xfs_io(8) System Manager's Manual xfs_io(8)
xfs_io - debug the I/O path of an XFS filesystem
xfs_io [ -c cmd ] ... [ -p prog ] [ -adFfmrRstx ] file
xfs_io is a debugging tool like xfs_db(8), but is aimed at examining
the regular file I/O path rather than the raw XFS volume itself.
The options to xfs_io are:
-c cmd xfs_io commands may be run interactively (the default) or as
arguments on the command line. Multiple -c arguments may be
given. The commands are run in the sequence given, then the
-p prog Set the program name for prompts and some error messages, the
default value is xfs_io.
-F Allow file to reside in non-XFS (foreign) filesystems. This
mode has a restricted set of commands.
-f Create file if it does not already exist.
-r Open file read-only, initially.
-x Expert mode. Dangerous commands are only available in this
mode. These commands also tend to require additional privi-
The other open(2) options described below are also available from the
xfs_io maintains a number of open files and memory mappings. Files can
be initially opened on the command line (optionally), and additional
files can also be opened later.
xfs_io commands can be broken up into three groups. Some commands are
aimed at doing regular file I/O - read, write, sync, space prealloca-
The second set of commands exist for manipulating memory mapped regions
of a file - mapping, accessing, storing, unmapping, flushing, etc.
The remaining commands are for the navigation and display of data
structures relating to the open files, mappings, and the filesystems
where they reside.
Many commands have extensive online help. Use the help command for
more details on any command.
FILE I/O COMMANDS
file [ N ]
Display a list of all open files and (optionally) switch to an
alternate current open file.
open [ -FacdfrstR ] [ path ]
Closes the current file, and opens the file specified by path
instead. Without any arguments, displays statistics about
the current file - see the stat command.
The -F option allows non-XFS (foreign) files to be opened and
operated on with a restricted command set.
The -a option opens append-only (O_APPEND).
The -d option opens for direct I/O (O_DIRECT).
The -f option creates the file if it doesn't already exist
The -r option opens read-only (O_RDONLY).
The -s option opens for synchronous I/O (O_SYNC).
The -t option truncates on open (O_TRUNC).
The -R option marks the file as a realtime XFS file after
opening it, if it is not already marked as such.
o See the open command.
close Closes the current open file, marking the next open file as
current (if one exists).
c See the close command.
pread [ -b bsize ] [ -v ]
Reads a range of bytes in a specified blocksize from the
The -b option can be used to set the blocksize into which the
read(2) requests will be split. The default blocksize is
The -v option will dump the contents of the buffer after
reading, by default only the count of bytes actually read is
r See the pread command.
pwrite [ -i file ] [ -d ] [ -s skip ] [ -b size ] [ -S seed ]
Writes a range of bytes in a specified blocksize from the
given offset. The bytes written can be either a set pattern
or read in from another file before writing.
The -i option allows an input file to be specified as the
source of the data to be written.
The -d option will cause direct I/O, rather than the usual
buffered I/O, to be used when reading the input file.
The -s options specifies the number of bytes to skip from the
start of the input file before starting to read.
The -b option can be used to set the blocksize into which the
write(2) requests will be split. The default blocksize is
4096 bytes. The -S option is used to set the (repeated) fill
pattern which is used when the data to write is not coming
from a file. The default buffer fill pattern value is 0xcd-
w See the pwrite command.
bmap [ -adlpv ] [ -n nx ]
Prints the block mapping for the current open file. Refer to
xfs_bmap(8) for complete documentation.
allocsp offset length
Allocates zeroed space for part of a file using the
XFS_IOC_ALLOCSP system call described in xfs(5).
freesp offset length
Frees space for part of a file using the XFS_IOC_FREESP sys-
tem call described in xfs(5).
fadvise [ -dnrsw ]
On platforms which support it, allows hints be given to the
system regarding the expected I/O patterns on the file. The
hints are similar to those of the madvise command, discussed
fdatasync Calls fdatasync(2) to flush the file's in-core data to disk.
fsync Calls fsync(2) to flush all in-core file state to disk.
s See the fsync command.
resvsp offset length
Allocates reserved, unwritten space for part of a file using
the XFS_IOC_RESVSP system call described in xfs(5).
unresvsp offset length
Frees reserved space for part of a file using the
XFS_IOC_UNRESVSP system call described in xfs(5).
Truncates the current file at the given offset using ftrun-
sendfile -i infile | -f N [ offset length ]
On platforms which support it, allows a direct in-kernel copy
between two file descriptors. The current open file is the
target, the source must be specified as another open file
(-f) or by path (-i).
MEMORY MAPPED I/O COMMANDS
mmap [ -rwx ] [[ N ] | [ offset length ]]
With no arguments, mmap shows the current mappings. Specifying
a single numeric argument sets the current mapping. If two
arguments are specified (a range), a new mapping is created
spanning the range, and the protection mode can be given as a
combination of PROT_READ (-r), PROT_WRITE (-w), and PROT_EXEC
mm See the mmap command.
munmap Unmaps the current memory mapping.
mu See the munmap command.
mread [ -frv ]
Accesses a segment of the current memory mapping, optionally
dumping it to the standard output stream (with -v or -f option)
for inspection. The accesses are performed sequentially from
the start offset by default, but can also be done from the end
backwards through the mapping if the -r option in specified.
The two verbose modes differ only in the relative offsets they
display, the -f option is relative to file start, whereas -v
shows offsets relative to the start of the mapping.
mr See the mread command.
mwrite [ -r ] [ -S seed ]
Stores a byte into memory for a range within a mapping. The
default stored value is 'X', repeated to fill the range speci-
fied, but this can be changed using the -S option. The memory
stores are performed sequentially from the start offset by
default, but can also be done from the end backwards through the
mapping if the -r option in specified.
mw See the mwrite command.
msync Writes all modified copies of pages over the specified range (or
entire mapping if no range specified) to their backing storage
locations. Also, optionally invalidates (-i) so that subsequent
references to the pages will be obtained from their backing
storage locations (instead of cached copies). The flush can be
done synchronously (-s) or asynchronously (-a).
ms See the msync command.
madvise [ -drwsw ] [ offset length ]
Modifies page cache behavior when operating on the current map-
ping. The range arguments are required by some advise commands
([*] below). With no arguments, the POSIX_MADV_NORMAL advice is
implied (default readahead). The -d option says the pages will
not be needed (POSIX_MADV_DONTNEED[*]). The -r option says to
expect random page references (POSIX_MADV_RANDOM), which sets
readahead to zero. The -s option says to expect sequential page
references (POSIX_MADV_SEQUENTIAL), which doubles the default
readahead on the file. The -w option advises the specified
pages will be needed again (POSIX_MADV_WILLNEED[*]) which forces
the maximum readahead.
Dumps a list of pages or ranges of pages that are currently in
core, for the current memory mapping.
print Display a list of all open files and memory mapped regions. The
current file and current mapping are distinguishable from any
p See the print command.
quit Exit xfs_io.
q See the quit command.
lsattr List extended inode flags on the currently open file.
chattr [ +/-riasAd ]
Change extended inode flags on the currently open file.
freeze Suspend all write I/O requests to the filesystem of the current
file. Only available in expert mode and requires privileges.
thaw Undo the effects of a filesystem freeze operation. Only avail-
able in expert mode and requires privileges.
inject [ tag ]
Inject errors into a filesystem to observe filesystem behavior
at specific points under adverse conditions. Without an argu-
ment, displays the list of error tags available. Only available
in expert mode and requires privileges.
resblks [ blocks ]
Get and/or set count of reserved filesystem blocks using the
XFS_IOC_GET_RESBLKS or XFS_IOC_SET_RESBLKS system calls, as
described in xfs(5). Note -- this can be useful for exercising
out of space behavior. Only available in expert mode and
shutdown [ -f ]
Force the filesystem to shutdown (with or without flushing the
log). Only available in expert mode and requires privileges.
stat [ -v ]
Selected statistics from stat(2) and the XFS_IOC_GETXATTR system
call from xfs(5) on the current file. If the -v option is spec-
ified, the atime (last access), mtime (last modify), and ctime
(last change) timestamps are also displayed.
statfs Selected statistics from statfs(2) and the XFS_IOC_FSGEOMETRY
system call from xfs(5) on the filesystem where the current file
fdatasync(2), fstat(2), fstatfs(2), fsync(2), ftruncate(2),
mkfs.xfs(8), mmap(2), open(2), pread(2), pwrite(2), xfs_bmap(8),