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Re: [dm-devel] Error writing to linear mapped device...



I don't have a solid explanation, but I would liken it to having multiple processes writing to the same file at the same time - corruption ensues. I don't think there is any level at which the two mounts could coordinate - they have separate block devices, separate caches, etc. Even if you sync, who flushes first? This would explain your mixed results.

I don't think you can prevent the two simultaneous mounts. However, you should avoid doing this.

 brassow

On May 23, 2005, at 12:21 AM, Manu Tayal wrote:

Hi All,
I tried the following steps:
1) Set up a loopback device
	# dd if=/dev/zero of=/tmp/store1 bs=1024 seek=2047 count=1
	# losetup /dev/loop1 /tmp/store1
	# mke2fs /dev/loop1
	# mount /dev/loop1 /mnt/loopdev

2) Create a linearly mapped device for the loopback device
	# echo 0 $(blockdev --getsize /dev/loop1) linear /dev/loop1 0 |
dmsetup create new
	# mount /dev/mapper/new /mnt/new

Both the devices created and mounted now refer to the same physical device.

3) Write to both the devices separately:
      # touch /mnt/loopdev/a.txt
      # touch /mnt/new/b.txt

OBSERVATION:
===========
1) The file created in /mnt/loopdev is not visible under /mnt/new and
vice-versa. Thought, it might be due to the buffers maintained by devices.
"sync" did not help in flushing.
2) On unmounting the devices and mounting it again gives unpredictable
results:
a. Sometimes, a.txt is visible at both the places, with b.txt missing
at both places.
b. Sometimes, b.txt is visible at both the places, with a.txt missing
at both places.
c. Sometimes, I get "Input/output" error in one of the mount points.


Questions:
=========
1) Can somebody explain the behavior?
2) Once a linear mapping is created for /dev/loop1 as /dev/mapper/new, how
can I prevent the user from using /dev/loop1 and always use /dev/mapper/new
for all the reads and writes?
3) What happens to all the open connections to files being used for
/dev/loop1 after I have mapped it to /dev/mapper/new?



Thanks and Regards, Manu.

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