losetup: support for physical devices

BUG Lucio Crusca 4 participants 5 messages Jan 14, 2015 – Jan 26, 2015 Util-Linux package

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Lucio Crusca

author

2015-01-14 18:05 UTC #0 lore raw

During a disaster recovery I found that losetup either

1. does not support creating loop devices that target physical devices. In this case I think there exists a compelling reason to support them
(http://serverfault.com/questions/659484) and this bug report is a wishlist bug

2. it does support physical devices, but it is an undocumented feature (at least in the manpage) and the warning it issues is misleading beacuse it makes you think it doesn't support them ("losetup: /dev/sdc2: warning: file smaller than 512 bytes, the loop device maybe be useless or invisible for system tools"). In this case this could be a documentation bug.

I haven't tested which of the two possibilities is the real situation, but I'm pretty sure you already know.

Thanks for your attention,
Lucio.

Karel Zak

maintainer

2015-01-14 20:08 UTC #1 lore raw

Quoted 7 lines · Lucio Crusca

During a disaster recovery I found that losetup either
1. does not support creating loop devices that target physical devices. In this case I think there exists a compelling reason to support them
(http://serverfault.com/questions/659484) and this bug report is a wishlist bug

Quoted 7 lines

or
2. it does support physical devices, but it is an undocumented feature (at least in the manpage) and the warning it issues is misleading beacuse it makes you think it doesn't support them ("losetup: /dev/sdc2: warning: file smaller than 512 bytes, the loop device maybe be useless or invisible for system tools"). In this case this could be a documentation bug.

commit b048b8af3a5568c90f6e0c2d56f6cb399dedb0d1

Author: Karel Zak <[email protected]>
Date:   Mon May 13 11:00:47 2013 +0200

losetup: use warn_size() for regular files only

Signed-off-by Karel Zak <[email protected]>

Karel

Signature · 2 lines

 Karel Zak  <[email protected]>
 http://karelzak.blogspot.com

Karel Zak

maintainer

2015-01-14 20:36 UTC #2 lore raw

In reply to Lucio Crusca

Quoted 1 line · Lucio Crusca

http://serverfault.com/questions/659484

BTW, it should be possible to use delpart(8) to release sdc1 partition and use addpart(8) to define the partition from another offset. Then you avoid /deb/loopN at all.

Karel

Signature · 2 lines

 Karel Zak  <[email protected]>
 http://karelzak.blogspot.com

Dale R. Worley

2015-01-26 03:29 UTC #3 lore raw

In reply to Lucio Crusca

Quoted 9 lines · Lucio Crusca

During a disaster recovery I found that losetup either
1. does not support creating loop devices that target physical devices. In this case I think there exists a compelling reason to support them
(http://serverfault.com/questions/659484) and this bug report is a wishlist bug
or

Quoted 8 lines

2. it does support physical devices, but it is an undocumented feature (at least in the manpage) and the warning it issues is misleading beacuse it makes you think it doesn't support them ("losetup: /dev/sdc2: warning: file smaller than 512 bytes, the loop device maybe be useless or invisible for system tools"). In this case this could be a documentation bug.
I haven't tested which of the two possibilities is the real situation, but I'm pretty sure you already know.

I don't recall what I've done that convinced me, but I'm *sure* that you can do

    losetup -o [offset] {-f[--show]|loopdev} /dev/sdXXX

This is implicit throughout Linux, because any block device is also a "file", and can be read/written as such. (You just can't change its size.)

What command are you executing? What error are you seeing?

Dale

Peter Cordes

2015-01-26 05:06 UTC #4 lore raw

Quoted 16 lines · Dale R. Worley

Quoted 15 lines · Lucio Crusca
During a disaster recovery I found that losetup either
1. does not support creating loop devices that target physical devices. In this case I think there exists a compelling reason to support them
(http://serverfault.com/questions/659484) and this bug report is a wishlist bug
or
2. it does support physical devices, but it is an undocumented feature (at least in the manpage) and the warning it issues is misleading beacuse it makes you think it doesn't support them ("losetup: /dev/sdc2: warning: file smaller than 512 bytes, the loop device maybe be useless or invisible for system tools"). In this case this could be a documentation bug.

Quoted 7 lines

Quoted 2 lines
I haven't tested which of the two possibilities is the real situation, but I'm pretty sure you already know.
I don't recall what I've done that convinced me, but I'm *sure* that you can do
    losetup -o [offset] {-f[--show]|loopdev} /dev/sdXXX

Yup, it works for me. I posted an answer on
http://serverfault.com/questions/659484, gist of it is that

sudo losetup -o 32256 -f /dev/sdb3
sets up a loopback dev. Test with:
sudo tail -c 32257 /dev/sdb3 | sudo cmp - /dev/loop5

If you leave out -f or a specific arg like /dev/loop0, then the
command fails because losetup /dev/sdb3 means you're asking losetup to print info on a loopback device called /dev/sdb3.

loop: can't get info on device /dev/sdb3: Inappropriate ioctl for device

IDK how you got it to print

losetup: /dev/sdc2: warning: file smaller than 512 bytes, the loop device maybe be useless or invisible for system tools

Signature · 6 lines

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Peter Cordes ;  e-mail: X(peter@cor , des.ca)

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