ext4 and data consistency [LWN.net]

ext4 and data consistency

Posted May 22, 2010 19:15 UTC (Sat) by anton (subscriber, #25547)
In reply to: ext4 and data consistency by Duncan
Parent article: The Next3 filesystem

What bothers me is how they reduced the guarantees and stability of the long mature ext3 filesystem in the aftermath of all this, by defaulting it to data=writeback, a change from the old default data=ordered.

Yes, that's what was at the back of my mind when I wrote about "backporting the data loss feature from ext4 to ext3".

Presumably you used tun2fs or simply fstab to ensure your ext3 mounts remain stable with data=ordered after the kernel in question (was it 2.6.30 or 2.6.31?), right?

The youngest kernel we have is 2.6.30, and according to /proc/mounts it mounts our ext3 file systems with data=ordered. I guess we will go the fstab route once we get a kernel that defaults to data=writeback.

I am a little worried, though, because of what happened after data=journal was no longer the default; I then read that using data=journal resulted in corrupt file systems; I read that for a significant amount of time, and never read that this bug has been fixed (but haven't seen such reports for some time).

So if they made data=ordered non-default in 2.6.31 or some kernel, will they really care if it works? My confidence is limited. We should probably better stick with 2.6.30 until we migrate off extx file systems completely.

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ext4 and data consistency

Posted May 22, 2010 20:36 UTC (Sat) by nix (subscriber, #2304) [Link] (4 responses)

Sticking with 2.6.30 is foolish. Bugs are fixed in ext[34] all the time, sometimes data loss bugs: by sticking with 2.6.30, you're depriving yourself of all of those.

(btw, you can put mount options in the superblock, and avoid modifying /etc/fstab.)

ext4 and data consistency

Posted May 23, 2010 11:44 UTC (Sun) by anton (subscriber, #25547) [Link] (3 responses)

And new bugs are introduced, and if they are for a non-default option like (now) data=ordered, they won't get noticed in time, and they won't get fixed for quite some time; at least that's what the non-default data=journal episode teaches. So what's higher: the risk of data loss from a well-known kernel, or from a new kernel in a non-default setting? Choosing the latter seems foolish to me.

Modifying fstab is not a big deal, why would I want to avoid it. The problem with doing it in the superblock is that I have to do it again when I transfer the system to another disk.

ext4 and data consistency

Posted May 23, 2010 11:50 UTC (Sun) by cortana (subscriber, #24596) [Link] (1 responses)

How can I check whether my distribution has changed the default value of the option in its kernels?

ext4 and data consistency

Posted May 23, 2010 13:29 UTC (Sun) by anton (subscriber, #25547) [Link]

One way is to mount such a file system with the default value (without overriding the default with tune2fs or in fstab), and the checking the actual options in /proc/mounts. That is what I do.

Another way would be to check CONFIG_EXT3_DEFAULTS_TO_ORDERED in the kernel config file.

ext4 and data consistency

Posted May 23, 2010 13:55 UTC (Sun) by nix (subscriber, #2304) [Link]

Well, OK, you're quite within your rights to stick with an old kernel: but I hope you encounter no other security bugs, or stability bugs, or new hardware, or *anything* else that might require a new kernel!