In this article, learn to:
- Verify the integrity of filesystems
- Monitor free space and inodes
- Repair simple filesystem problems
This article covers standard and journaling (also called journaling) filesystems with an emphasis is on ext2 (standard filesystem) and ext3 (journaling filesystem), but tools for other filesystems are mentioned too. Most of this material applies to both 2.4 and 2.6 kernels. Most examples in this article use Fedora 12, with a 2.6.32 kernel. Your results on other systems may differ.
This article helps you prepare for Objective 104.2 in Topic 104 of the Linux Professional Institute Certification level 1 (LPIC-1) exams. The objective has a weight of 2.
To get the most from the articles in this series, you should have a basic knowledge of Linux and a working Linux system on which you can practice the commands covered in this article. Sometimes different versions of a program will format output differently, so your results may not always look exactly like the listings and figures shown here.
You should also be familiar with the material in our article "Learn Linux 101: Create partitions and filesystems."
In cases when your system crashes or loses power, Linux may not be able to cleanly unmount your filesystems. Thus, your filesystems may be left in an inconsistent state, with some changes completed and some not. Operating with a damaged filesystem is not a good idea as you are likely to further compound any existing errors.
The main tool for checking filesystems is
mkfs, is really a front end to
filesystem-checking routines for the various filesystem types. Some of the
underlying check routines are shown in Listing 1.
Listing 1. Some of the fsck programs
[ian@echidna ~]$ ls /sbin/*fsck* /sbin/btrfsck /sbin/fsck /sbin/fsck.ext3 /sbin/fsck.msdos /sbin/dosfsck /sbin/fsck.cramfs /sbin/fsck.ext4 /sbin/fsck.vfat /sbin/e2fsck /sbin/fsck.ext2 /sbin/fsck.ext4dev /sbin/fsck.xfs
You may be surprised to learn that several of these files are hard links to just one file as shown in Listing 2. Remember that these programs may be used so early in the boot process that the filesystem may not be mounted and symbolic link support may not yet be available. See our article Learn Linux, 101: Create and change hard and symbolic links for more information about hard and symbolic links.
Listing 2. One fsck program with many faces
[ian@echidna ~]$ find /sbin -samefile /sbin/e2fsck /sbin/fsck.ext4dev /sbin/e2fsck /sbin/fsck.ext3 /sbin/fsck.ext4 /sbin/fsck.ext2
The system boot process use
fsck with the
-A option to check the root filesystem and any
other filesystems that are specified for checking in the /etc/fstab
control file. If the filesystem was not cleanly unmounted, a consistency
check is performed and repairs are made, if they can be done safely. This
is controlled by the pass (or passno) field (the sixth
field) of the /etc/fstab entry. Filesystems with pass set to zero are not
checked at boot time. The root filesystem has a pass value of 1 and is
checked first. Other filesystems will usually have a pass value of 2 (or
higher), indicating the order in which they should be checked.
fsck operations can run in parallel if the
system determines it is advantageous, so different filesystems are allowed
to have the same pass value, as is the case for the /grubfile and
//mnt/ext3test filesystems shown in Listing 3. Note that
fsck will avoid running multiple filesystem
checks on the same physical disk. To learn
more about the layout of /etc/fstab, check the man pages for
Listing 3. Boot checking of filesystems with /etc/fstab entries
filesystem mount point type options dump pass UUID=a18492c0-7ee2-4339-9010-3a15ec0079bb / ext3 defaults 1 1 UUID=488edd62-6614-4127-812d-cbf58eca85e9 /grubfile ext3 defaults 1 2 UUID=2d4f10a6-be57-4e1d-92ef-424355bd4b39 swap swap defaults 0 0 UUID=ba38c08d-a9e7-46b2-8890-0acda004c510 swap swap defaults 0 0 LABEL=EXT3TEST /mnt/ext3test ext3 defaults 0 2 /dev/sda8 /mnt/xfstest xfs defaults 0 0 LABEL=DOS /dos vfat defaults 0 0 tmpfs /dev/shm tmpfs defaults 0 0 devpts /dev/pts devpts gid=5,mode=620 0 0 sysfs /sys sysfs defaults 0 0 proc /proc proc defaults 0 0
Some journaling filesystems, such as ReiserFS and XFS, might have a pass
value of 0 because the journaling code, rather than
fsck, does the filesystem consistency check and
repair. On the other hand, some filesystems, such as /proc, are built at
initialization time and therefore do need to be checked.
You can check filesystems after the system is booted. You will need root
authority, and the filesystem you want to check should be unmounted first.
Listing 4 shows how to check two of our filesystems, using the device
name, label, or UUID. You can use the
command to find the device given a label or UUID, and the label and UUID,
given the device.
Listing 4. Using fsck to check filesystems
[root@echidna ~]# # find the device for LABEL=EXT3TEST [root@echidna ~]# blkid -L EXT3TEST /dev/sda7 [root@echidna ~]# # Find label and UUID for /dev/sda7 [root@echidna ~]# blkid /dev/sda7 /dev/sda7: LABEL="EXT3TEST" UUID="7803f979-ffde-4e7f-891c-b633eff981f0" SEC_TYPE="ext2" TYPE="ext3" [root@echidna ~]# # Check /dev/sda7 [root@echidna ~]# fsck /dev/sda7 fsck from util-linux-ng 2.16.2 e2fsck 1.41.9 (22-Aug-2009) EXT3TEST: clean, 11/7159808 files, 497418/28637862 blocks [root@echidna ~]# # Check it by label using fsck.ext3 [root@echidna ~]# fsck.ext3 LABEL=EXT3TEST e2fsck 1.41.9 (22-Aug-2009) EXT3TEST: clean, 11/7159808 files, 497418/28637862 blocks [root@echidna ~]# # Check it by UUID using e2fsck [root@echidna ~]# e2fsck UUID=7803f979-ffde-4e7f-891c-b633eff981f0 e2fsck 1.41.9 (22-Aug-2009) EXT3TEST: clean, 11/7159808 files, 497418/28637862 blocks [root@echidna ~]# # Finally check the vfat partition [root@echidna ~]# fsck LABEL=DOS fsck from util-linux-ng 2.16.2 dosfsck 3.0.9, 31 Jan 2010, FAT32, LFN /dev/sda9: 1 files, 1/513064 clusters
If you attempt to check a mounted filesystem, you will usually see a warning similar to the one in Listing 5 where we try to check our root filesystem. Heed the warning and do not do it!
Listing 5. Do not attempt to check a mounted filesystem
[root@echidna ~]# fsck UUID=a18492c0-7ee2-4339-9010-3a15ec0079bb fsck from util-linux-ng 2.16.2 e2fsck 1.41.9 (22-Aug-2009) /dev/sdb9 is mounted. WARNING!!! Running e2fsck on a mounted filesystem may cause SEVERE filesystem damage. Do you really want to continue (y/n)? no check aborted.
It is also a good idea to let
fsck figure out
which check to run on a filesystem; running the wrong check can corrupt
the filesystem. If you want to see what
fsck would do for a given filesystem or set of
filesystems, use the
-N option as shown in
Listing 6. Finding what fsck would do to check /dev/sda7, /dev/sda8, and /dev/sda9
[root@echidna ~]# fsck -N /dev/sda7 /dev/sda fsck from util-linux-ng 2.16.2 [/sbin/fsck.ext3 (1) -- /mnt/ext3test] fsck.ext3 /dev/sda7 [/sbin/fsck.xfs (2) -- /mnt/xfstest] fsck.xfs /dev/sda8 [/sbin/fsck.vfat (3) -- /dos] fsck.vfat /dev/sda9
So far, we have checked ext and vfat filesystems. Let's now check the XFS
filesystem on /dev/sda8. As you can see in Listing 7, the
fsck command simply tells us that we should use
xfs_check command. If there are no errors,
xfs_check does not display any output.
There is a
-v option for verbose output, but it
is much too verbose for a simple check.
Listing 7. Using fsck with XFS
[root@echidna ~]# fsck /dev/sda8 fsck from util-linux-ng 2.16.2 If you wish to check the consistency of an XFS filesystem or repair a damaged filesystem, see xfs_check(8) and xfs_repair(8). [root@echidna ~]# xfs_check /dev/sda8
Monitoring free space
On a storage device, a file or directory is contained in a collection of blocks. Information about a file is contained in an inode, which records information such who the owner is, when the file was last accessed, how large it is, whether it is a directory, and who can read from or write to it. The inode number is also known as the file serial number and is unique within a particular filesystem. See our article Learn Linux, 101: File and directory management for more information on files and directories.
Data blocks and inodes each take space on a filesystem, so you need to monitor the space usage to ensure that your filesystems have space for growth.
The df command
df command displays information about
mounted filesystems. If you add the
the filesystem type is included in the display; otherwise, it is not.
The output from
df for the Fedora 12 system
that we used above is shown in Listing 8.
Listing 8. Displaying filesystem usage
[ian@echidna ~]$ df -T Filesystem Type 1K-blocks Used Available Use% Mounted on /dev/sdb9 ext3 45358500 24670140 18384240 58% / tmpfs tmpfs 1927044 808 1926236 1% /dev/shm /dev/sda2 ext3 772976 17760 716260 3% /grubfile /dev/sda8 xfs 41933232 4272 41928960 1% /mnt/xfstest /dev/sda7 ext3 112754024 192248 106834204 1% /mnt/ext3test /dev/sda9 vfat 2052256 4 2052252 1% /dos
Notice that the output includes the total number of blocks as
well as the number used and free. Also notice the filesystem,
such as /dev/sbd9, and its mount point: / /dev/sdb9. The
entry is for a virtual memory filesystem. These exist only in RAM or swap
space and are created when mounted without need for a
mkfs command. You can read more about tmpfs in
"Common threads: Advanced filesystem implementor's guide,
For specific information on inode usage, use the
-i option on the
command. You can exclude certain filesystem types using the
-x option, or restrict information to just
certain filesystem types using the
Use these multiple times if necessary. See the examples in Listing 9.
Listing 9. Displaying inode usage
[ian@echidna ~]$ df -i -x tmpfs Filesystem Inodes IUsed IFree IUse% Mounted on /dev/sdb9 2883584 308920 2574664 11% / /dev/sda2 48768 41 48727 1% /grubfile /dev/sda8 20976832 3 20976829 1% /mnt/xfstest /dev/sda7 7159808 11 7159797 1% /mnt/ext3test /dev/sda9 0 0 0 - /dos [ian@echidna ~]$ df -iT -t vfat -t ext3 Filesystem Type Inodes IUsed IFree IUse% Mounted on /dev/sdb9 ext3 2883584 308920 2574664 11% / /dev/sda2 ext3 48768 41 48727 1% /grubfile /dev/sda7 ext3 7159808 11 7159797 1% /mnt/ext3test /dev/sda9 vfat 0 0 0 - /dos
You may not be surprised to see that the FAT32 filesystem does not have inodes. If you had a ReiserFS filesystem, its information would also show no inodes. ReiserFS keeps metadata for files and directories in stat items. And since ReiserFS uses a balanced tree structure, there is no predetermined number of inodes as there are, for example, in ext2, ext3, or xfs filesystems.
There are several other options you may use with
df to limit the display to local filesystems or
control the format of output. For example, use the
-H option to display human readable sizes, such
as 1K for 1024, or use the
--si) option to get sizes in powers of 10
If you aren't sure which filesystem a particular part of your directory
tree lives on, you can give the
df command a
parameter of a directory name or even a filename as shown in Listing 10.
Listing 10. Human readable output for df
[ian@echidna ~]$ df --si ~ian/index.html Filesystem Size Used Avail Use% Mounted on /dev/sdb9 47G 26G 19G 58% /
The tune2fs command
The ext family of filesystems also has a utility called
tune2fs, which can be used to inspect
information about the block count as well as information about whether the
filesystem is journaled (ext3 or ext4) or not (ext2). The command can
also be used to set many parameters or convert an ext2 filesystem to ext3
by adding a journal. Listing 11 shows the output for a near-empty ext3
filesystem using the
-l option to simply
display the existing information.
Listing 11. Using tune2fs to display ext4 filesystem information
[root@echidna ~]# tune2fs -l /dev/sda7 tune2fs 1.41.9 (22-Aug-2009) Filesystem volume name: EXT3TEST Last mounted on: <not available> Filesystem UUID: 7803f979-ffde-4e7f-891c-b633eff981f0 Filesystem magic number: 0xEF53 Filesystem revision #: 1 (dynamic) Filesystem features: has_journal ext_attr resize_inode dir_index filetype needs_recovery sparse_super large_file Filesystem flags: signed_directory_hash Default mount options: (none) Filesystem state: clean Errors behavior: Continue Filesystem OS type: Linux Inode count: 7159808 Block count: 28637862 Reserved block count: 1431893 Free blocks: 28140444 Free inodes: 7159797 First block: 0 Block size: 4096 Fragment size: 4096 Reserved GDT blocks: 1017 Blocks per group: 32768 Fragments per group: 32768 Inodes per group: 8192 Inode blocks per group: 512 Filesystem created: Mon Aug 2 15:23:34 2010 Last mount time: Tue Aug 10 14:17:53 2010 Last write time: Tue Aug 10 14:17:53 2010 Mount count: 3 Maximum mount count: 30 Last checked: Mon Aug 2 15:23:34 2010 Check interval: 15552000 (6 months) Next check after: Sat Jan 29 14:23:34 2011 Reserved blocks uid: 0 (user root) Reserved blocks gid: 0 (group root) First inode: 11 Inode size: 256 Required extra isize: 28 Desired extra isize: 28 Journal inode: 8 Default directory hash: half_md4 Directory Hash Seed: 2438df0d-fa91-4a3a-ba88-c07b2012f86a Journal backup: inode blocks
For XFS filesystems you can display the same information that
mkfs.xfs displayed when the filesystem was
created using the
xfs_info as shown in Listing
12. You need to use
xfs_info on a mounted
Listing 12. Using xfs_info to display XFS filesystem information
[root@echidna ~]# xfs_info /dev/sda8 meta-data=/dev/sda8 isize=256 agcount=4, agsize=2622108 blks = sectsz=512 attr=2 data = bsize=4096 blocks=10488429, imaxpct=25 = sunit=0 swidth=0 blks naming =version 2 bsize=4096 ascii-ci=0 log =internal bsize=4096 blocks=5121, version=2 = sectsz=512 sunit=0 blks, lazy-count=1 realtime =none extsz=4096 blocks=0, rtextents=0
The du command
df command gives information about a
whole filesystem. Sometimes you might want to know how much space is used
by your home directory, or how big a partition to use if you
wanted to move /usr to its own filesystem. To answer this kind of
question, use the
du command displays information about the
filename (or filenames) given as parameters. If a directory name is given,
du recurses and calculates sizes for every
file and subdirectory of the given directory. The result can be a lot of
output. Fortunately, you can use the
to request just a summary for a directory. If you use
du to get information for multiple directories,
then add the
-c option to get a grand total.
You can also control output format with the same set of size options
--si, and so on) that are used for
df. Listing 13 shows two views of the home
directory of a newly created user who has logged in once and created an
Listing 13. Using du
[testuser1@echidna ~]$ du -hc * 4.0K Desktop 4.0K Documents 4.0K Downloads 16K index.html 4.0K Music 4.0K Pictures 4.0K Public 4.0K Templates 4.0K Videos 48K total [testuser1@echidna ~]$ du -hs . 1.1M .
The reason for the difference between the 48K total from
du -c * and the 1.1M
du -s is that the
latter includes the entries starting with a dot, such as .bashrc, while
the former does not.
One other thing to note about
du is that you
must be able to read the directories that you are running it against.
So now, let's use
du to display the total space
used by the /usr tree and each of its first-level subdirectories. The
result is shown in Listing 14. Use root authority to make sure you have
appropriate access permissions.
Listing 14. Using du on /usr
[root@echidna ~]# du -shc /usr/* 394M /usr/bin 4.0K /usr/etc 4.0K /usr/games 156M /usr/include 628K /usr/kerberos 310M /usr/lib 1.7G /usr/lib64 110M /usr/libexec 136K /usr/local 30M /usr/sbin 2.9G /usr/share 135M /usr/src 0 /usr/tmp 5.7G total
Occasionally, very occasionally we hope, the worst will happen and you will
need to repair a filesystem because of a crash or other failure to unmount
fsck command that you saw above
can repair filesystems as well as check them.
Usually the automatic boot-time check will fix the problems and you can proceed.
If the automatic boot-time check of filesystems is unable to restore
consistency, you are usually dumped into a single user shell with some
instructions to run
fsck manually. For an ext2
filesystem, which is not journaled, you may be presented with a series of
requests asking you to confirm proposed actions to fix particular blocks
on the filesystem. You should generally allow
fsck to attempt to fix problems, by responding
y (for yes). When the system reboots, check for
any missing data or files.
If you suspect corruption, or want to run a check manually, most of the
checking programs require the filesystem to be unmounted, or at least
mounted read-only. Because you can't unmount the root filesystem on a
running system, the best you can do is drop to single user mode (using
telinit 1) and then remount the
root filesystem read-only, at which time you should be able to perform a
consistency check. A better way to check a filesystem is to boot a
recovery system, such as a live CD or a USB memory key, and perform the
check of your unmounted filesystems from that.
fsck cannot fix the problem, you do have some
other tools available, although you will generally need advanced
knowledge of the filesystem layout to successfully fix it.
fsck scan of an ext2 disk can take quite a
while to complete, because the internal data structure (or
metadata) of the filesystem must be scanned completely. As
filesystems get larger and larger, this takes longer and longer, even
though disks also keep getting faster, so a full check may take one or
This problem was the impetus for journaled, or journaling,
filesystems. Journaled filesystems keep a log of recent changes to the
filesystem metadata. After a crash, the filesystem driver inspects the log
in order to determine which recently changed parts of the filesystem may
possibly have errors. With this design change, checking a journaled
filesystem for consistency typically takes just a matter of seconds,
regardless of filesystem size. Furthermore, the filesystem driver will
usually check the filesystem on mounting, so an external
fsck check is generally not required. In fact,
for the xfs filesystem,
fsck does nothing!
If you do run a manual check of a filesystem, check the man pages for the
reiserfsck, and so on) to determine the
appropriate parameters. The
-p option, when
used with ext2, ext3, or ext4 filesystems will cause
fsck to automatically fix all problems that can
be safely fixed. This is, in fact, what happens at boot time.
We'll illustrate the use of
xfs_check by first running
e2fsck on an empty XFS filesystem and then
xfs_check to fix it. Remember we suggested
that you use the
fsck front end to be sure you
are using the right checker, and we warned you that failure to do so may
result in filesystem corruption.
In Listing 15, we start running
/dev/sda8, which contains an XFS filesystem. After a few interactions we
use ctrl-Break to break out, but it is too late. Warning:
Do NOT do this unless you are willing to destroy your
Listing 15. Deliberately running e2fsck manually on an XFS filesystem
[root@echidna ~]# e2fsck /dev/sda8 e2fsck 1.41.9 (22-Aug-2009) /dev/sda8 was not cleanly unmounted, check forced. Resize inode not valid. Recreate<y>? yes Pass 1: Checking inodes, blocks, and sizes Pass 2: Checking directory structure Pass 3: Checking directory connectivity Pass 4: Checking reference counts Pass 5: Checking group summary information Free blocks count wrong for group #0 (31223, counted=31224). Fix<y>? ctrl-Break /dev/sda8: e2fsck canceled. /dev/sda8: ***** FILE SYSTEM WAS MODIFIED *****
Even if you broke out at the first prompt, your XFS filesystem would still have been corrupted. Repeat after me. Do NOT do this unless you are willing to destroy your filesystem.
Now let's use
xfs_check to repair the XFS
xfs_check command is quite
verbose, but it has a
-s option which reports
only serious errors. The output is shown in Listing 16.
Listing 16. Repairing the XFS filesystem using xfs_check
[root@echidna ~]# xfs_check -s /dev/sda8 cache_node_purge: refcount was 1, not zero (node=0x1cf3ee0) xfs_check: cannot read root inode (117) cache_node_purge: refcount was 1, not zero (node=0x1cf7400) xfs_check: cannot read realtime bitmap inode (117) bad magic # 0x1040000 in btbno block 0/1 bad magic # 0x4000 in btcnt block 0/2 bad magic # 0x58465342 in inobt block 0/0
You can also use
xfs_repair to repair an XFS
xfs_check, it is quite
and it does not have an
-s option. If you'd like
just to see what needs repair without actually repairing it, use
You may be wondering how all these checking and repairing tools know where to
start. Linux and UNIX filesystems usually have a superblock,
which describes the filesystem metadata, or data describing the
filesystem itself. This is usually stored at a known location, frequently
at or near the beginning of the filesystem, and replicated at other
well-known locations. You can use the
mke2fs to display the superblock locations
for an existing filesystem. If you specified parameters such as the bytes
per inode ratio, you should invoke
the same parameters when you use the
Listing 17 shows the location of the superblocks on /dev/sda7.
Listing 17. Finding superblock locations
[root@echidna ~]# mke2fs -n /dev/sda7 mke2fs 1.41.9 (22-Aug-2009) Filesystem label= OS type: Linux Block size=4096 (log=2) Fragment size=4096 (log=2) 7159808 inodes, 28637862 blocks 1431893 blocks (5.00%) reserved for the super user First data block=0 Maximum filesystem blocks=4294967296 874 block groups 32768 blocks per group, 32768 fragments per group 8192 inodes per group Superblock backups stored on blocks: 32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208, 4096000, 7962624, 11239424, 20480000, 23887872
There are several more advanced tools that you can use to examine or repair a filesystem. Check the man pages for the correct usage and the Linux Documentation Project (see Resources) for how-to information. Almost all of these commands require a filesystem to be unmounted, although some functions can be used on filesystems that are mounted read-only. A few of the commands are described below.
You should always back up your filesystem before attempting any repairs.
Tools for ext2 and ext3 filesystems
- Adjusts parameters on ext2 and ext3 filesystems. Use this to add a journal to an ext2 system, making it an ext3 system, as well as display or set the maximum number of mounts before a check is forced. You can also assign a label and set or disable various optional features.
- Prints the super block and block group descriptor information for an ext2 or ext3 filesystem.
- Is an interactive file system debugger. Use it to examine or change the state of an ext2 or ext3file system.
Tools for Reiserfs filesystems
- Displays and adjusts parameters on ReiserFS filesystems.
- Performs similar functions to dumpe2fs and debugfs for ReiserFS filesystems.
Tools for XFS filesystems
- Displays XFS filesystem information.
- Expands an XFS filesystem (assuming another partition is available).
- Changes the parameters of an XFS filesystem.
- Repairs an XFS filesystem when the mount checks are not sufficient to repair the system.
- Examines or debugs an XFS filesystem.
We will wrap up our tools review with an illustration of the
debugfs command, which allows you to explore the
inner workings of an ext family filesystem. By default, it opens the
filesystem in read-only mode. It does have many commands that allow you to
attempt undeletion of files or directories, as well as other operations
that require write access, so you will specifically have to enable write
access with the
-w option. Use it with extreme
care. Listing 18 shows how to open the root filesystem on my system;
navigate to my home directory; display information, including the inode
number, about a file called index.html; and finally, map that inode number
back to the pathname of the file.
Listing 18. Using debugfs
[root@echidna ~]# debugfs /dev/sdb9 debugfs 1.41.9 (22-Aug-2009) debugfs: cd home/ian debugfs: pwd [pwd] INODE: 165127 PATH: /home/ian [root] INODE: 2 PATH: / debugfs: stat index.html Inode: 164815 Type: regular Mode: 0644 Flags: 0x0 Generation: 2621469650 Version: 0x00000000 User: 1000 Group: 1000 Size: 14713 File ACL: 0 Directory ACL: 0 Links: 1 Blockcount: 32 Fragment: Address: 0 Number: 0 Size: 0 ctime: 0x4bf1a3e9 -- Mon May 17 16:15:37 2010 atime: 0x4c619cf0 -- Tue Aug 10 14:39:44 2010 mtime: 0x4bf1a3e9 -- Mon May 17 16:15:37 2010 Size of extra inode fields: 4 Extended attributes stored in inode body: selinux = "unconfined_u:object_r:user_home_t:s0\000" (37) BLOCKS: (0-2):675945-675947, (3):1314836 TOTAL: 4 debugfs: ncheck 164815 Inode Pathname 164815 /home/ian/index.html debugfs: q
We've covered many tools you can use for checking, modifying, and repairing your filesystems. Remember to always use extreme care when using the tools discussed in this article or any other tools. Data loss may be only a keystroke away.
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