Handling open files
Posted by Sven Koester on 17 March 2008 16:51
In order to save away the data of a file, this file has to be opened. All of the operating systems allow files to be opened, read and written simultaneously from different programs. This practice is however more common on Darwin, Linux and Unix, than on a Windows.

Simultaneous access to files can however poise a problem, since the saved data may end up not being consistent, if one program is writing while at the same time the other is reading the same file. While this may not be very tragic when it comes to a textual log file, it can be quite disastrous when it comes to database files. Furthermore, database engines normally manage not one, but a set of files. So even when a backup product like PresSTORE does not conflict with the database engine at reading a file, it still may end up saving an inconsistent state of the database because two or more of the database files have not been saved at the same moment.

Normally, database engines are designed to cope with sudden power loss, which can also result in corrupted or un-paired files. However, this can mean a lengthy repair process, just as it would mean if PresTORE stored in an inconsistent state. In a worst case scenario, even a repair may be impossible.

On Solaris, Darwin and  Linux,  Simultaneous access to files is usually possible thus resulting in the effects described above. In order to avoid these effects, PresSTORE allows to configure pre- and post job execution scripts to bring down a database before executing backup or synchronize. The following article describes how to set up these scripts:
http://portal.archiware.com/support/index.php?/Knowledgebase/Article/View/5/0/configure-per-job-scripts

Windows, however, has a mechanism with which the first program to open a file can prevent any other from reading and/or writing to it. If PresSTORE encounters such a file during its backup process, it will report it with a "short data read".

Since Windows XP, there is an operating system level mechanism name VSS (Virtual Shadow Copy Service) that enables creating a snapshot of the file system at any given time and enable access to the entire file system without restrictions in a consistent state - including databases, Exchange and the registry. PresTORE 3 makes use of this service to enable uninterrupted and complete backup. To turn VSS use in PresSTORE on, enter the Client area, select the particular client, enter the Additional Options and chose the drive you would like to map and its destination drive letter.

The following sketch illustrates how VSS works:

Windows YSS Scheme



PresSTORE creates a temporary VSS-Snapshot when reading data for backup or synchronize.
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