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Filesystem mount options

News Unix filesystems Recommended Links Ext2/Ext3 File System  Ext2-Ext3-Ext4 Attributes
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The ext3/ext4 file system supports several mount options. For example, the acl parameter enables access control lists, while the user_xattr parameter enables user extended attributes. To enable both options, use their respective parameters with -o, as in: 

mount -o acl,user_xattr /dev/device /mount/point

The tune2fs utility  allows administrators to set default mount options in the file system superblock. For more information on this, refer to man tune2fs.

By default, ext4 uses write barriers to ensure file system integrity even when power is lost to a device with write caches enabled. For devices without write caches, or with battery-backed write caches, disable barriers using the nobarrier option, as in:

mount -o nobarrier /dev/device /mount/point

For more information about write barriers, refer to Chapter 16, Write Barriers.

Pervasive Code » Recommended mount options for ext3


if you disable atime updates, using the noatime mount option you can get a performance boost.

This is done by adding noatime to the appropriate lines in /etc/fstab (do it once for each ext3 filesystem that’s listed), in the fourth column, which probably says defaults now.

To make this change to a live, running filesystem, remount the drive (adjust this so that the right disk device is specified at the end of the line:

sudo mount -o noatime,nodiratime,remount,rw /dev/xvda1

(My understanding is that the noatime implies the nodiratime option, but I decided to add it just in case this was not true.)

atime is a relative of the well known file modification and creation timestamps, but it tracks access to file data. That means that if you read one byte from a file, even if it’s cached in RAM, you’re actually also triggering a write to the directory entry for that file, so that its atime can be updated. (If you want to slap your forehead now in disbelief, be my guest.) And if you read a ton of little files (which happens rather often in the unix world), that means a ton of writes to update all of their directory entries. You don’t want that, right?

But do you need it? Almost certainly not. It’s required by the POSIX standard, and the need for it to be present and turned on is well debated by people more knowledgeable about this in this thread from the Linux kernel mailing list. The summary of their argument is that it’s the kernel’s job to remain standards compliant, and only the distributor or user has enough information to know that they don’t care about that part of the standard and can safely disable it. I can understand that point of view.

Well, I did the reading, and you can safely disable it, unless you’re using mutt. If you’re using mutt, or if you’re just nervous about disabling something that somebody somewhere says you might maybe need someday, then disable atime for every filesystem that doesn’t have your mail spool on it, and use the relatime mode on that drive. (relatime is a clever hack that simulates atime behavior while skipping the disk write in certain cases.)

You can turn it dynamically using remount: Attempt to remount an already-mounted file system. This is commonly used to change the mount flags for a file system, especially to make a readonly file system writeable. It does not change device or mount point.


auto -- The filesystem can be mounted automatically (at bootup, or when mount is used with the -a option). This is really unnecessary as this is the default action of mount -a anyway.

mount(8) mount a file system - Linux man page

Options are specified with a -o flag followed by a comma separated string of options. Some of these options are only useful when they appear in the /etc/fstab file. The following options apply to any file system that is being mounted (but not every file system actually honors them - e.g., the sync option today has effect only for ext2, ext3 and ufs):
All I/O to the file system should be done asynchronously.
Update inode access time for each access. This is the default.
Can be mounted with the -a option (this is a default)
Use default options: rw, suid, dev, exec, auto, nouser, and async.
Interpret character or block special devices on the file system.
Permit execution of binaries.
The filesystem resides on a device that requires network access (used to prevent the system from attempting to mount these filesystems until the network has been enabled on the system).
Do not update inode access times on this file system (e.g, for faster access on the news spool to speed up news servers).
Can only be mounted explicitly (i.e., the -a option will not cause the file system to be mounted).
Do not interpret character or block special devices on the file system.
Do not allow execution of any binaries on the mounted file system. This option might be useful for a server that has file systems containing binaries for architectures other than its own.
Do not allow set-user-identifier or set-group-identifier bits to take effect. (This seems safe, but is in fact rather unsafe if you have suidperl(1) installed.)
Forbid an ordinary (i.e., non-root) user to mount the file system. This is the default.
Attempt to remount an already-mounted file system. This is commonly used to change the mount flags for a file system, especially to make a readonly file system writeable. It does not change device or mount point.
Mount the file system read-only.
Mount the file system read-write.
Allow set-user-identifier or set-group-identifier bits to take effect.
All I/O to the file system should be done synchronously.
All directory updates within the file system should be done synchronously. This affects the following system calls: creat, link, unlink, symlink, mkdir, rmdir, mknod and rename.
Allow an ordinary user to mount the file system. The name of the mounting user is written to mtab so that he can unmount the file system again. This option implies the options noexec, nosuid, and nodev (unless overridden by subsequent options, as in the option line user,exec,dev,suid).
Allow every user to mount and unmount the file system. This option implies the options noexec, nosuid, and nodev (unless overridden by subsequent options, as in the option line users,exec,dev,suid).

There are a number of interesting options which can be included using the -o parameter. While these are probably most useful in the context of /etc/fstab (see later), there are occasions where it may be helpful to be aware of them.

For more detail, check out the mount manual (man mount).

-etc-fstab under Linux

As the filesystems in /etc/fstab will eventually be mounted using mount(8) it isn't surprising that the options field simply contains a comma-seperated list of options which will be passed directly to mount when it tries to mount the filesystem.

The options common to all filesystems are:

sync / async
All I/O to the file system should be done (a)synchronously.
The filesystem can be mounted automatically (at bootup, or when mount is passed the -a option). This is really unnecessary as this is the default action of mount -a anyway.
The filesystem will NOT be automatically mounted at startup, or when mount passed -a. You must explicitly mount the filesystem.
dev / nodev
Permit any user to mount the filesyste. This automatically implies noexec,
exec / noexec
Permit/Prevent the execution of binaries from the filesystem.
suid / nosuid
Permit/Block the operation of suid, and sgid bits.
Mount read-only.
Mount read-write.
Permit any user to mount the filesystem. This automatically implies noexec, nosuid,nodev unless overridden.
Only permit root to mount the filesystem. This is also a default setting.
Use default settings. Equivalent to rw,suid,dev,exec,auto,nouser,async.
There are numerous options for the specific filesystes supported by mount.
However these are some of the more useful, for the full list check out the man page for `mount`.
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Old News ;-)

Mount options to improve ext4 file system performance

Web Development Advice and Tips

I recently boosted my rails test suite running time by around 30% by adding certain mount options for my ext4 partition (works for ext3 too). I thought I’d blog about it because the first time I tried my system wouldn’t boot! So here are the step by step instructions:

2) Run:
> tune2fs -o journal_data_writeback /dev/sdXY
Where /dev/sdXY is replaced by the partition that you want to boost

4) Edit fstab

> nano -w /mnt/sdXY/etc/fstab

Find the line that references sdXY. It will look something like:

# /dev/sda2
UUID=be2f0ac2-4683-4550-bcd1-704a1a840b3e / ext4 relatime,errors=remount-ro 0 1

The first entry is the UUID (although on your system this could just be /dev/sdXY). The second entry is the path (/ for me). Third is the fstype (ext3/4). Fourth are the options. Fifth is for dump and sixth is pass. See man fstab(5) for more info.

Change the options to:


(you can leave all of yours in place, if they weren’t the same as mine.

The main ones are replacing atime/relatime with noatime. This causes the FS to not write read-times to a file when read. Think about it. Writing to the FS for every read of the FS? crazy!

Next is data=writeback. This means that metadata for files can be written lazily after the file is written. This will not cause file system corruption, but it may cause the most recent changes to be lost in the event of a crash (so you may jump back into the past a bit).

Next is barrier, which is slightly more dangerous:

barrier=<0|1(*)> This enables/disables the use of write barriers in
the jbd code. barrier=0 disables, barrier=1 enables.
This also requires an IO stack which can support
barriers, and if jbd gets an error on a barrier
write, it will disable again with a warning.
Write barriers enforce proper on-disk ordering
of journal commits, making volatile disk write caches
safe to use, at some performance penalty. If
your disks are battery-backed in one way or another,
disabling barriers may safely improve performance.

Next is nobh:

bh (*) ext4 associates buffer heads to data pages to
nobh (a) cache disk block mapping information
(b) link pages into transaction to provide
ordering guarantees.
“bh” option forces use of buffer heads.
“nobh” option tries to avoid associating buffer
heads (supported only for “writeback” mode).

You can skip barrier and nobh if you’d like. noatime and data=writeback are the big ones.

6) Reboot to your system.

If you have any trouble booting, just boot a recovery disk and revert the fstab changes.

EDIT: Updated to no longer require recovery disk booting thanks to Nicolas Alpi’s response post.

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