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VMS FILES11 Hard Drive Data Recovery

About VMS FILES11 (OSD)

FILES-11 is the the general name for the file system used by the Digital Equipment Corporation (DEC) MicroVAX with its VMS operating system. VMS is now owned by HP and is the OpenVMS operating system. Files-11 is also known as ODS (On Disk Structure) with levels ranging from the earliest ODS-1 which was a single level file structure, then levels 2 through to 5 which are sophisticated hierarchical file systems supporting a raft of features to enable it to be useful in a multi-user environment.

FILES-11 ODS level 2 is the file system found on the VAX, level 5 is an extended version of level 2 for use with Alpha systems.

FILES-11 is based around one central file named INDEXF.SYS, within this file resides an entry for each entity within the file system. HPFS and NTFS employ an identical methodology, the NTFS MFT is a file with the same purpose as INDEXF.SYS. Paths under FILES-11 are described as per the following layout [000000.userdata.files]filename.dat;2, the equivalent under UNIX being /000000/userdata/files/filename.dat. The value following the semicolon is a file version number as FILE-11 can support multiple versions of a files, which does add a level of complexity if transferring data to NTFS or one of the UNIX/LINUX file systems. The top level of the FILES-11 hierarchy is the ROOT directory (000000), here there are file entries for the other system data structures such as the bad block map. Below this will exist a directory structure just as would be found under UNIX or Windows.

VMS FILES-11 differs from other file systems in that it also describes the contents of some data files. Some files are simply of type UNDEFINED, but for other file types the record structure is controlled within the file system rather then being purely at the mercy of a third party application. An example of this would be a text file. Use an editor under VMS to open a text file and what will appear in the screen is a sequence of records, just as if you used Notepad under Windows. Look at the raw data though and there is a different story. Using a hex editor to view a WINDOWS or UNIX text file and what will be seen is the characters that form the textual records, each delimited by a control code (0x0d 0x0a, or a single 0x0d or 0x0a). (These latter two type of file can be supported under FILES-11 as type STRMCR and STRMLF). Look at a similar file under VMS and what you will see is the text information, but instead of delimitation each record begins with a binary value which is the length of the record rounded up to 2 bytes, and a NULL vale in the final byte position if the number of textual characters is an odd value. This is a file of record structure type VARIABLE. In order that the text editor under VMS will know not just how long each record is, but also to treat the end of each record as if there is a "new-line" control, FILES-11 also supports a further file attribute which is known as the "record attribute". In the example instance the FILES11 record attribute is "CR", so there is an implicit carriage return.

VMS FILES11 (ODS) Data Recovery

The FILES-11 file system is just as vulnerable to corruption as any other file system. Read errors caused by a failing hard drive, system problem causing data to be incorrectly flushed, and the deletion of files can all cause problem for which data recovery is the answer. It is practicable to recover the objects from a FILES11 file system even following some quite major problems. The default starting position for the INDEXF.SYS file is the middle of the disks (although optionally it can begins at the start or the end of the disk) and so any corruption or overwriting of data at the beginning of the physical disk will not cause major damage to the file system. The contents of the INDEXF.SYS file can be trawled and processed to rebuild a virtual VMS file system that can then be used to access all available files.

Complications do arise when dealing with the VMS approach to file attributes if the recovery is carried out to a different file system as the data then has to be transferred back to the VMS system in a form that is usable. Any data recovery process that simply recovers the allocated data for the files will not be adequate as once these files are transferred back to a FILES-11 system they will have lost their record structure and record attribute information. The VMS data recovery process must also provide the information back that allows the file attributes to be re-set to their original values.

Last Updated (Tuesday, 09 June 2009 15:52)

 
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