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Old 07-07-2004, 09:00 AM   #1
qwijibow
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need short paragraph of techno babble translated (about use of /dev/loop)


Quote:
2.2. Use of journaling file systems on loop device
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Don't use a journaling file system on top of file backed loop device. Device
backed loop device can be used with journaling file systems as device backed
loops guarantee that writes reach disk platters in order required by
journaling file system (write caching must be disabled on the disk drive, of
course). With file backed loop devices, correct write ordering may extend
only to page cache (which resides in RAM) of underlying file system. VM can
write such pages to disk in any order it wishes, and thus break write order
expectation of journaling file system.
im going to use loop-aes from loop-aes.sourceforge.net to make an encrypted filesystem image.

am i right, in thinking that the about, is telling me that after i have setup the encrypted loop (pointing to /home/user/filesystemimage)

i should format it as a NON journalling file system (like ext2)
im sure last time i did this i used ext3 !

or maybe its saying dont make a jourrnaling file ssytem withing the encrypted file system ?
AGH !!!!!
help !

thanKs
 
Old 07-07-2004, 09:09 AM   #2
Dark_Helmet
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Yeah, it's saying if you loopback to a file, then you should not use a journalling file system, because the order of the writes cannot be guaranteed.

If you're using a loopback to an actual device, then any filesystem is fine as long as write caching is turned off fo the device.
 
Old 07-07-2004, 09:56 AM   #3
qwijibow
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lol,, my head is still spinning....

OK.. my root filesystem is reiserfs.
and on this root filesystem i have a file BIGFILE.fs
i pointed loop1 at it with losetup.
and i formated it with mkfs.ext2 /dev/loop1

im not using a journalling filesystem with the loop.. but the whole lot is built on top of the root file system. from what i understand, this setup IS okay ?
 
Old 07-07-2004, 10:45 AM   #4
Dark_Helmet
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Yes, you should be ok with that setup. As far as I can tell, the documentation above applies only to the filesystem you are creating on the loopback file (ext2 in this case); not the actual filesystem the loopback file lives in (reiserfs).
 
Old 07-07-2004, 11:43 AM   #5
qwijibow
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great.

just to be sure, i made a 100meg file from /dev/urandom in the encrypted file system, and noted its md5sum.
after many read / writes mounts and re-boots, the md5 has remained constant, which has to be a good sign.

thanks for the help.
 
Old 07-07-2004, 11:58 AM   #6
Dark_Helmet
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lol... I nearly had a heart attack. I thought you meant you had an md5sum of the file you were using as a loopback, and after writing to the loopback file, the md5 hash stayed the same. That would definitely not be a good thing.

It took a second for it to sink in that the md5sum was for the random file within the loopback file and that you were writing to other files in the loopback.

Hehehe... time for me to cut back on the caffeine
 
  


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