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OpenAFS

OpenAFS

The Andrew Filesystem (or AFS) is a global distributed filesystem. With a single mountpoint, clients can access any site on the Internet which is running AFS as if it were a local filesystem.

OpenAFS is an open source implementation of the AFS services and utilities.

A collection of AFS servers and volumes that are collectively administered within a site is called a cell. The OpenStack project runs the openstack.org AFS cell, accessible at /afs/openstack.org/.

At a Glance

Hosts
  • afsdb01.openstack.org (a vldb and pts server in DFW)
  • afsdb02.openstack.org (a vldb and pts server in ORD)
  • afs01.dfw.openstack.org (a fileserver in DFW)
  • afs01.ord.openstack.org (a fileserver in ORD)
Puppet
Projects
Bugs
Resources

OpenStack Cell

AFS may be one of the most thoroughly documented systems in the world. There is plenty of very good information about how AFS works and the commands to use it. This document will only cover the mininmum needed to understand our deployment of it.

OpenStack runs an AFS cell called openstack.org. There are three important services provided by a cell: the volume location database (VLDB), the protection database (PTS), and the file server (FS). The volume location service answers queries from clients about which fileservers should be contacted to access particular volumes, while the protection service provides information about users and groups.

Our implementation follows the common recommendation to colocate the VLDB and PTS servers, and so they both run on our afsdb* servers. These servers all have the same information and communicate with each other to keep in sync and automatically provide high-availability service. For that reason, one of our DB servers is in the DFW region, and the other in ORD.

Fileservers contain volumes, each of which is a portion of the file space provided by that cell. A volume appears as at least one directory, but may contain directories within the volume. Volumes are mounted within other volumes to construct the filesystem hierarchy of the cell.

OpenStack has one fileserver in DFW and one in ORD. They do not automatically contain copies of the same data. A read-write volume in AFS can only exist on exactly one fileserver, and if that fileserver is out of service, the volumes it serves are not available. However, volumes may have read-write copies which are stored on other fileservers. If a client requests a read-only volume, as long as one site with a read-only volume is online, it will be available.

Client Configuration

To use OpenAFS on a Debian or Ubuntu machine:

sudo apt-get install openafs-client openafs-krb5 krb5-user

Debconf will ask you for a default realm, cell and cache size. Answer:

Default Kerberos version 5 realm: OPENSTACK.ORG
AFS cell this workstation belongs to: openstack.org
Size of AFS cache in kB: 500000

The default cache size in debconf is 50000 (50MB) which is not very large. We recommend setting it to 500000 (500MB -- add a zero to the default debconf value), or whatever is appropriate for your system.

The OpenAFS client is not started by default, so you will need to run:

sudo service openafs-client start

When it's done, you should be able to cd /afs/openstack.org.

Most of what is in our AFS cell does not require authentication. However, if you have a principal in kerberos, you can get an authentication token for use with AFS with:

kinit
aklog

Administration

The following information is relevant to AFS administrators.

All of these commands have excellent manpages which can be accessed with commands like man vos or man vos create. They also provide short help messages when run like vos -help or vos create -help.

For all administrative commands, you may either run them from any AFS client machine while authenticated as an AFS admin, or locally without authentication on an AFS server machine by appending the -localauth flag to the end of the command.

Adding a User

First, add a kerberos principal as described in addprinc. Have the username and UID from puppet ready.

Then add the user to the protection database with:

pts createuser $USERNAME -id UID

Admin UIDs start at 1 and increment. If you are adding a new admin user, you must run pts listentries, find the highest UID for an admin user, increment it by one and use that as the UID. The username for an admin user should be in the form username.admin.

Note

Any '/' characters in a kerberos principal become '.' characters in AFS.

Adding a Superuser

Run the following commands to add an existing principal to AFS as a superuser:

bos adduser -server afsdb01.openstack.org -user $USERNAME.admin
bos adduser -server afsdb02.openstack.org -user $USERNAME.admin
pts adduser -user $USERNAME.admin -group system:administrators

Creating a Volume

Select a fileserver for the read-write copy of the volume according to which region you wish to locate it after ensuring it has sufficient free space. Then run:

vos create $FILESERVER a $VOLUMENAME

The a in the preceding command tells it to place the volume on partition vicepa. Our fileservers only have one partition and therefore this is a constant.

Be sure to mount the volume in AFS with:

fs mkmount /afs/openstack.org/path/to/mountpoint $VOLUMENAME

You may want to create read-only sites for the volume with vos addsite and then vos release.

You should set the volume quota with fs setquota.

Adding a Fileserver

Put the machine's public IP on a single line in /var/lib/openafs/local/NetInfo (TODO: puppet this).

Copy /etc/openafs/server/KeyFile from an existing fileserver.

Create an LVM volume named vicepa from cinder volumes. See cinder for details on volume management. Then run:

mkdir /vicepa
echo "/dev/main/vicepa  /vicepa ext4  errors=remount-ro,barrier=0  0  2" >>/etc/fstab
mount -a

Finally, create the fileserver with:

bos create NEWSERVER dafs dafs \
  -cmd "/usr/lib/openafs/dafileserver -p 23 -busyat 600 -rxpck 400 -s 1200 -l  1200 -cb 65535 -b 240 -vc 1200" \
  -cmd /usr/lib/openafs/davolserver \
  -cmd /usr/lib/openafs/salvageserver \
  -cmd /usr/lib/openafs/dasalvager