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xfs_io tweaks - support multiple files and memory mapped IO, amongst others.

.TH xfs_io 8
.SH NAME
xfs_io \- debug the I/O path of an XFS filesystem
.SH SYNOPSIS
.nf
\f3xfs_io\f1 [ \f3\-c\f1 cmd ] ... [ \f3\-p\f1 prog ] [ \f3\-adFfmrRstx\f1 ] file
.fi
.SH DESCRIPTION
\f2xfs_io\f1 is a debugging tool like \f2xfs_db\f1(8), but is aimed
at examining the regular file I/O path rather than the raw XFS volume
itself.
.PP
The options to \f2xfs_io\f1 are:
.TP 10
\f3\-c\f1 \f2cmd\f1
\f2xfs_io\f1 commands may be run interactively (the default)
or as arguments on the command line.
Multiple \f3\-c\f1 arguments may be given.
The commands are run in the sequence given, then the program exits.
.TP
\f3\-p\f1 \f2prog\f1
Set the program name for prompts and some error messages,
the default value is \f2xfs_io\f1.
.TP
\f3\-F\f1
Allow \f2file\f1 to be ``foreign'', i.e. not in an XFS filesystem.
This mode has a restricted set of commands.
.TP
\f3\-f\f1
Create \f2file\f1 if it does not already exist.
.TP
\f3\-r\f1
Open \f2file\f1 read-only, initially.
.TP
\f3\-x\f1
Expert mode.
Dangerous commands are only available in this mode.
These commands also tend to require additional privileges.
.PP
The other \f2open\f1(2) options described below are also available
from the command line.
.SH CONCEPTS
\f2xfs_io\f1 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.
.PP
\f2xfs_io\f1 commands can be broken up into three groups.
Some commands are aimed at doing regular file I/O - read, write,
sync, space preallocation, etc.
.PP
The second set of commands exist for manipulating memory mapped regions
of a file - mapping, accessing, storing, unmapping, flushing, etc.
.PP
The remaining commands are for the navigation and display of data
structures relating to the open files, mappings, and the filesystems
where they reside.
.PP
Many commands have extensive online help.
Use the \f3help\f1 command for more details on any command.
.SH FILE I/O COMMANDS
.TP 10
\f3open\f1 [ \f2\-FacdfrstR\f1 ] [ \f2path\f1 ]
Closes the current file, and opens the file specified by \f2path\f1 instead.
Without any arguments, displays statistics about the current file \-
see the \f3stat\f1 command.
.br
The \f3\-F\f1 option allows non-XFS (foreign) files to be opened and
operated on with a restricted command set.
.br
The \f3\-a\f1 option opens append-only (O_APPEND).
.br
The \f3\-d\f1 option opens for direct I/O (O_DIRECT).
.br
The \f3\-f\f1 option creates the file if it doesn't already exist (O_CREAT).
.br
The \f3\-r\f1 option opens read-only (O_RDONLY).
.br
The \f3\-s\f1 option opens for synchronous I/O (O_SYNC).
.br
The \f3\-t\f1 option truncates on open (O_TRUNC).
.br
The \f3\-R\f1 option marks the file as a realtime XFS file after
opening it, if it is not already marked as such.
.TP
\f3o\f1
See the \f3open\f1 command.
.TP
\f3close\f1
Closes the current open file, marking the next open file as current
(if one exists).
.TP
\f3c\f1
See the \f3close\f1 command.
.TP
\f3pread\f1 [ \f2\-b bsize\f1 ] [ \f2\-v\f1 ]
Reads a range of bytes in a specified blocksize from the given offset.
.br
The \f3\-b\f1 option can be used to set the blocksize into which the
\f2read\f1(2) requests will be split.
The default blocksize is 4096 bytes.
.br
The \f3\-v\f1 option will dump the contents of the buffer after reading,
by default only the count of bytes actually read is dumped.
.TP
\f3r\f1
See the \f3pread\f1 command.
.TP
\f3pwrite\f1 [ \f2\-i file\f1 ] [ \f2\-d\f1 ] [ \f2\-s skip\f1 ] [ \f2\-b size\f1 ] [ \f2\-S seed\f1 ]
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.
.br
The \f3\-i\f1 option allows an input file to be specified as the source
of the data to be written.
.br
The \f3\-d\f1 option will cause direct I/O, rather than the usual buffered
I/O, to be used when reading the input file.
.br
The \f3\-s\f1 options specifies the number of bytes to skip from the
start of the input file before starting to read.
.br
The \f3\-b\f1 option can be used to set the blocksize into which the
\f2write\f1(2) requests will be split.
The default blocksize is 4096 bytes.
The \f3\-S\f1 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 0xcdcdcdcd.
.TP
\f3w\f1
See the \f3pwrite\f1 command.
.TP
\f3bmap\f1 [ \f2\-adlpv\f1 ] [ \f2\-n nx\f1 ]
Prints the block mapping for the current open file.
Refer to \f2xfs_bmap\f1(8) for complete documentation.
.TP
\f3allocsp\f1 \f2offset\f1 \f2length\f1
Allocates zeroed space for part of a file using the XFS_IOC_ALLOCSP
system call described in \f2xfs\f1(5).
.TP
\f3freesp\f1 \f2offset\f1 \f2length\f1
Frees space for part of a file using the XFS_IOC_FREESP
system call described in \f2xfs\f1(5).
.TP
\f3fadvise\f1 [ \f2\-dnrsw\f1 ]
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 \f3madvise\f1 command,
discussed later.
.TP
\f3fdatasync\f1
Calls \f2fdatasync\f1(2) to flush the file's in-core data to disk.
.TP
\f3fsync\f1
Calls \f2fsync\f1(2) to flush all in-core file state to disk.
.TP
\f3s\f1
See the \f3fsync\f1 command.
.TP
\f3resvsp\f1 \f2offset\f1 \f2length\f1
Allocates reserved, unwritten space for part of a file using the XFS_IOC_RESVSP
system call described in \f2xfs\f1(5).
.TP
\f3unresvsp\f1 \f2offset\f1 \f2length\f1
Frees reserved space for part of a file using the XFS_IOC_UNRESVSP
system call described in \f2xfs\f1(5).
.TP
\f3truncate\f1 \f2offset\f1
Truncates the current file at the given offset using \f2ftruncate\f1(2).

.SH MEMORY MAPPED I/O COMMANDS
.TP
\f3mmap\f1 [ \f2-rwx\f1 ] [ \f2offset\f1 \f2length\f1 ]
With no arguments, \f3mmap\f1 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 (\f2-r\f1), PROT_WRITE (\f2-w\f1), and PROT_EXEC (\f2-x\f1).
.TP
\f3mm\f1
See the \f3mmap\f1 command.
.TP
\f3munmap\f1
Unmaps the current memory mapping.
.TP
\f3mu\f1
See the \f3munmap\f1 command.
.TP
\f3mread\f1 [ \-\f2frv\f1 ]
Accesses a segment of the current memory mapping, optionally dumping it to
the standard output stream (with \f2-v\f1 or \f2-f\f1 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 \f2-r\f1 option in specified.
The two verbose modes differ only in the relative offsets they display,
the \f2-f\f1 option is relative to file start, whereas \f2-v\f1 shows
offsets relative to the start of the mapping.
.TP
\f3mr\f1
See the \f3mread\f1 command.
.TP
\f3mwrite\f1 [ \f2-r\f1 ] [ \f2-S seed\f1 ]
Stores a byte into memory for a range within a mapping.
The default stored value is 'X', repeated to fill the range specified,
but this can be changed using the \f2-S\f1 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 \-\f2r\f1
option in specified.
.TP
\f3mw\f1
See the \f3mwrite\f1 command.
.TP
\f3msync\f1
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 (\f2-i\f1) 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 (\f2-s\f1) or asynchronously (\f2-a\f1).
.TP
\f3ms\f1
See the \f3msync\f1 command.
.TP
\f3madvise\f1 [ \-\f2drwsw\f1 ] [ \f2offset\f1 \f2length\f1 ]
Modifies page cache behavior when operating on the current mapping.
The range arguments are required by some advise commands ([*] below).
With no arguments, the POSIX_MADV_NORMAL advice is implied (default readahead).
The \f2-d\f1 option says the pages will not be needed (POSIX_MADV_DONTNEED[*]).
The \f2-r\f1 option says to expect random page references (POSIX_MADV_RANDOM),
which sets readahead to zero.
The \f2-s\f1 option says to expect sequential page references
(POSIX_MADV_SEQUENTIAL), which doubles the default readahead on the file.
The \f2-w\f1 option advises the specified pages will be needed
again (POSIX_MADV_WILLNEED[*]) which forces the maximum readahead.
.TP
\f3mincore\f1
Dumps a list of pages or ranges of pages that are currently in core,
for the current memory mapping.

.SH OTHER COMMANDS
.TP
\f3print\f1
Display a list of all open files and memory mapped regions.
The current file and current mapping are distinguishable from
any others.
.TP
\f3p\f1
See the \f3print\f1 command.
.TP
\f3quit\f1
Exit \f2xfs_io\f1.
.TP
\f3q\f1
See the \f3quit\f1 command.
.TP
\f3lsattr\f1
List extended inode flags on the currently open file.
.TP
\f3chattr\f1 [ \f2+/\-riasAd\f1 ]
Change extended inode flags on the currently open file.
.TP
\f3freeze\f1
Suspend all write I/O requests to the filesystem of the current file.
Only available in expert mode and requires privileges.
.TP
\f3thaw\f1
Undo the effects of a filesystem freeze operation.
Only available in expert mode and requires privileges.
.TP
\f3inject\f1 [ \f2tag\f1 ]
Inject errors into a filesystem to observe filesystem behavior at
specific points under adverse conditions.
Without an argument, displays the list of error tags available.
Only available in expert mode and requires privileges.
.TP
\f3resblks\f1 [ \f2blocks\f1 ]
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 \f2xfs\f1(5).
Note \-\- this can be useful for exercising out of space behavior.
Only available in expert mode and requires privileges.
.TP
\f3shutdown\f1 [ \f2\-f\f1 ]
Force the filesystem to shutdown (with or without flushing the log).
Only available in expert mode and requires privileges.
.TP
\f3stat\f1 [ \f2\-v\f1 ]
Selected statistics from \f2stat\f1(2) and the XFS_IOC_GETXATTR
system call from \f2xfs\f1(5) on the current file.
If the \f2-v\f1 option is specified, the atime (last access), mtime
(last modify), and ctime (last change) timestamps are also displayed.
.TP
\f3statfs\f1
Selected statistics from \f2statfs\f1(2) and the XFS_IOC_FSGEOMETRY
system call from \f2xfs\f1(5) on the filesystem where the current
file resides.

.SH SEE ALSO
fdatasync(2),
fstat(2),
fstatfs(2),
fsync(2),
ftruncate(2),
mkfs.xfs(8),
mmap(2),
open(2),
pread(2),
pwrite(2),
xfs_bmap(8),
xfs_db(8),
xfs(5).