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NAME
ioctl_getfsmap - retrieve the physical layout of the filesystem
SYNOPSIS
#include <sys/ioctl.h>
#include <linux/fs.h>
#include <linux/fsmap.h>
int ioctl(int fd, FS_IOC_GETFSMAP, struct fsmap_head * arg);
DESCRIPTION
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 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 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.
RETURN VALUE
On error, -1 is returned, and errno is set to indicate the error.
ERRORS
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 request.
EOPNOTSUPP
The filesystem does not support this command.
EUCLEAN
The filesystem metadata is corrupt and needs repair.
VERSIONS
The FS_IOC_GETFSMAP operation first appeared in Linux 4.12.
CONFORMING TO
This API is Linux-specific. Not all filesystems support it.
EXAMPLE
See io/fsmap.c in the xfsprogs distribution for a sample program.
SEE ALSO
ioctl(2)
COLOPHON
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