Enscribe Programmer's Guide

(SCF) to label the disk, by estimating how many files you want each directory extent to hold. The
system translates this to an approximate extent size when it creates the actual directory file. The
actual number of files that fit in a directory extent varies according to the types of files involved,
because some file types need larger file labels than other types. Therefore, the actual capacity
might not always be precisely what you specified.
The disk process can potentially create as many as 987 directory extents, so the creation of too
many files to fit in the currently allocated directory extent space merely causes the disk process to
allocate another directory extent.
CAUTION: Never write to a file directory. Any attempt by an application process to alter the
content of a file directory can cause directory corruption and/or a DP2 halt.
Audited Files
In a system with TMF, any database file can be designated as an audited file. To help maintain
database consistency, TMF audits all transactions involving files designated as audited files. That
is, TMF maintains images (in an audit trail) of the database changes made by those transactions.
If necessary, TMF can use the audit trail later to back out failed transactions or to restore audited
files that some system failure has rendered inconsistent.
TMF also uses a record-locking mechanism to perform concurrency control for audited files. This
feature ensures that none of a given transaction's changes are visible to other, concurrent
transactions until all the given transaction's changes are either committed or aborted and backed
out.
Access Coordination
Several different processes can have access to one file at the same time. For coordination of
simultaneous access, each process must indicate, when opening a file, how it intends to use that
file. Each process must specify both an access mode and an exclusion mode.
The access mode specifies which operations are to be performed. The access mode can specify
read/write (the default access mode), read-only, or write-only access.
The exclusion mode specifies how much access is granted to other processes. The exclusion mode
can specify shared, exclusive, or protected access.
The access and exclusion modes operate on a file from the time it is opened until the time it is
closed. To prevent concurrent access to a disk file for short periods of time, two locking mechanisms
are provided: file locking and record locking. TMF enforces a special set of locking rules for audited
files.
Waited and Nowait I/O
The Enscribe software allows an application process to execute concurrently with its file operations
by means of nowait I/O.
The default is waited I/O; when designated file operations are performed, the application process
is suspended until the operation completes.
Nowait I/O means that when designated file operations are performed, the application process
is not suspended. The application process executes concurrently with the file operation. The
application process waits for an I/O completion in a separate file system procedure call.
Waited and nowait I/O are described in the Guardian Programmer's Guide. For more information,
also see the descriptions of the FILE_OPEN_, READ, FILE_READ64_, AWAITIO and
FILE_AWAITIO64_ procedures in the Guardian Procedure Calls Reference Manual.
22 Introducing the Enscribe Record Manager