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
At a Glance¶
afsdb01.openstack.org (a vldb and pts server in DFW)
afsdb02.openstack.org (a vldb and pts server in ORD)
afsdb03.openstack.org (a second vldb and pts server in DFW)
afs01.dfw.openstack.org (a fileserver in DFW)
afs02.dfw.openstack.org (a second fileserver in DFW)
afs01.ord.openstack.org (a fileserver in ORD)
mirror-update.opendev.org (host running mirror update jobs)
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 minimum 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. As described in https://docs.openafs.org/AdminGuide/HDRWQ101.html the Ubik protocol requires three servers to maintain availability; for that reason, two of our DB servers are 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 two fileservers 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.
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
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:
If not running on Debian or Ubuntu you can install openafs client packages as well as Kerberos5 packages on your distro of choice.
Then to kinit, use your fully qualified user id:
Or for admin access:
Then aklog, specifying the openstack.org cell:
aklog -cell openstack.org
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
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 Adding A User Principal. 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
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:
pts adduser -user $USERNAME.admin -group system:administrators
After this, you should update the system-config: playbooks/roles/openafs-server-config/files/UserList file to ensure the new username is authorized to issue privileged commands.
This is a basic example of write operations for AFS-hosted content, so applies more generally to manually adding or changing files as well. As we semi-regularly get requests to delete subtrees of documentation, this serves as a good demonstration.
First, as a prerequisite, make sure you’ve followed the Client
Configuration and Adding a Superuser steps for yourself and
that you know the password for your
principal. Safely authenticate your superuser’s principal in a new
PAG as follows:
pagsh -c /bin/bash export KRB5CCNAME=FILE:`mktemp` kinit $USERNAME/admin aklog
If this is a potentially destructive change (perhaps you’re worried you might mistype a deletion and remove more content than you intended) you can first create a copy-on-write backup snapshot like so:
vos backup docs
When deleting files, note that you should use the read-write
/afs/.openstack.org path rather than the read-only
/afs/openstack.org path, but normal Unix file manipulation
commands work as expected (do _not_ use
sudo for this):
rm -rf /afs/.openstack.org/docs/project-install-guide/baremetal/draft
If you don’t want to have to wait for a volume release to happen (so that your changes to the read-write filesystem are reflected immediately in the read-only filesystem), you can release it now too:
vos release docs -verbose
Now you can clean up your session, destroy your ticket and exit the temporary PAG thusly:
unlog kdestroy exit
Creating a Volume¶
You should not run these operations on the fileservers where the
volumes or replicas are to be created (
`vos will resolve the ipv4 address of the fileserver
host from the command-line. If you are using the tool on the
fileserver, Debuntu’s use of
127.0.1.1 for localhost and having
the hostname in
/etc/hosts can thus result in the
not correctly filtering the loopback address and setting the server
address for the volume as
127.0.1.1 – making it effectively
inaccessible. A similar problem can occur for NAT servers, if we
were to use them. Running on an external host means the lookups
shouldn’t return local addresses and avoids this issue. The other
option is to specify the fileservers as the IP address, rather than
the hostname, to avoid any lookup issues.
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 read-write 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
addsite and then
If the volume’s mountpoint lies within another volume, you may also
vos release that parent volume before it will show up in
the read-only path.
You should set the volume quota with
Deleting a Volume¶
Remove the mountpoint(s) of the volume:
fs rmmount /afs/.openstack.org/path/to/mountpoint
Be sure to release the parent volume (with
vos release) if
necessary after removing the mountpoint.
vos examine to see a list of volume sites. Identify the
read-write and read-only sites.
Remove the read-only sites first; repeat this command for each one:
vos remove -server $FILESERVER -partition $PARTITION -id $VOLUME.readonly
Remove the read-write volume:
vos remove -id $VOLUME
Adding a Fileserver¶
Put the machine’s public IP on a single line in /var/lib/openafs/local/NetInfo (TODO: puppet this).
/etc/openafs/server/* from an existing fileserver.
Create an LVM volume named
vicepa from cinder volumes. See
Cinder Volume Management 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 -server NEWSERVER -instance dafs -type dafs \ -cmd "/usr/lib/openafs/dafileserver -L -p 242 -busyat 600 -rxpck 700 \ -s 1200 -l 1200 -cb 2000000 -b 240 -vc 1200 \ -udpsize 131071 -sendsize 131071" \ -cmd /usr/lib/openafs/davolserver \ -cmd /usr/lib/openafs/salvageserver \ -cmd /usr/lib/openafs/dasalvager
It is worth evaluating these settings periodically
-Lselects the large size, which ups a number of defaults
-pdefines the worker threads for processing incoming calls. Since they block until there is work to do, we should leave this at around the maximum (which may increase across versions; see documentation)
-sendsizeshould be increased above their default
-cbdefines the callbacks. For our use case, with a single mirror writer, this should be around the number of files the client is configured to cache (
-dcache) multiplied by the number of clients.
The helper script system-config: tools/afs-server-restart.sh is a helper
script to restart AFS servers, and optionally enable audit logging on
the servers which is sometimes useful for debugging afs clients. You
can edit settings in the script and run
If you wish to update the settings for an existing server manually,
you can stop and remove the existing
bnode (the collection of
processes the overseer is monitoring, created via
above) and recreate it.
bos stop -server afs01.dfw.openstack.org \ -instance dafs \ -wait
Then remove the server with
bos delete -server afs01.dfw.openstack.org \ -instance dafs
Finally run the
bos create command above with any modified
parameters to restart the server.
Recovering a Failed Fileserver¶
If a fileserver crashes, take the following steps to ensure it’s usable after recovery:
Pause mirror updates and volume release cron jobs
Reboot the server; fix any filesystem errors and check the salvager logs
Check for any stuck volume transactions; remedy as appropriate
Perform a manual release of every volume from a terminal on a server using “-localauth” in case OpenAFS decides it can’t do an incremental update.
Re-enable cron jobs
We host mirrors in AFS so that we store only one copy of the data, but mirror servers local to each cloud region in which we operate serve that data to nearby hosts from their local cache.
All of our mirrors are housed under
Each mirror is on its own volume, and each with a read-only replica.
This allows mirrors to be updated and then the read-only replicas
atomically updated. Because mirrors are typically very large and
replication across regions is slow, we place both copies of mirror
data on two fileservers in the same region. This allows us to perform
maintenance on fileservers hosting mirror data as well deal with
outages related to a single server, but does not protect the mirror
system from a region-wide outage.
In order to establish a new mirror, do the following:
The following commands need to be run authenticated on a host with kerberos and AFS setup (see afs_client; admins can run the commands on
mirror-update.opendev.org). See the note above about not doing this on the actual fileservers. Firstly
aklogto get tokens.
Create the mirror volume. See Creating a Volume for details. The volume should be named
mirror.foo, where foo is descriptive of the contents of the mirror. Example:
vos create afs01.dfw.openstack.org a mirror.foo
Create read-only replicas of the volume. One replica should be located on the same fileserver (it will take little to no additional space), and at least one other replica on a different fileserver. Example:
vos addsite afs01.dfw.openstack.org a mirror.foo vos addsite afs02.dfw.openstack.org a mirror.foo
Release the read-only replicas:
vos release mirror.foo
See the status of all volumes with:
When traversing from a read-only volume to another volume across a mountpoint, AFS will first attempt to use a read-only replica of the destination volume if one exists. In order to naturally cause clients to prefer our read-only paths for mirrors, the entire path up to that point is composed of read-only volumes:
/afs [root.afs] /openstack.org [root.cell] /mirror [mirror] /bar [mirror.bar]
In order to mount the
mirror.foo volume under
mirror we need
to modify the read-write version of the
mirror volume. To make
this easy, the read-write version of the cell root is mounted at
/afs/.openstack.org. Following the same logic from earlier,
traversing to paths below that mount point will generally prefer
Mount the volume into afs using the read-write path:
fs mkmount /afs/.openstack.org/mirror/foo mirror.foo
mirrorvolume so that the (currently empty) foo mirror itself appears in directory listings under
vos release mirror
Create a principal for the mirror update process. See Adding A User Principal for details. The principal should be called
kadmin: addprinc -randkey service/foo-mirror@OPENSTACK.ORG kadmin: ktadd -k /path/to/foo.keytab service/foo-mirror@OPENSTACK.ORG
ktaddis run, the key is rotated and previous keytabs are invalidated.
Add the service principal’s keytab to Ansible secrets. Copy the binary key to
bridge.openstack.organd then use
hieraeditto update the files
root@bridge:~# /home/zuul/src/opendev.org/opendev/system-config/tools/hieraedit.py \ --yaml /etc/ansible/hosts/host_vars/mirror-update01.opendev.org.yaml \ -f /path/to/foo.keytab KEYNAME
(don’t forget to
git commitand save the change; you can remove the copies of the binary key too). The key will be base64 encoded in the heira database. If you need to examine it for some reason you can use
cat /path/to/foo.keytab | base64
Ensure the values in this new variable are written to disk as the keytab on
mirror-update.opendev.orgby adding it to the
mirror-updaterole for the mirror scripts to use during update. You should check this with
testinfra/test_mirror-update.py(note this involves defining a “dummy” keytab for testing; see the other examples).
Create an AFS user for the service principal:
pts createuser service.foo-mirror
Because mirrors usually have a large number of directories, it is best to avoid frequent ACL changes. To this end, we grant access to the mirror directories to a group where we can easily modify group membership if our needs change.
Create a group to contain the service principal, and add the principal:
pts creategroup foo-mirror pts adduser service.foo-mirror foo-mirror
View users, groups, and their membership with:
pts listentries pts listentries -group pts membership foo-mirror
Grant the group access to the mirror volume:
fs setacl /afs/.openstack.org/mirror/foo foo-mirror write
Grant anonymous users read access:
fs setacl /afs/.openstack.org/mirror/foo system:anyuser read
Set the quota on the volume (e.g., 100GB):
fs setquota /afs/.openstack.org/mirror/foo 100000000
Because the initial replication may take more time than we allocate in our mirror update cron jobs, manually perform the first mirror update:
In screen, obtain the lock on
flock -n /var/run/foo-mirror/mirror.lock bash
Leave that running while you perform the rest of the steps.
Also in screen on
mirror-update, run the initial mirror sync. If using one of the mirror update scripts (from
/usr/local/bin) be aware that they generally run the update process under
timeoutwith shorter periods than may be required for the initial full sync. e.g. for
NO_TIMEOUT=1 /usr/local/bin/reprepro-mirror-update /etc/reprepro/ubuntu mirror.ubuntu
screen. Within that session, periodically during the sync, and once again after it is complete, run:
vos release mirror.foo -localauth
It is important to do this from an AFS server using
-localauthrather than your own credentials and inside of screen because if
vos releaseis interrupted, it will require some manual cleanup (data will not be corrupted, but clients will not see the new volume until it is successfully released). Additionally,
vos releasehas a bug where it will not use renewed tokens and so token expiration during a vos release may cause a similar problem.
Once the initial sync and
vos releaseare complete, release the lock file on mirror-update.
Removing a mirror¶
If you need to remove a mirror, you can do the following:
Unmount the volume from the R/W location:
fs rmmount /afs/.openstack.org/mirror/foo
Release the R/O mirror volume to reflect the changes:
vos release mirror
Check what servers the volumes are on with
VLDB entries for all servers ... mirror.foo RWrite: 536870934 ROnly: 536870935 number of sites -> 3 server afs01.dfw.openstack.org partition /vicepa RW Site server afs01.dfw.openstack.org partition /vicepa RO Site server afs01.ord.openstack.org partition /vicepa RO Site ...
Remove the R/O replicas (you can also see these with
vos listvol -server afs0[1|2].dfw.openstack.org):
vos remove -server afs01.dfw.openstack.org -partition a -id mirror.foo.readonly vos remove -server afs02.dfw.openstack.org -partition a -id mirror.foo.readonly
Remove the R/W volume:
vos remove -server afs02.dfw.openstack.org -partition a -id mirror.foo
Reverse Proxy Cache¶
Each of the region-local mirror hosts exposes a limited reverse HTTP proxy on port 8080. These proxies run within the same Apache setup as used to expose AFS mirror contents. mod_cache is used to expose a white-listed set of resources (currently just RDO).
Currently they will cache data for up to 24 hours (Apache default)
with pruning performed by
htcacheclean once an hour to keep the
cache size at or under 2GB of disk space.
The reverse proxy is provided because there are some hosted resources that are not currently able to be practically mirrored. Examples of this include RDO (rsync from RDO is slow and they update frequently) and docker images (which require specialized software to run a docker registry and then sorting out how to run that on a shared filesystem).
Apache was chosen because we already had configuration management in place for Apache on these hosts. This avoids management overheads of a completely new service deployment such as Squid or a caching docker registry daemon.
No Outage Server Maintenance¶
We have redundant AFS DB servers. You can take one down without causing a service outage as long as the others remain up. To do this safely:
root@afsdb01:~# bos shutdown afsdb01.openstack.org -wait -localauth root@afsdb01:~# bos status afsdb01.openstack.org -localauth Instance ptserver, temporarily disabled, currently shutdown. Instance vlserver, temporarily disabled, currently shutdown.
Then perform your maintenance on afsdb01. When done a reboot will automatically restart the bos service or you can manually restart the openafs-fileserver service:
root@afsdb01:~# service openafs-fileserver start
Finally check that the service is back up and running:
root@afsdb01:~# bos status afsdb01.openstack.org -localauth Instance ptserver, currently running normally. Instance vlserver, currently running normally.
Now you can repeat the process against afsdb02 or afsdb03.
Taking down the actual fileservers is slightly more complicated but works similarly. Basically what we need to do is make sure that either no one needs the RW volumes hosted by a fileserver before taking it down or move the RW volume to another fileserver. When taking down afs01.dfw.openstack.org we must also ensure that the vos releases that are performed on it by mirror-update are stopped.
To ensure nothing needs the RW volumes you can hold the various file locks on hosts that publish to AFS and/or remove cron entries that perform vos releases or volume writes.
If instead you need to move the RW volume first step is checking where the volumes live:
root@afsdb01:~# vos listvldb -localauth VLDB entries for all servers mirror RWrite: 536870934 ROnly: 536870935 number of sites -> 3 server afs01.dfw.openstack.org partition /vicepa RW Site server afs01.dfw.openstack.org partition /vicepa RO Site server afs01.ord.openstack.org partition /vicepa RO Site
We see that if we want to allow write to the mirror volume and take down afs01.dfw.openstack.org we will have to move the volume to one of the other servers:
root@afsdb01:~# screen # use screen as this may take quite some time. root@afsdb01:~# vos move -id mirror -toserver afs01.ord.openstack.org -topartition vicepa -fromserver afs01.dfw.openstack.org -frompartition vicepa -localauth
When that is done (use listvldb command above to check) it is now safe to take down afs0X.dfw.openstack.org while having writers to the mirror volume. If operating on afs01.dfw.openstack.org you should also hold all mirror update locks and the release-volumes lock. This ensures we do not interrupt any vos releases on afs01.dfw.openstack.org that are run by mirror-update remotely. We use the same process as for the db server:
root@afsdb01:~# bos shutdown afs01.dfw.openstack.org -localauth root@afsdb01:~# bos status afs01.dfw.openstack.org -localauth Auxiliary status is: file server shut down.
Perform maintenance, then restart as above and check the status again:
root@afsdb01:~# bos status afs01.dfw.openstack.org -localauth Auxiliary status is: file server running.
AFS uses the following DNS entries which indicate an even balance:
_afs3-prserver._udp.openstack.org. 300 IN SRV 10 10 7002 afsdb01.openstack.org. _afs3-prserver._udp.openstack.org. 300 IN SRV 10 10 7002 afsdb02.openstack.org. _afs3-prserver._udp.openstack.org. 300 IN SRV 10 10 7002 afsdb03.openstack.org. _afs3-vlserver._udp.openstack.org. 300 IN SRV 10 10 7003 afsdb01.openstack.org. _afs3-vlserver._udp.openstack.org. 300 IN SRV 10 10 7003 afsdb02.openstack.org. _afs3-vlserver._udp.openstack.org. 300 IN SRV 10 10 7003 afsdb03.openstack.org.
Be sure to update them if volume location and PTS servers change. Also note that only A (IPv4 address) records are used in the SRV data. Since OpenAFS lacks support for IPv6, avoid entering corresponding AAAA (IPv6 address) records for these so that it won’t cause fallback delays for other v6-supporting AFS client implementations.