Microsoft KB Archive/96741

= How Fault Tolerance Is Implemented =

Article ID: 96741

Article Last Modified on 10/31/2006



This article was previously published under Q96741



SUMMARY
Fault tolerance is implemented (and detected by its utilities) by three types of disk changes: added lines in the CONFIG.SYS file, hidden/system files, and modifications to the partition tables.



MORE INFORMATION
These changes are explained below, but before you read about them, keep in mind two system aspects that are important to fault tolerance:

First, FDISKPM presents the view that there is only one extended partition and one or more logical drives in it, but, each logical drive is in fact an extended partition with its own partition table.

Second, when a drive is mirrored, a partition is created whose system type is unknown to most of the OS/2 system. It does not receive a drive letter, and is virtually inaccessible to the user. For instance, if a mirrored drive does not take up the whole extended area of its disk, FDISKPM honors the allocation of the mirrored partition, but does not display a logical drive. When asked to create a logical drive, the space available is reduced to account for the mirrored partition.

Means of Fault Tolerance Implementation
  Two lines in CONFIG.SYS:      device=c:\ \netprog\diskft.sys run=c:\ \netprog\ftmonit.exe where is the LAN Manager directory (default: LANMAN)  A hidden/system file (FTCFG.DRV) in the root directory on each fault tolerant drive that contains fault tolerance information about that drive. A hidden/system file (FTCFG.SYS) in the root directory of the boot volume that contains system-wide configuration information. FTPEND.SYS, FTPENDF.SYS, and FTPENDCP.SYS, which contain hold information about drives pending mirroring.

Mirrored Partitions
The mirror partitions are created when mirroring is done.

Mirrored partitions appear as one partition to HPFS. For example, HPFS does not know whether it is hotfixing a master or duplicate partition--a hotfix is made on both the master and duplicate partitions.

Mirroring is not done on the file level. It is done on the sector level on a partition basis. Some other operating systems, by contrast, mirror on a sector level on a physical disk basis.

LAN Man Files Involved in Fault Tolerance

 * For LAN Manager 2.0: DISK01.SYS (and DISK02.SYS for ABIOS machines) or a driver supplied by the OEM.
 * For LAN Manager 2.1 and later: FT.VSD (a LADDR system module) provides device-independent support. For every LADDR system disk I/O request, FT.VSD calls DISKFT.SYS to see if the request should be mirrored, and submits the requests to the proper board interface driver (.BID) for a mirrored write. When the .BID completes the request, FT.VSD reports to DISKFT, which checks the request's error status field. DISKFT keeps track of the mirror configurations and statuses, monitors errors on all system drives, and supplies memory for duplicated requests (not memory for request data, only for request headers).

Other Fault Tolerance Files and Programs

 * FTSETUP detects the condition of fault tolerance by looking at the hidden/system files and at the partition tables.
 * FTADMIN uses the hidden/system configuration files to track errors and does not rely as heavily as FTSETUP does on the partition tables.
 * The TABLES.EXE program displays partition table information. You can find it on Utilities Disk 2 of the &quot;LAN Manager Resource Kit.&quot;

Additional query words: 2.10 2.10a 2.20 2.1 2.1a 2.2

Keywords: KB96741

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