fs_mkmount - Online in the Cloud

PROGRAM:

NAME

fs_mkmount - Creates a mount point for a volume

SYNOPSIS

fs mkmount -dir <directory> -vol <volume name>
[-cell <cell name>] [-rw] [-fast] [-help]

fs mk -d <directory> -v <volume name>
[-c <cell name>] [-r] [-f] [-h]

DESCRIPTION

The fs mkmount command creates a mount point for the volume named by the -vol argument at
the location in the AFS file space specified by the -dir argument. The mount point looks
like a standard directory element, and serves as the volume's root directory, but is
actually a special file system object that refers to an AFS volume. When the Cache Manager
first encounters a given mount point during pathname traversal, it contacts the VL Server
to learn which file server machines house the indicated volume, then fetches a copy of the
volume's root directory from the appropriate file server machine.

It is possible, although not recommended, to create more than one mount point to a volume.
The Cache Manager can become confused if a volume is mounted in two places along the same
path through the filespace.

The Cache Manager observes three basic rules as it traverses the AFS filespace and
encounters mount points:

Rule 1: Access Backup and Read-only Volumes When Specified
When the Cache Manager encounters a mount point that specifies a volume with either a
".readonly" or a ".backup" extension, it accesses that type of volume only. If a mount
point does not have either a ".backup" or ".readonly" extension, the Cache Manager
uses Rules 2 and 3.

For example, the Cache Manager never accesses the read/write version of a volume if
the mount point names the backup version. If the specified version is inaccessible,
the Cache Manager reports an error.

Rule 2: Follow the Read-only Path When Possible
If a mount point resides in a read-only volume and the volume that it references is
replicated, the Cache Manager attempts to access a read-only copy of the volume; if
the referenced volume is not replicated, the Cache Manager accesses the read/write
copy. The Cache Manager is thus said to prefer a read-only path through the filespace,
accessing read-only volumes when they are available.

The Cache Manager starts on the read-only path in the first place because it always
accesses a read-only copy of the root.afs volume if it exists; the volume is mounted
at the root of a cell's AFS filespace (named /afs by convention). That is, if the
"root.afs" volume is replicated, the Cache Manager attempts to access a read-only copy
of it rather than the read/write copy. This rule then keeps the Cache Manager on a
read-only path as long as each successive volume is replicated. The implication is
that both the "root.afs" and "root.cell" volumes must be replicated for the Cache
Manager to access replicated volumes mounted below them in the AFS filespace. The
volumes are conventionally mounted at the /afs and /afs/cellname directories,
respectively.

Rule 3: Once on a Read/write Path, Stay There
If a mount point resides in a read/write volume and the volume name does not have a
".readonly" or a ".backup" extension, the Cache Manager attempts to access only the
read/write version of the volume. The access attempt fails with an error if the
read/write version is inaccessible, even if a read-only version is accessible. In this
situation the Cache Manager is said to be on a read/write path and cannot switch back
to the read-only path unless mount point explicitly names a volume with a ".readonly"
extension. (Cellular mount points are an important exception to this rule, as
explained in the following discussion.

There are three types of mount points, each appropriate for a different purpose because of
the manner in which the Cache Manager interprets them.

· When the Cache Manager crosses a regular mount point, it obeys all three of the mount
point traversal rules previously described. To create a regular mount point, include
only the required -dir and -vol arguments to the fs mkmount command.

· When the Cache Manager crosses a read/write mount point, it attempts to access only
the volume version named in the mount point. If the volume name is the base
(read/write) form, without a ".readonly" or ".backup" extension, the Cache Manager
accesses the read/write version of the volume, even if it is replicated. In other
words, the Cache Manager disregards the second mount point traversal rule when
crossing a read/write mount point: it switches to the read/write path through the
filespace.

To create a read/write mount point, include the -rw flag on the fs mkmount command. It
is conventional to create only one read/write mount point in a cell's filespace, using
it to mount the cell's "root.cell" volume just below the AFS filespace root (by
convention, /afs/.cellname). See the OpenAFS Quick Start Guide for instructions and
the chapter about volume management in the OpenAFS Administration Guide for further
discussion.

Creating a read/write mount point for a read-only or backup volume is acceptable, but
unnecessary. The first rule of mount point traversal already specifies that the Cache
Manager accesses them if the volume name in a regular mount point has a ".readonly" or
".backup" extension.

· When the Cache Manager crosses a cellular mount point, it accesses the indicated
volume in the specified cell, which is normally a foreign cell. (If the mount point
does not name a cell along with the volume, the Cache Manager accesses the volume in
the cell where the mount point resides.) The Cache Manager disregards the third mount
point traversal rule when crossing a regular cellular mount point: it accesses a read-
only version of the volume if it is replicated, even if the volume that houses the
mount point is read/write. Switching to the read-only path in this way is designed to
avoid imposing undue load on the file server machines in foreign cells.

To create a regular cellular mount point, include the -cell argument on the fs mkmount
command. It is conventional to create cellular mount points only at the second level
in a cell's filespace, using them to mount foreign cells' root.cell volumes just below
the AFS filespace root (by convention, at /afs/foreign_cellname). The mount point
enables local users to access the foreign cell's filespace, assuming they have the
necessary permissions on the ACL of the volume's root directory and that there is an
entry for the foreign cell in each local client machine's /etc/openafs/CellServDB
file. In the output of the fs lsmount command, the cell name and a colon (":") appear
between the initial number sign and the volume name in a regular cellular mount point
name.

OPTIONS

-dir <directory>+
Names the directory to create as a mount point. The directory must not already exist.
Relative pathnames are interpreted with respect to the current working directory.

Specify the read/write path to the directory, to avoid the failure that results from
attempting to create a new mount point in a read-only volume. By convention, the
read/write path is indicated by placing a period before the cell name at the
pathname's second level (for example, /afs/.abc.com). For further discussion of the
concept of read/write and read-only paths through the filespace, see DESCRIPTION.

-vol <volume name>
Specifies the name or volume ID number of the volume to mount. If appropriate, add the
".readonly" or ".backup" extension to the name, or specify the appropriate volume ID
number.

-cell <cell name>
Names the cell in which the volume resides (creates a cellular mount point). Provide
the fully qualified domain name, or a shortened form that disambiguates it from the
other cells listed in the local /etc/openafs/CellServDB file.

If this argument is omitted, no cell indicator appears in the mount point. When the
Cache Manager interprets it, it assumes that the volume named in the mount point
resides in the same cell as the volume that houses the mount point.

-rw Creates a read/write mount point. Omit this flag to create a regular mount point.

-fast
Prevents the Volume Location (VL) Server from checking that the volume has a VLDB
entry and printing a warning message if it does not. Whether or not this flag is
included, the File Server creates the mount point even when the volume has no VLDB
entry.

-help
Prints the online help for this command. All other valid options are ignored.

EXAMPLES

The following command creates a regular mount point, mounting the volume "user.smith" at
/afs/abc.com/usr/smith:

% cd /afs/abc.com/usr
% fs mkmount -dir smith -vol user.smith

The following commands create a read/write mount point and a regular mount point for the
ABC Corporation cell's "root.cell" volume in that cell's file tree. The second command
follows the convention of putting a period at the beginning of the read/write mount
point's name.

% fs mkmount -dir /afs/abc.com -vol root.cell
% fs mkmount -dir /afs/.abc.com -vol root.cell -rw

The following command mounts the State University cell's "root.cell" volume in the ABC
Corporation cell's file tree, creating a regular cellular mount point called
/afs/stateu.edu. When a ABC Corporation Cache Manager encounters this mount point, it
crosses into the State University cell on a read-only path.

% fs mkmount -dir /afs/stateu.edu -vol root.cell -c stateu.edu

PRIVILEGE REQUIRED

The issuer must have the "i" (insert) and "a" (administer) permissions on the ACL of the
directory that is to house the mount point.

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