ahead Log (WAL)

           To maintain database consistency in case of an operating system
           crash, previous releases of PostgreSQL have forced all data
           modifications to disk before each transaction commit. With WAL,
           only one log file must be flushed to disk, greatly improving
           performance. If you have been using -F in previous releases to
           disable disk flushes, you may want to consider discontinuing its


   The next benefit is consistency of the data pages. The truth is that,
   before WAL, PostgreSQL was never able to guarantee consistency in the case
   of a crash. Before WAL, any crash during writing could result in:

   1. index tuples pointing to nonexistent table rows

   2. index tuples lost in split operations

   3. totally corrupted table or index page content, because of partially
      written data pages

 .... WAL saves the entire data page content in the log if
 that is required to ensure page consistency for after-crash recovery.

 IMHO vyvoj pujde v tomto smeru jeste dal nekam k on-line logum:

   WAL offers the opportunity for a new method for database on-line backup
   and restore (BAR). To use this method, one would have to make periodic
   saves of data files to another disk, a tape or another host and also
   archive the WAL log files

 vice viz:

 Pochopitelne zalezi co s tim systemem delate. Znicit se da vse :-)


 Karel Zak  <zakkr na>
 C, PostgreSQL, PHP, WWW,,

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