Editing Stratus VOS (section)

==File system==
VOS supports a number of unique file types:
* Stream files: a stream of binary data, corresponding directly with the concept of a file on other operating systems.
* Fixed files: a sequence of records of a fixed size.
* Relative files: a sequence of records of a fixed maximum record length supporting random access
* Sequential files: a sequence of records of variable size
* Queue files: file-system based backup for [[message queue]]s
* Pipes: [[named pipe]]s for [[inter-process communication]]
* Transaction files: these provide support for journal based rollback
The VOS API allows the creation of multiple indexes per file, sorting according to the contents of a record, or an external key, or an internal key, or a well-defined set of multiple keys.  A VOS file with one or more indexes can be used as a C-ISAM database table.

VOS uses a proprietary file naming syntax which includes the system name, disk name, and directory, with components separated by the ">" symbol. Typically the system disk will be housed in the same module as the CPU, <code>#m1</code>, so a system file for a VOS cluster would be referenced as

 (%system)#m1_d01>system>devices.table

VOS disk allocation  and memory is organised in "blocks", each block being 4,096 bytes. Memory takes the form of RAM or paging. VOS systems support paging partitions and paging files. In modern versions of VOS, paging files can be created dynamically by the SysAdmin (but not removed without a reboot). These paging files can in theory consist of more than one extent (which is viewed by the kernel as a mini-paging partition) which may or may not be contiguous. However, non-contiguous extents are NOT recommended as they greatly increase disk activity. Admin should create the largest possible extent for the paging files as early as possible after the system has been booted.<ref name="Paging Discussion">{{cite web|url=http://groups.google.ie/group/comp.sys.stratus/browse_thread/thread/0cd3ca08e000cd2e?hl=en |title=Google Groups |publisher=Groups.google.ie |date= |accessdate=2016-05-16}}</ref>

===File system security===
VOS supports write, read, execute, and null (no) access to all files, directories and devices (although directories and files have slightly different access lists). Access can be assigned to users, groups, or the world. Only read access is required to run an executable program, provided that the user has "status" access for the directory in which that program resides.

VOS inherited access control lists from [[Multics]] and also implements directory access control lists. If a file does not have an access control list, the containing directory's default access control list applies.

Access to devices is typically controlled by creating a file which is linked to the device by the administrator. (This may be true in OpenVOS, but does not apply to the original operating system.) Access is then given to this file, and this sets the access on the device.