ecryptfs team mailing list archive

[Daniel Colascione <dan.colascione@xxxxxxxxx>]

The risk isn't data loss; if you forgo fsync, you accept the risk of
some data loss. The issue that started this whole debate is consistency.

The risk here is of the system ending up in an invalid state with zero-
length files *THAT NEVER APPEARED ON THE RUNNING SYSTEM* suddenly
cropping up. A zero-length file in a spot that is supposed to be
occupied by a valid configuration file can cause problems --- an absent
file might indicate default values, but an empty file might mean
something completely different, like a syntax error or (famously)
"prevent all users from logging into this system."

When applications *really* do is create a temporary file, write data to
it, and rename that temporary file to its final name regardless of
whether the original exists. If the filesystem doesn't guarantee
consistency for a rename to a non-existing file, the application's
expectations will be violated in unusual cases causing hard-to-discover
bugs.

Why should an application that atomically updates a file have to check
whether the original exists to get data consistency?

Allocate blocks before *every* rename. It's a small change from the
existing patch. The performance downsides are minimal, and making this
change gives applications the consistency guarantees they expect.

Again: if you accept that you can give applications a consistency
guarantee when using rename to update the contents of a file, it doesn't
make sense to penalize them the first time that file is updated (i.e.,
when it's created.) Unless, of course, you just want to punish users and
application developers for not gratuitously calling fsync.

-- 
Ext4 data loss
https://bugs.launchpad.net/bugs/317781
You received this bug notification because you are a member of eCryptfs,
which is subscribed to ecryptfs-utils in ubuntu.

Status in “ecryptfs-utils” source package in Ubuntu: Invalid
Status in “linux” source package in Ubuntu: Fix Released
Status in ecryptfs-utils in Ubuntu Jaunty: Invalid
Status in linux in Ubuntu Jaunty: Fix Released

Bug description:
I recently installed Kubuntu Jaunty on a new drive, using Ext4 for all my data.

The first time i had this problem was a few days ago when after a power loss ktimetracker's config file was replaced by a 0 byte version . No idea if anything else was affected.. I just noticed ktimetracker right away.

Today, I was experimenting with some BIOS settings that made the system crash right after loading the desktop. After a clean reboot pretty much any file written to by any application (during the previous boot) was 0 bytes.
For example Plasma and some of the KDE core config files were reset. Also some of my MySQL databases were killed...

My EXT4 partitions all use the default settings with no performance tweaks. Barriers on, extents on, ordered data mode..

I used Ext3 for 2 years and I never had any problems after power losses or system crashes.

Jaunty has all the recent updates except for the kernel that i don't upgrade because of bug #315006

ProblemType: Bug
Architecture: amd64
DistroRelease: Ubuntu 9.04
NonfreeKernelModules: nvidia
Package: linux-image-2.6.28-4-generic 2.6.28-4.6
ProcCmdLine: root=UUID=81942248-db70-46ef-97df-836006aad399 ro rootfstype=ext4 vga=791 all_generic_ide elevator=anticipatory
ProcEnviron:
 LANGUAGE=
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcVersionSignature: Ubuntu 2.6.28-4.6-generic
SourcePackage: linux