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Is it possible to export a block device such as a DVD or CDROM and make it so that it's mountable on another computer as a block device?

NOTE: I'm not interested in doing this using NFS or Samba, I actually want the optical drive to show up as a optical drive on a remote computer.

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4 Answers 4

up vote 25 down vote accepted

I think you might be able to accomplish what you want using network block devices (NBD). Looking at the wikipedia page on the subject there is mention of a tool called nbd. It's comprised of a client and server component.

Example

In this scenario I'm setting up a CDROM on my Fedora 19 laptop (server) and I'm sharing it out to an Ubuntu 12.10 system (client).

installing
$ apt-cache search ^nbd-
nbd-client - Network Block Device protocol - client
nbd-server - Network Block Device protocol - server

$ sudo apt-get install nbd-server nbd-client
sharing a CD

Now back on the server (Fedodra 19) I do a similar thing using its package manager YUM. Once complete I pop a CD in and run this command to share it out as a block device:

$ sudo nbd-server 2000 /dev/sr0

** (process:29516): WARNING **: Specifying an export on the command line is deprecated.

** (process:29516): WARNING **: Please use a configuration file instead.
$

A quick check to see if it's running:

$ ps -eaf | grep nbd
root     29517     1  0 12:02 ?        00:00:00 nbd-server 2000 /dev/sr0
root     29519 29071  0 12:02 pts/6    00:00:00 grep --color=auto nbd
Mounting the CD

Now back on the Ubuntu client we need to connect to the nbd-server using nbd-client like so. NOTE: the name of the nbd-server is greeneggs in this example.

$ sudo nbd-client greeneggs 2000 /dev/nbd0
Negotiation: ..size = 643MB
bs=1024, sz=674983936 bytes

We can confirm that there's now a block device on the Ubuntu system using lsblk:

$ sudo lsblk -l
NAME                 MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda                    8:0    0 465.8G  0 disk 
sda1                   8:1    0   243M  0 part /boot
sda2                   8:2    0     1K  0 part 
sda5                   8:5    0 465.5G  0 part 
ubuntu-root (dm-0)   252:0    0 461.7G  0 lvm  /
ubuntu-swap_1 (dm-1) 252:1    0   3.8G  0 lvm  [SWAP]
sr0                   11:0    1 654.8M  0 rom  
nbd0                  43:0    0   643M  1 disk 
nbd0p1                43:1    0   643M  1 part 

And now we mount it:

$ sudo mount /dev/nbd0p1 /mnt/
mount: block device /dev/nbd0p1 is write-protected, mounting read-only
$
did it work?

The suspense is killing me, and we have liftoff:

$ sudo ls /mnt/
EFI  GPL  isolinux  LiveOS

There's the contents of a LiveCD of CentOS that I mounted in the Fedora 19 laptop and was able to mount it as a block device of the network on Ubuntu.

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I get this: "WARNING Specifying an export on the command line is deprecated. Please use a configuration file instead". Version: 3.3 –  guettli Aug 11 at 13:41
    
@guettli - thanks for the feedback. What distro was this on? –  slm Aug 11 at 14:30
    
@sim openSUSE 13.1 –  guettli Aug 11 at 14:54
    
Note: greeneggs is the server's host name. –  devius Nov 10 at 16:34
    
@devius - thanks added a note to that effect in the A. –  slm Nov 10 at 16:38

You obviously prefer the SAN solution. Beside the already mentioned iSCSI and NBD, you have also the AoE (ATA over ethernet) approach.

This is very easy to do:

On the serving side you need to

modprobe aoe
vbladed 0 0 eth0 /dev/sdc

On the client side

modprobe aoe
aoe-discover
aoe-stat
e0.0      1000.204GB       eth0 1024  up

Your devices are in

ls -l /dev/etherd/
c-w--w----  1 root disk 152, 3 Mar 12 22:47 discover
brw-rw----  1 root disk 152, 0 Mar 12 22:47 e0.0
brw-rw----  1 root disk 152, 1 Mar 12 22:47 e0.0p1
cr--r-----  1 root disk 152, 2 Mar 12 22:47 err
c-w--w----  1 root disk 152, 6 Mar 12 22:47 flush
c-w--w----  1 root disk 152, 4 Mar 12 22:47 interfaces
c-w--w----  1 root disk 152, 5 Mar 12 22:47 revalidate

Where e0.0 is your /dev/sdc and e0.0.p1 is /dev/sdc1

dmesg on server:

[221384.454447] aoe: AoE v85 initialised.

dmesg output on client:

[ 1923.225832] aoe: AoE v85 initialised.
[ 1923.226379] aoe: e0.0: setting 1024 byte data frames
[ 1923.226910] aoe: 38607725d8b1 e0.0 v4014 has 1953525168 sectors
[ 1923.653820]  etherd/e0.0: p1

Pretty easy.

Additional Notes

  • vbladed is part of the package vblade on Fedora & Ubuntu, likely the same in other distros as well.
  • aoe-discover & aoe-stat are part of the package aoetools on Fedora & Ubuntu as well.
  • Device shows up in fdisk as a block device, for example, /dev/etherd/e0.0.
  • The version of vblade that's available in the F19 and F20 repositories is pretty dated, it's version 14. The ATAoE project page has version 21 available. There's a updated RPM available for Fedora 19 x86_64 here.
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Tried doing this b/w Fedora and Ubuntu, it's not working. –  slm Mar 12 at 22:23
    
I usually check everything before I post. Q: is aoe loaded? Did you give proper arguments? On receiver: did aoe found the opponent? What says dmesg? –  user55518 Mar 12 at 22:27
    
I know you do, I wasn't implying that it was wrong only that there might be some extra steps 8-) –  slm Mar 12 at 22:30
1  
ROTFL, and did you count the smartphone too? –  user55518 Mar 12 at 23:19
1  
aaahh, well, thanks for the votes :) –  user55518 Mar 12 at 23:20

One alternative to nbd (if you're interested) is using iSCSI. tgtd can be configured to have a /dev device as its backing storage for a particular iSCSI IQN.

You're probably on a RHEL system so you just need to install scsi-target-utils and then configure/start tgtd on the source system. Configuration of tgtd can get involved but Red Hat provides plenty of different examples for the various scenarios.

For Example:

<target iqn.2008-09.com.example:server.target4>
    direct-store /dev/sdb      # Becomes LUN 1
    direct-store /dev/sdc      # Becomes LUN 2
    direct-store /dev/sdd      # Becomes LUN 3
    write-cache off
    vendor_id MyCompany Inc.
</target>

You would install iscsi-initiator-utils on the client system and use iscsiadm to send targets then to "log into" the enumerated targets. For Example:

# iscsiadm -m discovery -t sendtargets -p <remoteHost>
# iscsiadm -m node -T <Complete Target IQN> -l -p <remoteHost>:3260

The iSCSI LUN's will then present to the system as regular block devices. On RHEL, you can check the transport a particular device is coming over you can just do an ls -l /dev/disk/by-path | grep iscsi to see what storage is coming over iSCSI. The path will also list the IQN of the target you logged into above.

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2  
If you want you can add your A to this AU Q too, it's what prompted me to write this one up: askubuntu.com/questions/433231/… –  slm Mar 12 at 16:45
    
I would, but I don't really have an Ubuntu system handy to ensure the package names are the same or if the configuration examples I referenced are upstream or just something Red Hat does. The configuration itself should be the same, though. –  Bratchley Mar 12 at 16:54
1  
Joel you're the best...no Ubuntu system 8-) –  slm Mar 12 at 17:05

Although this solution works, it is presented only as a method to show that TMWTDI.

For this setup we need a unix inspired system to host the block device, and a system with a loop back block device for the system to view it. Linux will work for both.

Next we need a dumb network file system (it must not make optimizations of the data storage), like samba or webdav. Specifically NFS, and AFS will not work.

Place the block device in a folder that is exported. (man mknod will help.) disable block device checking if applicable. mount the exported filesystem on the client computer, create a loop device using the imported file as it's backing store and mount as normal. (losetup or mount -o loop)

How and why it works, and why the performance sucks: One of the core ideas behind unix is that everything is a file, so devices are files only they are a little special (that is why they are called special files). There are several types of special files that mknod can produce, but we are only going to look at block devices. block devices exist in the file system like any other file and like any other file they have owners, permissions, inodes and names; but they have two differences: they do not store their data in the filesystem, and they have extra capabilities so they can be mounted. If the file server is dumb enough not to pay attention to these differences, you can ignore them and export them as regular files. Next the loop device takes an ordinary file (local or remote) and adds the special properties and capabilities so that you can mount regular files as if they are block devices. The big drawback is performance.

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