Manual browser: fss(4)

FSS(4) Kernel Interfaces Manual FSS(4)


fssfile system snapshot device


pseudo-device fss 4


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).


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.

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.
Unconfigures a fss device.
Sets the flags of a fss device. Possible flags are:
Unconfigure the fss device on the last close.
Unlink the backing file before the fss device is created.
Gets the flags of a fss device.


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.




The fss device appeared in NetBSD 2.0.
February 24, 2011 NetBSD 7.0