Files-11 History Overview Directory Layout Disk Organization

Files-11 is similar to, but significantly more advanced than, the filesystems used in previous Digital Equipment Corporation operating systems such as TOPS-20 and RSTS/E.

1 History

The native OpenVMS filesystem is descended from older DEC operating systems, and is similar in many ways. One of the major differences, however, is the layout of directories. These filesystems all provided some form of rudimentary non-hierarchical directory structure, typically based on assigning one directory per user account. Under RSTS/E, each user account was represented by two numbers, a [project,programmer] pair, and had an associated directory. Special system files, such as program executables and the OS itself, were stored in the directory of a reserved system account.

While this was suitable for PDP-11 systems, which possessed limited permanent storage capacity, the advent of VAX systems with very large hard drives (for the time) required a more flexible method of file storage—hierarchical directory layout in particular, the most notable improvement in ODS-2.

2 Overview

"Files-11" is the general term for five separate filesystems, known as on-disk structure (ODS) levels 1, through 5. Support for each filesystem is via individual ancillary control processes (ACPs), one per each of the ODS levels.

ODS-1 is the flat filesystem used by the RSX-11 OS, supported on older VAX systems for RSX compatibility, but never used to support VMS itself; it has been largely superceded by ODS-2 and ODS-5.

ODS-2 is the standard VMS filesystem, and remains the most common filesystem for system disks (the disk on which VMS itself is installed).

Although seldom referred to by their ODS level designations, ODS-3 and ODS-4 are the Files-11 support for the CDROM ISO 9660ISO 9660 a standard published by the International Organization for Standardization, defines a file system for CD-ROM media. It aims at supporting different computer operating systems such as Microsoft Windows, Mac OS, and systems that follow the UNIX spe and High Sierra filesystems, respectively.

ODS-5 is an extended version of ODS-2 available on AlphaAXP" redirects here. AXP also is the stock symbol for American Express. The DEC Alpha also known as the Alpha AXP is a 64-bit RISC microprocessor originally developed and fabricated by Digital Equipment Corp. DEC), which used it in its own line of worksta and ItaniumIn computing, the Itanium is an IA-64 microprocessor developed jointly by Hewlett-Packard and Intel. The first version, code named Merced shipped in June 2001. Manufactured in a 180 nm process, it was offered at speeds of 733 and 800MHz, with a choice of platforms which adds support for case-preserving filenames with non- ASCIIASCII A merican S tandard C ode for I nformation I nterchange , generally pronounced 'aski', is a character set and a character encoding based on the Roman alphabet as used in modern English and other Western European languages. It is most commonly used b characters and improvements to the hierarchical directory support. It was originally intended for file serving to Microsoft WindowsImage use policy. Microsoft Windows is a range of commercial operating environments for personal computers. The range was first introduced by Microsoft in 1985 and eventually has come to dominate the world personal computer market. All recent versions of or other non-VMS systems as part of the " NT affinity " project, but is also used on user disks and InternetThis article is about the Internet the extensive, worldwide computer network available to the public. An internet is a more general term for a set of interconnected computer networks that are connected by internetworking''. WWW information network structu servers.

2.1 Directory layout

A typical Files-11 directory hierarchy.

All files and directories in a Files-11 filesystem are contained inside one or more parent directories, and eventually under the root directory, the master file directory (see below). The filesystem is therefore organised in a tree-like structure.

In this example (see right), File 2 has a directory entry under both Dir 2 and Dir 3; it is "in" both directories simultaneously. Even if deleted from one, it would still exist in the other directory until deleted from there also.

2.2 Disk organization and naming

An operational VMS system has access to one or more online disks, each of which contains a complete, independent filesystem. These are either local storage or, in the case of a cluster, storage shared with remote systems.

Figure 1: Sample OpenVMS cluster disk configuration.

In an OpenVMS cluster configuration, non-private disks are shared between all nodes in the cluster (see figure 1). In this configuration, the two system disks are accessible to both nodes via the network, but the private disk is not shared: it is mounted for use only by a particular user or process on that machine. Access to files across a cluster is managed by the OpenVMS Distributed Lock Manager, an integral part of the filesystem.

Multiple disks can be combined to form a single large logical disk, or volume set. Disks can also be automatically replicated into shadow sets for data security or faster read performance.

A disk is identified by either its physical name or (more often) by a user-defined logical name. For example, the boot device (system disk) may have the physical name $3$DKA100, but it is generally referred to by the logical name SYS$SYSDEVICE.

Filesystems on each disk (with the exception of ODS-1) are hierarchical. The standard filename format consists of a nodename, a username and password, a device name, directory, filename, file type, and a version number, in the format:

NODE"user pass"::device:[dir.subdir]filename.type;ver

For example, [DIR1.DIR2]FILE.EXT refers to the latest version of FILE.EXT, on the current default disk, in directory [DIR1.DIR2]. DIR1 is a subdirectory of the master file directory (MFD), or root directory, and DIR2 is a subdirectory of DIR1. A disk's MFD is identified by [000000].

Any part of the filename can be omitted, in which case it will be taken from the current default file specification. The default file specification replaces the concept of "current directory" in other operating systems by providing a set of defaults for node, device name, directory and filename. All processes have a default file specification which includes disk name and directory, and most RMS filesystem routineA routine can be any activity that recurs. Patterns of living among people are studied in Ethnomethodology. Routines studied can be as mundane as when, how often, and how people wash their hair, drive their cars, and conversational and talk-related issuess accept a default file specification which can also include the file type; the TYPE command, for example, defaults to ".LIS" as the file type, so the command TYPE F, with no extention, will attempt to open the file F.LIS.

Every file has a version, which is 1 at file creation, and is incremented every time a file is written to; old versions are only deleted when the file's version limit (as set by SET FILE/VERSION_LIMIT) is reached. Old versions are thus not overwritten, but are kept on disk and may be retrieved at any time. The architectural limit on version numbers is 32767.

ODS-2 only supports eight levels of subdirectories, and only uppercase, alphanumeric filenames (plus the underscore, dash, and dollar sign) which are limited to 39 characters for the filename and another 39 for the extension. ODS-5 expands the character set to lowercase letters and most other printable ASCII characters, as well as ISO Latin-1 and Unicode characters, increases the maximum filename length and allows unlimited levels of subdirectories. When constructing a pathname for an ODS-5 file which uses characters not allowed under ODS-2, a special "^" syntax is used to preserve backwards compatibility; the file "file.tar.gz;1" on an ODS-5 disk, for example, would be referred to as "file^.tar.gz"—the file's name is "file.tar", and the extension is ".gz".

2.3 File security: protection and ACLs

VMS file security is defined by two mechanisms, UIC-based access control and ACL-based access control. UIC access control is based on the owner of the file and the UIC, or user, accessing the file. Access is determined by four groups of permissions:

System
Owner
Group
World

And four permission bits:

Read
Write
Execute
Delete

The "system" access applies to any user whose UIC group code is less than or equal to the SYSGEN parameter MAXSYSGROUP (typically 8, or 10 octal) (for example the SYSTEM user); "owner" and "group" apply to the owner of the file and that user's user group, and "world" applies to any other user. There is also a fifth permission bit, "Control", which is used to determine access to change file metadata such as protection. This group cannot be set explicitly; it is always set for System and Owner, and never for Group or World.

UIC-based access control is also affected by four system privileges , which allow users holding them to override access controls:

BYPASS: user implicitly has RWED access to all files, regardless of file protection;
READALL: user implicitly has R access to all files;
SYSPRV: user may access files based on System protection;
GRPPRV: user may access files based on System protection if their UIC group matches the file's group.

ACLs allow additional privileges to be assigned on a user– or group–specific basis; for example, a web server's UIC could be granted read access to all files in a particular directory. ACLs can be marked as inherited, where a directory file's ACL applies to all files underneath it. ACLs are modified using the EDIT/ACL command, and take the form of identifier/access pairs. For example, the ACL entry

(IDENTIFIER=HTTP$SERVER,ACCESS=READ+EXECUTE)

would allow the user HTTP$SERVER to read and execute the file.

2.4 Logical names

A logical name is an additional name given to a disk, or a name used to refer to a particular directory as if it were a disk. For example, the logical SYS$SYSDEVICE is assigned to the system's boot device at startup. A logical name normally refers to a single directory or disk, e.g. SYS$LOGIN: which is the user's login (home) directory (or directories); these logicals cannot be used as true disk names—SYS$LOGIN:[DIR]FILE is not a valid file specification. However, concealed logical names, defined by DEFINE/TRANSLATION=CONCEALED, can be used in that way; these are defined with a trailing "." on the directory specification:

$ DEFINE/TRANS=CONCEAL HOME DISK$USERS:[username.]

would allow HOME:[DIR]FILE to be used.

3 Record-oriented I/O: Record Management Services

Record Management Services is the structured I/O layer of the VMS operating system. RMS provides comprehensive program support for managing structured files, such as record-based and indexed database files. The VMS filesystem, in conjunction with RMS, extends files access past simple byte-streams and allows OS-level support for a variety of rich files types. Each file in the VMS filesystem may be thought of as a database, containing a series of records , each of which has one of more individual fields . A text file, for example, is a list of records (lines) separated by a newline character.

There are four record formats defined by RMS:

Fixed length - all records in the file have the same length.
Variable length - records vary in length, and every record is prefixed by a count byte giving its length.
Variable record length with fixed-length control - records vary in length, but are preceded by a fixed-length control block.
Stream - record vary in length, and every record is separated from the next one by a termination character. A text file is an example of a stream-format file using line feed or carriage return to separate records.

There are four record access methods, or methods to retrieve extant records from files:

Sequential Access - starting with a particular records, subsequent records are retrieved in order until the end of the file.
Relative Record Number Access - records are retrived via a record number relative to the beginning of the file.
Record File Address Access - records are retrieved directly by their location in the file (RFA, or Record File Address).
Indexed Access - records are retrieved via a key, in a form of key-value mapping.

4 Physical layout: the On-Disk Structure

At the disk level, ODS represents the filesystem as an array of blocks, a block being 512 bytes, spanning one or more physical disks (volumes). Groups of blocks are organised into clusters. Each file on the volume is spread across one or more contiguous allocated clusters, called extents. Ideally, each file will have only one extent, but fragmentation issues and growth of files result in additional extents being allocated for single files.

Every file on a Files-11 disk structure has a unique file identification (FID), composed of three numbers: the file number (NUM), the file sequence number (SEQ), and the relative volume number (RVN). The NUM and SEQ combined indicate where in the INDEXF.SYS file (see below) the metadata for the file is located; the RVN indicates the volume number on which the file is stored when using multi-volume file systems.

4.1 Directories

The structural support of an ODS volume is provided by the directory file—a special file containing a list of file names and their associated FIDs. At the root of the directory structure is the master file directory (MFD), the root directory which contains (directly or indirectly) every file on the volume.

This diagram shows an example directory containing 3 files, and the way each filename is mapped to the INDEXF.SYS entry (each INDEXF entry contains more information; only the first few items are shown here).

4.2 The Master File Directory

At the top level of an ODS filesystem is the master file directory (MFD), which contains all top-level directory files (including itself), and several system files used to store filesystem information. On ODS-1 volumes, a two-level directory structure is used: each user identifiation code (UIC) has an associated user file directory (UFD), of the form [GROUP.USER]. On ODS-2 and later volumes, the layout of directories under the MFD is free-form, subject to a limit on the nesting of directories (8 levels on ODS-2 and unlimited on ODS-5). On multi-volume sets, the MFD is always stored on the first volume, and contains the subdirectories of all volumes.

The following system files are present in the ODS MFD:

INDEXF.SYS;1—Index file
BITMAP.SYS;1—Storage bitmap file
BADBLK.SYS;1—Bad block file
000000.DIR;1—The MFD directory file itself
CORIMG.SYS;1—Core image file
VOLSET.SYS;1—Volume set list file (ODS-2/5 only)
CONTIN.SYS;1—Continuation file (ODS-2/5 only)
BACKUP.SYS;1—Backup log file (ODS-2/5 only)
BADLOG.SYS;1—Pending bad block (ODS-2/5 only)
SECURITY.SYS;1—Volume security profile (ODS-2/5 only)
QUOTA.SYS;1—Quota file (ODS-2/5 only)

4.2.1 Index file: INDEXF.SYS

The index file contains the most basic information about a Files-11 volume set. Block 1 is the boot block, which contains the location of the primary boostrap image, used to load the VMS operating system. This is nearly always located at physical block 0 on the disk, so that the hardware firmware can read it. This block is present even on non-system (non-bootable) volumes.

After the boot block is the home block. This contains the volume name, the location of the location of the extents comprising the remainder of the index file, the volume owner's UIC, and the volume protection information. There are normally several copies of the home block, to allow recovery of the volume if it is lost or damaged.

The rest of the index file is composed of file headers, which describe the extents allocated to the files residing on the volume, and file metadata such as the owner UIC, ACLs and protection information. Each file is described by one or more file headers—more than one can be required when a file has a large number of extents. The file header is a fixed-length block, but contains both fixed– and variable–length sections:

The header contains the NUM and SEQ, the protection (security) information, and the location(s) of the rest of the file header.
The ident section contains the accounting metadata: the filename, creation and modification times, and the time of the last backup.
The map describes which physical disk blocks (extents) map to each virtual block of the file.
The access control list contains the ACL information for the file.
The reserved area is space at the end of the file header which is not used by the operating system. This can be used by for customer- or vendor-specific information.
The last two bytes of the header are a checksum of the previous 255 words, to verify the validity of the header.

If possible, the map and ACL sections of the header are contained completely in the primary header. However, if the ACL is too long, or the file contains too many extents, there will not be enough space in the primary header to store them. In this case, an extension header is allocated to store the overflow information.

Layout of the INDEXF.SYS header.

The file header begins with 4 offsets (IDOFFSET, MPOFFSET, ACOFFSET and ROFFSET). Since the size of the areas after the fixed-length header may vary (such as the map and ACL areas), the offsets are required to locate these additional areas. Each offset is the number of 16-bit words from the beginning of the file header to the beginning of that area.

If the file requires multiple headers, the extension segment number (SEGNUM) contains the sequence number of this header, beginning at 0.

STRUCLEV contains the current structure level (in the high byte) and version (in the low byte) of the filesystem; ODS-2 being strucure level 2. An increase in the version number indicates a backwards-compatible change that older software may ignore; changes in the structure level itself are incompatible.

W_FID (containing three values: FID_NUM, FID_SEQ and FID_RVN, corresponding to the file, sequence, and relative volume number) contains the ID of this file; EXT_FID (again composed of three values) holds the location of the next extension header, if any. In both of these values, the RVN is specified as 0 to represent the "current" volume (0 is not normally a valid RVN).

FILECHAR contains several flags which affect how the file is handled or organised:

NOBACKUP causes this file to be ignored when a backup is run.
WRITEBACK enables cached (delayed) writes to the file.
READCHECK causes all reads of the file to be done twice, and compared to ensure data integrity.
WRITCHECK results in all writes being verified by a subsequent read and compare.
CONTIGB causes the OS to attempt to allocate storage for the file in as contiguous a manner as possible.
LOCKED is set of the file is deaccess-locked. If set, this indicates that the file was not properly closed after its last use, and the contents may be inconsistent.
CONTIG indicates that the file is stored contiguously on disk; that is, each virtual block is mapped to the logical (physical) block , for some constant .
BADACL if set if the file has an invalid access control list.
SPOOL is set if the file is a spool file, such as an intermediate file used during printing.
DIRECTORY is set if the file is a directory.
BADBLOCK is set if the file contains bad blocks.
MARKDEL is set if the file has been marked for deletion, but is still in use; it will be deleted once closed by the last user.
NOCHARGE, if set, causes space used by the file to not be taken from the owner's storage quota.
ERASE causes the file's contents to be overwritten when it is deleted.

ACCMODE describes the privilege level at which a process must be running in order to access the file. VMS defines four privilege levels: user, supervisor, exec, and kernel. Each type of access - read, write, execute and delete - is encoded as a 2-bit integer.

FILEPROT contains the discretionary access control information for the file. It is divided into 4 groups of 4 bits each: system, owner, group and world. Bit 0 corresponds to read access, 1 to write, 2 to execute and 3 to delete. Setting a bit denies a particular access to a group; clearing it allows it.

If the file header is an extension header, BACKLINK contains the file ID of the primary header; otherwise, it contains the file ID of the directory file containing the primary entry for the file.

4.2.2 Other files

Storage bitmap file: BITMAP.SYS

The bitmap file is responsible for storing information regarding used and available space on a volume. It contains the storage control block (SCB), which includes summary information detailing ???, and the bitmap, an array of bits representing each block and whether it is free or allocated.

Bad block file: BADBLK.SYS

The bad block file contains a list of bad block on the physical volume, so that the system can avoid allocating them to files.

Volume set list file: VOLSET.SYS

The volume set list is located on volume 1 of a volume set, and contains a list of labels of all volumes in the set, and the set's volume name.

Continuation file: CONTIN.SYS

When a file on a multi-volume set crosses the boundary of two constituent volumes, the continuation file is used as its extension header and describes the volume where the rest of the file can be found.

Quota file: QUOTA.SYS

The quota file contains information of each UIC's disk space usage on a volume. It contains a record for each UIC with space allocated to it on a volume, along with information on how much space is being used by that UIC. NOTE: The DISK QUOTA feature is optional and the file will only exist if the feature was ever enabled.

Volume security profile: SECURITY.SYS

The volume security profile contains the volume's owner UIC, the volume protection mask, and its access control list.

5 References

Files-11 On-Disk Structure Specification, Andrew C. Goldstein, VAX/VMS Software Development, 11-Jan-1985.
OpenVMS System Manager's Manual, Volume 2: Tuning, Monitoring, and Complex Systems, Appendix A: Files-11 Disk Structure. Hewlett-Packard Development Company, L.P., September 2003.
Kirby McCoy, VMS File System Internals, Digital Press, 1990. BooksEnthsiast.com.

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