fs_vxfs(4)						     Kernel Interfaces Manual							fs_vxfs(4)

NAME

       fs_vxfs - format of a VxFS file system volume

SYNOPSIS

DESCRIPTION

       The  VxFS super-block always begins at byte offset 8192 from the start of the file system.  The super-block location is fixed so that vari-
       ous system utilities know where to locate it.

       Super-block fields contain the following fundamental sizes and offsets:

	      The block size of the file system.
				VxFS supports block sizes of 1024, 2048, 4096, and 8192 bytes.	The default block size depends on the size of  the
				file system when you create it. See the bsize option in mkfs_vxfs(1M) for specific values.

	      The creation date of the file system.
				The system call supplies the time.

	      The default size, in blocks, for indirect data extents.
				Currently set to 64 by default.

	      The number of data blocks in the file system.
				A data block is a block which may be allocated to a file in the file system.

	      A placeholder in instances when the creation
				date for a file system is expanded for more precision.	Currently zero.

	      The size, in bytes, of the immediate data area in each inode.
				Currently 96.

	      The block address of the last log area block.
				You  can  specify the log area size with the command.  If not specified, the default size is 256 blocks for a file
				system less than 8 megabytes in size, 1024 blocks for a file system from 8 megabytes to 512 megabytes,	and  16384
				blocks	for  file systems greater than 512 megabytes in size.  For smaller file systems, the default is reduced to
				avoid wasting space.

	      The block address of the first log area block.
				Currently two.

	      The magic number	for the file system.  This number identifies the file system as a VxFS file system type.

	      The number of file system allocation units.

	      The number of direct extents supported by the
				mapping type (see the section describing inode list).  Currently 10.

	      The number of blocks,
				of size fs_bsize, in the file system.  is an unsigned 32-bit number, so the maximum number of  blocks  in  a  VxFS
				file system is limited to 32 bits.

	      The version number
				of the file system disk layout.

       The  preceding  fields define the size and makeup of the file system.  To reduce the calculations required in utilities, some of the values
       are derived from the fundamental values and placed in the super-block.

       The super-block contains the following derived offsets:

	      The number of data blocks in an allocation unit.

	      The address, in blocks, of the first allocation unit.
				There can be a gap between the end of the intent log and the first allocation unit.  This gap is used to align the
				first allocation unit on a desired boundary.

	      The length of a free extent map in blocks.

	      The length of an allocation unit in blocks.

	      The length, in blocks, of the allocation unit alignment padding.

	      A mask value such that
				(byte_offset fs_bmask) rounds the offset to the nearest smaller block boundary.

	      A mask value such that
				(byte_offset fs_boffmask) yields the offset from the start of the nearest smaller block boundary.

	      The log base 2 of fs_bsize.  Used to convert a byte offset into a block offset.

	      The offset, in blocks, of the first data block
				from the start of an allocation unit.  An allocation unit header may contain padding to align the first data block
				to a specific boundary.

	      A checksum of the above fields.
				A macro, verifies or calculates the checksum.

	      The offset, in blocks, of the free extent map (emap)
				from the start of an allocation unit.

	      The offset, in blocks, of the first data block
				from the start of the file system.

	      The offset, in blocks, of the first free extent map (emap)
				from the start of the file system.

	      The offset, in blocks, of the first free inode map (imap)
				from the start of the file system.

	      The size, in blocks, of an indirect address block.
				An indirect address block is 8K bytes.	This field is set to (8K / fs_bsize).

	      The offset, in blocks, of the free inode map (imap)
				from the start of an allocation unit.

	      The number of inodes in an allocation unit.

	      The number of inode entries per
				block in the inode list.  The VxFS inode is currently 256 bytes long.

	      The log base 2 of fs_inopb.
				Used to convert an inode number into a block offset in the inode list.

	      The log base 2 of fs_aublocks.

	      The number of entries in an indirect address extent.
				An indirect address extent is currently 8192 bytes in length, making the value for 2048.

       The preceding fields are initialized when the file system is created and do not change unless the file system is resized.  These fields are
       replicated in each allocation unit header.

       The following are additional fields which are dynamic:

	      Set to		when  a file system is mounted for read/write access.  Set to on a or successful The file system cannot be mounted
				for read/write access unless the field is

	      An array of the current number of free extents of each extent size in the
				file system.

	      Initial log ID for when the file system is mounted.

	      The following flags are recognized:
				Set when a file system requires a full structural check to recover from an error.  If this flag  is  set,  a  full
				check is performed after the replay recovery completes.

				Set when the file system was mounted with the
				       option.	If this flag is set, then no log replay recovery is performed.

				Set when an I/O error invalidated the log.
				       If this flag is set, no log replay recovery is performed.

				Set when the log ID runs over
				       ( 2^30 ).  The log ID is reset at the next appropriate time (such as a mount or system sync).

				Set when a file system resizing is in progress.
				       If  an detects this flag, it performs a resize recovery.  See fsadm_vxfs(1M) for a description of file sys-
				       tem expansion.

				Set when a file system upgrade is in progress. If an
				       detects this flag, it performs an upgrade recovery.

	      File system name (6 characters).

	      File system pack label (6 characters).

	      The number of free data blocks.

	      The version number of the log format. Set by the
				kernel on each mount to ensure that an running log replay understands the log format written by the kernel.

				The log format may change with each release, so all file systems  should  be  clean  before  upgrading	to  a  new
				release.

	      Set whenever a mounted file system is modified.
				It indicates whether the super-block requires rewriting when a sync operation is performed.

	      Reserved for future use.

	      Last time the super-block was written to disk, indicated as the
				number of seconds and microseconds that have elapsed since 0:00:00 GMT January 1, 1970.

       The  following  fields  are required by VxFS Version 2 disk layouts and later.  These fields are set when the file system is created and do
       not change. They are replicated in each allocation unit header.

	      A checksum of fields.

	      The size, in bytes, of a disk inode. Currently 256 bytes.

	      The number of indirect address levels per inode.

	      The length, in blocks, of a free inode map in an inode allocation unit.

	      The size, in blocks, of an inode allocation unit.

	      The size, in blocks, of the object location table extents pointed to
				by

	      An array of two extent addresses. These extent addresses
				point to the two replicated copies of the first object location table extent.

SEE ALSO

       fsadm(1M), fsadm_vxfs(1M), fsck(1M), fsdb(1M), mkfs(1M), mount(2), time(2), inode_vxfs(4).

																	fs_vxfs(4)