How can I mount a block device from one computer to another via the network as a block device?

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

(On some systems - e.g. Fedora - one has to modprobe nbd first.)

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.

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.

If you're 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>

To start it up on Fedora/RHEL:

# systemctl start tgtd.service
# firewall-cmd --add-service iscsi-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>

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.

When the iscsi device is not needed anymore one can remove it via:

# iscsiadm -m node -T <Complete Target IQN> -u -p <remoteHost>

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.