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).

$ 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

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

Now back on the Ubuntu client we need to connect to the nbd-server using nbd-client like so:

$ 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
$

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.

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

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

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.

References

• Using ATA Over Ethernet (AoE) On Ubuntu 12.04 (Initiator And Target)