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https://github.com/chaos/9nbd

network block device over 9P transport
https://github.com/chaos/9nbd

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network block device over 9P transport

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README 

        
### 9nbd



`9nbd` is a network block device driver for the linux kernel.



The [nbd driver](http://en.wikipedia.org/wiki/Network_block_device)

that has been in the Linux kernel since 2.1.101 associates a block

special file, `/dev/nbdX`, with a remote file or block device.

A remote file system image is mounted from a block device backed

by `nbd` just as from a block device backed by a local disk.

`nbd-server` runs in user space and communicates with `nbd`

using a purpose-built TCP protocol.



`9nbd` is a replacement for `nbd` that uses the 9P transport instead

of the purpose-built TCP protocol.  9P has the advantage of

abstracting the protocol away from the network block device

implementation; leveraging a proven design and implementation;

supporting TCP, Infiniband RDMA, virtio, and file descriptor passing;

and allowing any 9P server,

[diod](https://github.com/chaos/diod) for example, to serve

file system images.



In addition, for read-only block device images, `9nbd` has the ability

to survive a server reboot and, if configured, to fail over to an alternate

server that has the identical block device image, without returning

errors in the block layer.   `9nbd` behavior in this regard is similar

to the familiar NFS client, except for the client-orchestrated failover

which goes beyond NFS client capabilities.



### Scalability



On clusters, serving read-only root file systems with a network block

device really shines.  Specifically, the inability of network file systems

like NFS and Lustre to cache full directories creates an environment

that is pathalogical for common idioms such as path search.  With

a network block device, the buffer cache is active, and buffer cache

entries never need to be invalidated.  This means the network can be

_dead silent_ while an application is running or when loading shared

libraries or searching for executables after a working set has been

pulled in.



### Security



The 9P transport can use a privileged port if desired, and servers

such as `diod` can restrict access based on that and an access list.

The 9P AUTH mechanism is available for extending this at the transport

level.  At LLNL we have experimented with MUNGE authentication over 9P,

though this is not currently implemented in `9nbd`.



### Diskless



`9nbd` works great for root file systems.  More details will appear here

as we integrate `9nbd` with dracut in our RHEL 7 based clusters.



### Quick How-to



Create a 4GB disk image image, bound to a loopback device:

```

# dd if=/dev/zero of=/tmp/image bs=4k count=$((1024*1024))

1048576+0 records in

1048576+0 records out

4294967296 bytes (4.3 GB) copied, 32.3354 s, 133 MB/s

# losetup /dev/loop0 /tmp/image

```



Create an ext4 file system on it:

```

# yes | mkfs.ext4 -m 0 -N 1000000 /dev/loop0

mke2fs 1.41.12 (17-May-2010)

Filesystem label=

...

180 days, whichever comes first.  Use tune2fs -c or -i to override.



# tune2fs -i 0 /dev/loop0

tune2fs 1.41.12 (17-May-2010)

Setting interval between checks to 0 seconds



# tune2fs -O ^has_journal,sparse_super /dev/loop0

tune2fs 1.41.12 (17-May-2010)



# fsck.ext4 -y /dev/loop0

e2fsck 1.41.12 (17-May-2010)

/dev/loop0: clean, 11/1000448 files, 64654/1048576 blocks

```

Mount `/dev/loop0` and copy in content, then unmount.

The file system image should not be exposed to writes while it is

being served as a network block device.



Export the image via `diod` by adding it to `/etc/diod.conf`.

```

exports = {

    { path="/tmp/image",  opts="ro,noauth" }

}

```

then `service diod reload`.



Load the `9nbd` module on the client, instantiate the block device entry,

and mount the remote file system.

```

insmod ./9nbd.ko

/sbin/mount.diod --9nbd-attach host:path /dev/9nbd0

mount -oro /dev/9nbd0 /mnt

...

umount /mnt

/sbin/mount.diod --9nbd-detach /dev/9nbd0

```