This module is to block devices what FUSE is to filesystems: it provides a small translation layer that redirects block device requests to a program running in userspace.
Right now, this code is alpha quality. My main reference for developing this is Linux Device Drivers, Third Edition -- a well written but, unfortunately, equally well-dated reference book; blog postings by Jonathan Corbet; reading/skimming through kernel source; and trial and error. Nobody else has reviewed this code for correctness or security issues.
If this doesn't scare you off production use... well, good luck with that.
The main purpose of this module is to make it easy to experiment with ideas for virtual block devices without having to roll your own kernel module. Go ahead, implement your own version of RAID 0 backed by Amazon S3, a floppy drive, and Gmail. (I don't recommend this, but if you do get it to work, I'd love to see the results.)
Currently tested only on Ubuntu 15.04 Server, but it's straightforward:
cd ubd/src
make
insmod ublkdev.ko
The client-side library is still being worked out; only Python is available
for now. test-fileserver.py gives a good example of a loop driver.
The smallest "hello world" example -- a block /dev/null driver:
import ublkdev
ubd = ublkdev.UserBlockDevice()
ubd.register(name="hello", size=(1<<20)) # 1 MB
for request in ubd:
replytype = request.msgtype | 0x80000000
replysize = ublkdev.UBDHeader.size + ublkdev.UBDReply.size
if request.msgtype == ublkdev.UBD_MSGTYPE_READ_REQUEST:
data = "\0" * (512 * request.n_sectors)
replysize += len(data)
else:
data = ""
ubd.send_reply(
ublkdev.UBDReply(replytype, replysize, request.tag,
request.n_sectors, data))
Given the number of context switches, validation, and copying going on, this driver will never threaten raw hardware. This is what I see on a 10 MB loop file:
root@ubddev:~# dd if=/dev/ubd/foo of=/dev/null
20480+0 records in
20480+0 records out
10485760 bytes (10 MB) copied, 0.0591159 s, 177 MB/s
Test builds may have excessive logging that prevents any serious performance. If you want a good laugh, take a look at this output -- this is on a 2013 vintage MacBook Pro with an SSD (with VMware running Ubuntu 15.04):
root@ubddev:~# dd if=/dev/ubd/foo of=/dev/null
20480+0 records in
20480+0 records out
10485760 bytes (10 MB) copied, 69.6044 s, 151 kB/s
This code is licensed under the BSD 2-Clause License.
Copyright (c) 2010-2015, David Cuthbert All rights reserved.
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