Linux – What does the ‘barrier’ mount flag mean in Linux

ext4linuxmount

The manual for the 'barrier' mount option is:

barrier=0 / barrier=1

This disables / enables the use of write barriers in the jbd code.barrier=0 disables, barrier=1 enables (default). This also requires an IO stack which can support barriers, and if jbd gets an error on a barrier write, it will disable barriers 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.

But I do not know what the sentence "proper on-disk ordering of journal commits" means.

Suppose normal order — journal 1,data 1; journal 2, data 2.
Which of the following ordering results will happen if I set barrier=0?

journal 2, data 2; journal 1, data 1;
data 1, journal 1; data 2, journal 2.

Best Answer

Most modern file systems are journaling file systems, which means that they keep track of changes that have not yet been written to disk in an internal data structure called a journal. In the event of a crash, this journal will be replayed, to make sure that all writes performed successfully, preventing file corruption.

When actually writing out the data to disk, the write cache will re-order the writes to try to maximize the throughput, but it must make sure that the actual file data is written to disk before the metadata, to make sure that if a crash occurs the metadata won't be out-of-date with the data.

The problem is that many disks have caches of their own that might re-order the writes as well. Some file systems will assume that that will happen and will force the disk to flush the cache at certain points to prevent this, and is called write barriers in e.g. ext4, and Linux in general.

For modern disks the performance sacrifice for doing this is negligible though, and you shouldn't disable write barriers unless absolutely necessary.

Related Solutions

What does ‘mount point’ mean in a Linux installer

The mount point specifies at which location in the directory hierarchy a device or disk partition appears.

If you want to move /home to a new partition, you have to create a new partition for it, say /dev/sda4 and format it, e.g. with ext4. Creating partitions and formatting them can be comfortably done using e.g. gparted.

Then you have to copy the old contents to the new partition and modify /etc/fstab so /home points to the new partition. As root do something like this after having the partition created and formatted. Again, I assume /dev/sda4 for the partition, this is just an example and you have to use your real partition device:

$ mkdir /mnt/tmphome
$ mount /dev/sda4 /mnt/tmphome
$ cd /home/
$ find . -depth -print0 | cpio --null --sparse -pvd /mnt/tmphome/
$ umount /mnt/newhome
$ mv /home /old_home
$ mkdir /home
$ mount /dev/sda4 /home

Now check if your system is still working correctly. If it does, add a line like this to /etc/fstab:

/dev/sda4 /home ext4    defaults        1 2

and delete the backup in /old_home

if however you find that something went wrong, you can move back by not adding respectively removing the above line in /etc/fstab and doing as root

$ umount /home
$ rmdir /home
$ mv /old_home /home

This answer is inspired by the howto on http://embraceubuntu.com/2006/01/29/move-home-to-its-own-partition/

Linux – What does this Linux kernel trace mean

kernel BUG at fs/ext4/inode.c:2118!
invalid opcode: 0000 [#1] SMP

Appears to be an issue with the ext4 driver in your kernel.

Process mythbackend (pid: 27841, threadinfo ffff88004262a000, task ffff88007fb83330)

mythbackend is triggering it.

[<ffffffff811731df>] mpage_da_map_and_submit+0x2c6/0x2dc
[<ffffffff8117390a>] ext4_da_writepages+0x2d4/0x465

this is the frame stack; what syscalls were being made that led to this.

Best Answer

Related Solutions

What does ‘mount point’ mean in a Linux installer

Linux – What does this Linux kernel trace mean

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