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 currently 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 indicated.

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.

If fss_bstore is a regular file on the file system at fss_mount, the snapshot will be persistent. In this case, if fss_bstore is already a snapshot file from a past state of the file system, the snapshot will be loaded from it; otherwise, fss_bstore will be truncated and a snapshot of the current state of the file system will be saved to it.

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?

HISTORY

The fss device appeared in NetBSD 2.0.