tune2fs – adjust tunable filesystem parameters

tune2fs command is one of the advanced unix commands which allows you to adjust various tunable parameters of the ext2/ext3 filesystems. Naturally, it also helps you confirm the existing parameters configured for your filesystems.

Confirm current filesystem parameters with tune2fs

The tunefs -l command will show you all the information contained in a filesystem's superblock. Here's how it typically looks:

ubuntu# tune2fs -l /dev/sda1
tune2fs 1.40-WIP (14-Nov-2006)
Filesystem volume name:   <none>
Last mounted on:          <not available>
Filesystem UUID:          d2ff8a06-74b7-4877-9d37-1873414e25b3
Filesystem magic number:  0xEF53
Filesystem revision #:    1 (dynamic)
Filesystem features:      has_journal filetype needs_recovery sparse_super
Default mount options:    (none)
Filesystem state:         clean
Errors behavior:          Continue
Filesystem OS type:       Linux
Inode count:              2490368
Block count:              4980736
Reserved block count:     249036
Free blocks:              3417990
Free inodes:              2401957
First block:              0
Block size:               4096
Fragment size:            4096
Blocks per group:         32768
Fragments per group:      32768
Inodes per group:         16384
Inode blocks per group:   512
Filesystem created:       Wed Sep 26 02:30:22 2007
Last mount time:          Tue Apr  1 00:17:16 2008
Last write time:          Tue Apr  1 00:17:16 2008
Mount count:              1
Maximum mount count:      29
Last checked:             Tue Apr  1 00:16:22 2008
Check interval:           15552000 (6 months)
Next check after:         Sun Sep 28 00:16:22 2008
Reserved blocks uid:      0 (user root)
Reserved blocks gid:      0 (group root)
First inode:              11
Inode size:               128
Journal inode:            8
Default directory hash:   tea
Directory Hash Seed:      c0c5742c-980a-49b2-ae0b-4e96895376b6
Journal backup:           inode blocks

Reserved space on a Unix filesystem

By default, every filesystem in Unix has some space reserved for the superuser (root). This means that no regular Unix user can fill your filesystem up to 100%, and so it's always going to have enough free space to continue normal function.

As a standard, each filesystem has 5% of space reserved in this way. If you look at the above output, you may notice the following lines there, which regulate the space reservation:

Reserved block count: 249036
Reserved blocks uid: 0 (user root)
Reserved blocks gid: 0 (group root)

Compared to the overall filesystem block count:

Block count: 4980736

this 249036 reserve is exactly 5%. The uid and gid confirm the Unix user id and Unix group id of the user who will be allowed to tap into the reserved space. As I said earlier, it's root.

If you have root access on your system, you can alter this reserved space allocation for any filesystem using tune2fs -m parameter, by specifying the percentage of the space to be reserved.

Here's how we change the default reserve to be 6% of the overall filesystem size:

ubuntu# tune2fs -m 6 /dev/sda1
tune2fs 1.40-WIP (14-Nov-2006)
Setting reserved blocks percentage to 6% (298844 blocks)

And here we change it back. Note how the number of block corresponding to 5% is exactly the figure we've seen earlier – 249036 blocks:

ubuntu# tune2fs -m 5 /dev/sda1
tune2fs 1.40-WIP (14-Nov-2006)
Setting reserved blocks percentage to 5% (249036 blocks)

Default block size for a filesystem

If you ever want to confirm the block size of any filesystem, tune2fs will help you do just that:

ubuntu# tune2fs -l /dev/sda1 | grep Block
Block count:              4980736
Block size:               4096
Blocks per group:         32768

From this example, you can see that the default block size for the filesystem on /dev/sda1 partition is 4096 bytes, or 4k. That's the default block size for ext3 filesystem.

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