[linux-lvm] dd on a lv?

[Michael Munger]

In preparing a lv for LUKS, I wanted to write random data to the
entirety of the logical volume.

(It's one of two lv's in the vg that are running LUKS, the other lv are
not encrypted, and all reside on the same pv, which is a linux-raid
(mdadm) device at /dev/md0).

I used dd if=/dev/urandom of=/dev/mygroup/mylogicalvolume to write
random data to the entirety of the lv device.

Here's the issue:

The lv is only 4TB (yes... this took days). I had estimated it would
stop some time early this morning. This afternoon when I checked it, dd
had claimed to have written 4.2 TB of data to a 4TB device.

I immediately thought: "How's that possible" and began to worry that it
escaped the logical volume and began to write other areas of the array.
But, dd if=/dev/mygroup/othervol1 bs=1K count=1 | hexdump -C showed a
LUKS header (as it should) and the same command on the other lv showed
the proper header as well. I took this to mean that those lv's were in tact.

How is it possible that I can write 4.2TB to a 4.0 TB logical volume?
Should I be worried that I have somehow overritten something somewhere
else? If so, how do I find and check that? (I have backups, so if I need
to restore, I need to do it now).

P.S. When I did this on the other lv, which is only 100GB, dd behaved as
expected, and wrote only 100GB of random data to the logical volume.

[Zdenek Kabelac]

Post by Michael Munger
In preparing a lv for LUKS, I wanted to write random data to the
entirety of the logical volume.
(It's one of two lv's in the vg that are running LUKS, the other lv are
not encrypted, and all reside on the same pv, which is a linux-raid
(mdadm) device at /dev/md0).
I used dd if=/dev/urandom of=/dev/mygroup/mylogicalvolume to write
random data to the entirety of the lv device.
The lv is only 4TB (yes... this took days). I had estimated it would
stop some time early this morning. This afternoon when I checked it, dd
had claimed to have written 4.2 TB of data to a 4TB device.
I immediately thought: "How's that possible" and began to worry that it
escaped the logical volume and began to write other areas of the array.
But, dd if=/dev/mygroup/othervol1 bs=1K count=1 | hexdump -C showed a
LUKS header (as it should) and the same command on the other lv showed
the proper header as well. I took this to mean that those lv's were in tact.
How is it possible that I can write 4.2TB to a 4.0 TB logical volume?
Should I be worried that I have somehow overritten something somewhere
else? If so, how do I find and check that? (I have backups, so if I need
to restore, I need to do it now).
P.S. When I did this on the other lv, which is only 100GB, dd behaved as
expected, and wrote only 100GB of random data to the logical volume.

Please check you compare sizes in same 'units'

dd reports sizes in 1000 units
while lvm2 normally in 1024


And basically if you want to hear any 'analysis' - you need to show us

'dmsetup table'


Zdenek

[Michael Munger]

Post by Zdenek Kabelac
Please check you compare sizes in same 'units'
dd reports sizes in 1000 units
while lvm2 normally in 1024
And basically if you want to hear any 'analysis' - you need to show us
'dmsetup table'

This makes sense, and may be what the other poster was saying, but I
mis-understood them to have it backwards. If dd is reporting TB (base
1000) and lvm2 is reporting TiB (base 1024), then that's the answer.

Thanks!