ecryptfs team mailing list archive

["Volodymyr M. Lisivka" <vlisivka@xxxxxxxxx>]

> OK, so let me explain what's going on a bit more explicitly. There are application programmers who are rewriting application files like this:
>
> 1.a) open and read file ~/.kde/foo/bar/baz
> 1.b) fd = open("~/.kde/foo/bar/baz", O_WRONLY|O_TRUNC|O_CREAT) --- this truncates the file
> 1.c) write(fd, buf-of-new-contents-of-file, size-of-new-contents-of-file)
> 1.d) close(fd)
>
> Slightly more sophisticated application writers will do this:
>
> 2.a) open and read file ~/.kde/foo/bar/baz
>2.b) fd = open("~/.kde/foo/bar/baz.new", O_WRONLY|O_TRUNC|O_CREAT)
>2.c) write(fd, buf-of-new-contents-of-file, size-of-new-contents-of-file)
>2.d) close(fd)
>2.e) rename("~/.kde/foo/bar/baz.new", "~/.kde/foo/bar/baz")
>
>What emacs (and very sophisticated, careful application writers) will do is this:
>
>3.a) open and read file ~/.kde/foo/bar/baz
>3.b) fd = open("~/.kde/foo/bar/baz.new", O_WRONLY|O_TRUNC|O_CREAT)
>3.c) write(fd, buf-of-new-contents-of-file, size-of-new-contents-of-file)
>3.d) fsync(fd) --- and check the error return from the fsync
>3.e) close(fd)
>3.f) rename("~/.kde/foo/bar/baz", "~/.kde/foo/bar/baz~") --- this is optional
>3.g) rename("~/.kde/foo/bar/baz.new", "~/.kde/foo/bar/baz")
>
>The fact that series (1) and (2) works at all is an accident. Ext3 in its default configuration happens to have the property that 5 seconds after (1) and (2) completes, the data is safely on disk. (3) is the ***only*** thing which is guaranteed not to lose data. For example, if you are using laptop mode, the 5 seconds is extended to 30 seconds.


The variant (1) is unsafe by design: data can be gone due to software failure. But variant (2) is correct. Both application developer and ext3 assuming following logic behind the scene:

2.a) open and read file ~/.kde/foo/bar/baz
2.b) fd = open("~/.kde/foo/bar/baz.new", O_WRONLY|O_TRUNC|O_CREAT)

transaction_start(fd); // Hidden logic

2.c) write(fd, buf-of-new-contents-of-file, size-of-new-contents-of-file)
2.d) close(fd)
2.e) rename("~/.kde/foo/bar/baz.new", "~/.kde/foo/bar/baz")

transaction_finish(fd);  // Hidden logic

While ext4 and XFS assumes following logic:

2.a) open and read file ~/.kde/foo/bar/baz
2.b) fd1 = open("~/.kde/foo/bar/baz.new", O_WRONLY|O_TRUNC|O_CREAT)
2.c) write(fd, buf-of-new-contents-of-file, size-of-new-contents-of-file)
2.d) close(fd)

transaction_start(); // Hidden logic

2.e) rename("~/.kde/foo/bar/baz.new", "~/.kde/foo/bar/baz")

transaction_finish();  // Hidden logic


Because of that, such problem might happen in many other areas. It cannot be fixed easily just by putting call to fsync(fd), (which is not available in every programming language, BTW).

IMHO, ext4 should respect these hidden transactions. I.e., it should not
reorder file and filesystem operations, which come from same process.

-- 
Ext4 data loss
https://bugs.launchpad.net/bugs/317781
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Status in “ecryptfs-utils” source package in Ubuntu: Invalid
Status in “linux” source package in Ubuntu: Confirmed
Status in ecryptfs-utils in Ubuntu Jaunty: Invalid
Status in linux in Ubuntu Jaunty: Confirmed

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