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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | EXAMPLES | SEE ALSO | COLOPHON |
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ioctl_fsmap(2) System Calls Manual ioctl_fsmap(2)
ioctl_fsmap, FS_IOC_GETFSMAP - retrieve the physical layout of
the filesystem
Standard C library (libc, -lc)
#include <linux/fsmap.h> /* Definition of FS_IOC_GETFSMAP,
FM?_OF_*, and *FMR_OWN_* constants */
#include <sys/ioctl.h>
int ioctl(int fd, FS_IOC_GETFSMAP, struct fsmap_head * arg);
This ioctl(2) operation retrieves physical extent mappings for a
filesystem. This information can be used to discover which files
are mapped to a physical block, examine free space, or find known
bad blocks, among other things.
The sole argument to this operation should be a pointer to a
single struct fsmap_head:
struct fsmap {
__u32 fmr_device; /* Device ID */
__u32 fmr_flags; /* Mapping flags */
__u64 fmr_physical; /* Device offset of segment */
__u64 fmr_owner; /* Owner ID */
__u64 fmr_offset; /* File offset of segment */
__u64 fmr_length; /* Length of segment */
__u64 fmr_reserved[3]; /* Must be zero */
};
struct fsmap_head {
__u32 fmh_iflags; /* Control flags */
__u32 fmh_oflags; /* Output flags */
__u32 fmh_count; /* # of entries in array incl. input */
__u32 fmh_entries; /* # of entries filled in (output) */
__u64 fmh_reserved[6]; /* Must be zero */
struct fsmap fmh_keys[2]; /* Low and high keys for
the mapping search */
struct fsmap fmh_recs[]; /* Returned records */
};
The two fmh_keys array elements specify the lowest and highest
reverse-mapping key for which the application would like physical
mapping information. A reverse mapping key consists of the tuple
(device, block, owner, offset). The owner and offset fields are
part of the key because some filesystems support sharing physical
blocks between multiple files and therefore may return multiple
mappings for a given physical block.
Filesystem mappings are copied into the fmh_recs array, which
immediately follows the header data.
Fields of struct fsmap_head
The fmh_iflags field is a bit mask passed to the kernel to alter
the output. No flags are currently defined, so the caller must
set this value to zero.
The fmh_oflags field is a bit mask of flags set by the kernel
concerning the returned mappings. If FMH_OF_DEV_T is set, then
the fmr_device field represents a dev_t structure containing the
major and minor numbers of the block device.
The fmh_count field contains the number of elements in the array
being passed to the kernel. If this value is 0, fmh_entries will
be set to the number of records that would have been returned had
the array been large enough; no mapping information will be
returned.
The fmh_entries field contains the number of elements in the
fmh_recs array that contain useful information.
The fmh_reserved fields must be set to zero.
Keys
The two key records in fsmap_head.fmh_keys specify the lowest and
highest extent records in the keyspace that the caller wants
returned. A filesystem that can share blocks between files
likely requires the tuple (device, physical, owner, offset,
flags) to uniquely index any filesystem mapping record. Classic
non-sharing filesystems might be able to identify any record with
only (device, physical, flags). For example, if the low key is
set to (8:0, 36864, 0, 0, 0), the filesystem will only return
records for extents starting at or above 36 KiB on disk. If the
high key is set to (8:0, 1048576, 0, 0, 0), only records below
1 MiB will be returned. The format of fmr_device in the keys
must match the format of the same field in the output records, as
defined below. By convention, the field fsmap_head.fmh_keys[0]
must contain the low key and fsmap_head.fmh_keys[1] must contain
the high key for the operation.
For convenience, if fmr_length is set in the low key, it will be
added to fmr_block or fmr_offset as appropriate. The caller can
take advantage of this subtlety to set up subsequent calls by
copying fsmap_head.fmh_recs[fsmap_head.fmh_entries - 1] into the
low key. The function fsmap_advance (defined in linux/fsmap.h)
provides this functionality.
Fields of struct fsmap
The fmr_device field uniquely identifies the underlying storage
device. If the FMH_OF_DEV_T flag is set in the header's
fmh_oflags field, this field contains a dev_t from which major
and minor numbers can be extracted. If the flag is not set, this
field contains a value that must be unique for each unique
storage device.
The fmr_physical field contains the disk address of the extent in
bytes.
The fmr_owner field contains the owner of the extent. This is an
inode number unless FMR_OF_SPECIAL_OWNER is set in the fmr_flags
field, in which case the value is determined by the filesystem.
See the section below about owner values for more details.
The fmr_offset field contains the logical address in the mapping
record in bytes. This field has no meaning if the
FMR_OF_SPECIAL_OWNER or FMR_OF_EXTENT_MAP flags are set in
fmr_flags.
The fmr_length field contains the length of the extent in bytes.
The fmr_flags field is a bit mask of extent state flags. The
bits are:
FMR_OF_PREALLOC
The extent is allocated but not yet written.
FMR_OF_ATTR_FORK
This extent contains extended attribute data.
FMR_OF_EXTENT_MAP
This extent contains extent map information for the
owner.
FMR_OF_SHARED
Parts of this extent may be shared.
FMR_OF_SPECIAL_OWNER
The fmr_owner field contains a special value instead
of an inode number.
FMR_OF_LAST
This is the last record in the data set.
The fmr_reserved field will be set to zero.
Owner values
Generally, the value of the fmr_owner field for non-metadata
extents should be an inode number. However, filesystems are
under no obligation to report inode numbers; they may instead
report FMR_OWN_UNKNOWN if the inode number cannot easily be
retrieved, if the caller lacks sufficient privilege, if the
filesystem does not support stable inode numbers, or for any
other reason. If a filesystem wishes to condition the reporting
of inode numbers based on process capabilities, it is strongly
urged that the CAP_SYS_ADMIN capability be used for this purpose.
The following special owner values are generic to all
filesystems:
FMR_OWN_FREE
Free space.
FMR_OWN_UNKNOWN
This extent is in use but its owner is not known or
not easily retrieved.
FMR_OWN_METADATA
This extent is filesystem metadata.
XFS can return the following special owner values:
XFS_FMR_OWN_FREE
Free space.
XFS_FMR_OWN_UNKNOWN
This extent is in use but its owner is not known or
not easily retrieved.
XFS_FMR_OWN_FS
Static filesystem metadata which exists at a fixed
address. These are the AG superblock, the AGF, the
AGFL, and the AGI headers.
XFS_FMR_OWN_LOG
The filesystem journal.
XFS_FMR_OWN_AG
Allocation group metadata, such as the free space
btrees and the reverse mapping btrees.
XFS_FMR_OWN_INOBT
The inode and free inode btrees.
XFS_FMR_OWN_INODES
Inode records.
XFS_FMR_OWN_REFC
Reference count information.
XFS_FMR_OWN_COW
This extent is being used to stage a copy-on-write.
XFS_FMR_OWN_DEFECTIVE:
This extent has been marked defective either by the
filesystem or the underlying device.
ext4 can return the following special owner values:
EXT4_FMR_OWN_FREE
Free space.
EXT4_FMR_OWN_UNKNOWN
This extent is in use but its owner is not known or
not easily retrieved.
EXT4_FMR_OWN_FS
Static filesystem metadata which exists at a fixed
address. This is the superblock and the group
descriptors.
EXT4_FMR_OWN_LOG
The filesystem journal.
EXT4_FMR_OWN_INODES
Inode records.
EXT4_FMR_OWN_BLKBM
Block bit map.
EXT4_FMR_OWN_INOBM
Inode bit map.
On error, -1 is returned, and errno is set to indicate the error.
The error placed in errno can be one of, but is not limited to,
the following:
EBADF fd is not open for reading.
EBADMSG
The filesystem has detected a checksum error in the
metadata.
EFAULT The pointer passed in was not mapped to a valid memory
address.
EINVAL The array is not long enough, the keys do not point to a
valid part of the filesystem, the low key points to a
higher point in the filesystem's physical storage address
space than the high key, or a nonzero value was passed in
one of the fields that must be zero.
ENOMEM Insufficient memory to process the operation.
EOPNOTSUPP
The filesystem does not support this operation.
EUCLEAN
The filesystem metadata is corrupt and needs repair.
Linux.
Not all filesystems support it.
Linux 4.12.
See io/fsmap.c in the xfsprogs distribution for a sample program.
ioctl(2)
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user-space interface documentation) project. Information about
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Linux man-pages 6.9.1 2024-06-13 ioctl_fsmap(2)
Pages that refer to this page: ioctl(2)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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