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FSS(4)			 BSD Kernel Interfaces Manual			FSS(4)

NAME
     fss — file system snapshot device

SYNOPSIS
     pseudo-device fss 4

DESCRIPTION
     The fss driver provides a read-only interface to the snapshot of a cur‐
     rently mounted file system.  Reading from a fss device gives the view of
     the file system when the snapshot was taken.  It can be configured via
     ioctl(2).

IOCTLS
     The ioctl(2) command codes below are defined in <sys/dev/fssvar.h>.

     The (third) argument to ioctl(2) should be a pointer to the type indi‐
     cated.

     FSSIOCSET(struct fss_set)
	       Configures a fss device.

		     struct fss_set {
			     char *fss_mount;
			     char *fss_bstore;
			     blksize_t fss_csize;
			     int fss_flags;
		     };

	       The struct element fss_mount is the mount point of the file
	       system.	The struct element fss_bstore is either a regular file
	       or a raw disk device where data overwritten on the file system
	       will be saved.  The struct element fss_csize is the preferred
	       size of this data.  The struct element fss_flags is the initial
	       set of flags.

     FSSIOCGET(struct fss_get)
	       Gets the status of a fss device.

		     struct fss_get {
			     char fsg_mount[MNAMELEN];
			     struct timeval fsg_time;
			     blksize_t fsg_csize;
			     blkcnt_t fsg_mount_size;
			     blkcnt_t fsg_bs_size;
		     };
	       The struct element fsg_mount is the mount point of the file
	       system.	The struct element fsg_time is the time this snapshot
	       was taken.  The struct element fsg_csize is the current size of
	       data clusters.  The struct element fsg_mount_size is the number
	       of clusters of the file system.	The struct element fsg_bs_size
	       is the number of clusters written to the backing store.

     FSSIOCCLR
	       Unconfigures a fss device.

     FSSIOFSET(int)
	       Sets the flags of a fss device.	Possible flags are:

	       FSS_UNCONFIG_ON_CLOSE
			     Unconfigure the fss device on the last close.

	       FSS_UNLINK_ON_CREATE
			     Unlink the backing file before the fss device is
			     created.

     FSSIOFGET(int)
	       Gets the flags of a fss device.

KERNEL THREADS
     For each active snapshot device there is a kernel thread that handles the
     backing store.  This thread is named fssN where N is the device minor
     number.

FILES
     /dev/rfss?
     /dev/fss?

SEE ALSO
     fssconfig(8), mount(8), umount(8)

HISTORY
     The fss device appeared in NetBSD 2.0.

BSD			       February 24, 2011			   BSD

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