Preventing data loss

GraphicsMagick does its best to avoid data loss.  Recently it came to 
my attention that a file update pattern used prior to GraphicsMagick 
1.2 may allow unexpected data loss (total loss of file data) on some 
Linux filesystems.  The problematic file update pattern is

   open tmp
   write tmp
   close tmp
   rename tmp original

and was used by 'mogrify'.  The problem is that some Linux filesystems 
prioritize the rename more than the write and close operations and so 
the file could be entirely lost, or corrupted after a system crash or 
power failure (see 
http://thunk.org/tytso/blog/2009/03/12/delayed-allocation-and-the-zero-length-file-problem/). 
It is always possible to lose pending unflushed updates on crash or 
power fail, but this Linux bug allows the rename to be recorded on 
disk before the file data is even written yet.

Starting with GraphicsMagick 1.2, a different file update patten is 
used which should not encounter this particular bug, but if the system 
crashes, the original file may be saved under a different name.  The 
scheme that 1.2 and later uses is

   rename original backup
   open original
   write original
   close original
   remove backup

It is still possible for data to be lost if 'remove backup' is given 
elevated priority similar to 'rename' so that the backup is removed 
before the file data has been written.

As an assurance of file integrity, in GraphicsMagick 1.3 or later a 
fsync() may be enabled prior to every file close by setting the 
environment variable MAGICK_IO_FSYNC to 'TRUE'.  This assures that 
file data is commited to disk before returning to the user.

Outside of 'mogrify' the problematic file update pattern is quite 
commonly used by image processing scripts and so care should be taken 
to not loose good images if the system crashes.  Using 
MAGICK_IO_FSYNC=TRUE is recommended if the file data is very important 
and the system might crash or lose power.

Bob
--
Bob Friesenhahn
[email protected], http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/

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