.TH mmcrsnapshot 02/16/06
mmcrsnapshot Command
.SH "Name"
.PP
\fBmmcrsnapshot\fR - Creates a snapshot of an entire GPFS file system
at a single point in time.
.SH "Synopsis"
.PP
\fBmmcrsnapshot\fR \fIDevice\fR \fIDirectory\fR
.SH "Description"
.PP
Use the \fBmmcrsnapshot\fR command to create a snapshot of an entire
GPFS file system at a single point in time.
.PP
A snapshot is a copy of the changed user data in the file system.
System data and existing snapshots are not copied. The snapshot
function allows a backup or mirror program to run concurrently with user
updates and still obtain a consistent copy of the file system as of the time
the copy was created. Snapshots are exact copies of changed data in the
active files and directories of a file system. Snapshots of a file
system are read-only and are stored in a \fB.snapshots\fR
directory. The files and attributes of the file system may be changed
only in the active copy.
.PP
There is a maximum limit of 31 snapshots per file system. Snapshots may
be deleted only by issuing the
\fBmmdelsnapshot\fR
command. The \fB.snapshots\fR directory cannot be
deleted.
.PP
If the \fBmmcrsnapshot\fR command is issued while a conflicting command
is running, the \fBmmcrsnapshot\fR command waits for that command to
complete. If the \fBmmcrsnapshot\fR command is running while a
conflicting command is issued, the conflicting command waits for the
\fBmmcrsnapshot\fR command to complete. Conflicting operations
include: 
.RS +3
.HP 3
1. Other snapshot commands
.HP 3
2. Adding, deleting, replacing disks in the file system
.HP 3
3. Rebalancing, repairing, reducing disk fragmentation in a file system
.RE
.PP
If quorum is lost before the \fBmmcrsnapshot\fR command completes, the
snapshot is considered partial and will be deleted when quorum is achieved
again.
.PP
Because snapshots are not copies of the entire file system, they should not
be used as protection against media failures. For protection against
media failures, see
\fIGeneral Parallel File
System: Concepts, Planning, and Installation Guide\fR and search
on \fIrecoverability considerations\fR.
.SH "Parameters"
.PP
.RS +3
\fB\fIDevice\fR
\fR
.RE
.RS +9
The device name of the file system for which the snapshot is to be
created. File system names need not be fully-qualified.
\fBfs0\fR is just as acceptable as \fB/dev/fs0\fR.
.PP
This must be the first parameter.
.RE
.PP
.RS +3
\fB\fIDirectory\fR
\fR
.RE
.RS +9
The subdirectory name where the snapshots are stored.
.PP
This is a subdirectory of the root directory and must be a unique name
within the root directory. If you do not want to traverse the root to
access the snapshot you can create a symbolic link to the snapshot by issuing
the
\fBmmsnapdir\fR command.
.RE
.SH "Options"
.PP
NONE
.SH "Exit status"
.PP
.PP
.RS +3
\fB0
\fR
.RE
.RS +9
Successful completion.
.RE
.PP
.RS +3
\fBnonzero
\fR
.RE
.RS +9
A failure has occurred.
.RE
.SH "Security"
.PP
You must have root authority to run the \fBmmcrsnapshot\fR
command.
.PP
You may issue the \fBmmcrsnapshot\fR command from any node in the GPFS
cluster.
.PP
When using the \fBrcp\fR and \fBrsh\fR commands for remote
communication, a properly configured \fB.rhosts\fR file must exist
in the root user's home directory on each node in the GPFS
cluster. If you have designated the use of a different remote
communication program on either the
\fBmmcrcluster\fR
or the
\fBmmchcluster\fR command, you must
ensure:
.RS +3
.HP 3
1. Proper authorization is granted to all nodes in the GPFS cluster.
.HP 3
2. The nodes in the GPFS cluster can communicate without the use of a
password, and without any extraneous messages.
.RE
.SH "Examples"
.PP
To create a snapshot \fBsnap1\fR, for the file system \fBfs1\fR,
issue this command:
.sp
.nf
mmcrsnapshot fs1 snap1
.fi
.sp
.PP
The output is similar to this:
.sp
.nf
Writing dirty data to disk
Quiescing all file system operations
Writing dirty data to disk again
Creating snapshot.
Resuming operations.
.fi
.sp
.PP
Before issuing the command, the directory structure would appear similar
to:
.sp
.nf
/fs1/file1
/fs1/userA/file2
/fs1/userA/file3
.fi
.sp
.PP
After the command has been issued, the directory structure would appear
similar to:
.sp
.nf
/fs1/file1
/fs1/userA/file2
/fs1/userA/file3
/fs1/.snapshots/snap1/file1
/fs1/.snapshots/snap1/userA/file2
/fs1/.snapshots/snap1/userA/file3
.fi
.sp
.PP
If a second snapshot were to be created at a later time, the first snapshot
would remain as is. Snapshots are made only of active file systems, not
existing snapshots. For example:
.sp
.nf
mmcrsnapshot fs1 snap2
.fi
.sp
.PP
The output is similar to this:
.sp
.nf
Writing dirty data to disk
Quiescing all file system operations
Writing dirty data to disk again
Creating snapshot.
Resuming operations.
.fi
.sp
.PP
After the command has been issued, the directory structure would appear
similar to:
.sp
.nf
/fs1/file1
/fs1/userA/file2
/fs1/userA/file3
/fs1/.snapshots/snap1/file1
/fs1/.snapshots/snap1/userA/file2
/fs1/.snapshots/snap1/userA/file3
/fs1/.snapshots/snap2/file1
/fs1/.snapshots/snap2/userA/file2
/fs1/.snapshots/snap2/userA/file3
.fi
.sp
.SH "See also"
.PP
mmdelsnapshot Command
.PP
mmlssnapshot Command
.PP
mmrestorefs Command
.PP
mmsnapdir Command
.SH "Location"
.PP
\fB/usr/lpp/mmfs/bin\fR
.PP