ioctl_getfsmap(2) — Linux manual page


ioctl_getfsmap(2)          System Calls Manual         ioctl_getfsmap(2)

NAME         top

       ioctl_getfsmap - retrieve the physical layout of the filesystem

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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);

DESCRIPTION         top

       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

       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.

       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 request.

       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

       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

       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:

                  The extent is allocated but not yet written.

                  This extent contains extended attribute data.

                  This extent contains extent map information for the

                  Parts of this extent may be shared.

                  The fmr_owner field contains a special value instead
                  of an inode number.

                  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

                  Free space.

                  This extent is in use but its owner is not known or
                  not easily retrieved.

                  This extent is filesystem metadata.

       XFS can return the following special owner values:

                  Free space.

                  This extent is in use but its owner is not known or
                  not easily retrieved.

                  Static filesystem metadata which exists at a fixed
                  address.  These are the AG superblock, the AGF, the
                  AGFL, and the AGI headers.

                  The filesystem journal.

                  Allocation group metadata, such as the free space
                  btrees and the reverse mapping btrees.

                  The inode and free inode btrees.

                  Inode records.

                  Reference count information.

                  This extent is being used to stage a copy-on-write.

                  This extent has been marked defective either by the
                  filesystem or the underlying device.

       ext4 can return the following special owner values:

                  Free space.

                  This extent is in use but its owner is not known or
                  not easily retrieved.

                  Static filesystem metadata which exists at a fixed
                  address.  This is the superblock and the group

                  The filesystem journal.

                  Inode records.

                  Block bit map.

                  Inode bit map.

RETURN VALUE         top

       On error, -1 is returned, and errno is set to indicate the error.

ERRORS         top

       The error placed in errno can be one of, but is not limited to,
       the following:

       EBADF  fd is not open for reading.

              The filesystem has detected a checksum error in the

       EFAULT The pointer passed in was not mapped to a valid memory

       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 request.

              The filesystem does not support this command.

              The filesystem metadata is corrupt and needs repair.

STANDARDS         top


       Not all filesystems support it.

HISTORY         top

       Linux 4.12.

EXAMPLES         top

       See io/fsmap.c in the xfsprogs distribution for a sample program.

SEE ALSO         top


Linux man-pages 6.04           2023-03-30              ioctl_getfsmap(2)

Pages that refer to this page: ioctl(2)