Setting up a Replica Server (Stratum 1)¶
While a CernVM-FS Stratum 0 repository server is able to serve clients directly, a large number of clients is better be served by a set of Stratum 1 replica servers. Multiple Stratum 1 servers improve the reliability, reduce the load, and protect the Stratum 0 master copy of the repository from direct accesses. Stratum 0 server, Stratum 1 servers and the site-local proxy servers can be seen as content distribution network. The figure below shows the situation for the repositories hosted in the cern.ch domain.
A Stratum 1 server is a standard web server that uses the
CernVM-FS server toolkit to create and maintain a mirror of a
CernVM-FS repository served by a Stratum 0 server. To this end, the
cvmfs_server utility provides the
add-replica command. This
command will register the Stratum 0 URL and prepare the local web
server. Periodical synchronization has to be scheduled, for instance
cron, using the
cvmfs_server snapshot command. The
advantage over general purpose mirroring tools such as rSync is that all
CernVM-FS file integrity verifications mechanisms from the Fuse client
are reused. Additionally, by the aid of the CernVM-FS file catalogs, the
cvmfs_server utility knows beforehand (without remote listing) which
files to transfer.
In order to prevent accidental synchronization from a repository, the
Stratum 0 repository maintainer has to create a
.cvmfs_master_replica file in the HTTP root directory. This file is
created by default when a new repository is created. Note that
replication can thrash caches that might exist between Stratum 1 and
Stratum 0. A direct connection is therefore preferable.
The vast majority of HTTP requests will be served by the site’s local proxy servers. Being a publicly available service, however, we recommend to install a Squid frontend in front of the Stratum 1 web server.
We suggest the following key parameters:
- RAID-protected storage. The
cvmfs_serverutility should have low latency to the storage because it runs a large number of system calls (
stat()) against it. For the local storage backends ext3/4 filesystems are preferred (rather than XFS).
- Web server
- A standard Apache server. Directory listing is not required. In addition, it is a good practice to exclude search engines from the replica web server by an appropriate robots.txt. The webserver should be close to the storage in terms of latency.
- Squid frontend
- Squid should be used as a frontend to Apache, configured as a reverse proxy. It is recommended to run it on the same machine as Apache to reduce the number of points of failure. Alternatively, separate Squid server machines may be configured in load-balance mode forwarding to the Apache server, but note that if any of them are down the entire service will be considered down by CernVM-FS clients. The Squid frontend should listen on ports 80 and 8000. The more RAM that the operating system can use for file system caching, the better.
- DNS cache
- A Stratum 1 does a lot of DNS lookups, so we recommend installing a
DNS caching mechanism on the machine such as
bind. We do not recommend
nscdsince it does not honor the DNS Time-To-Live protocol.
The Squid configuration differs from the site-local Squids because the Stratum 1 Squid servers are transparent to the clients (reverse proxy). As the expiry rules are set by the web server, Squid cache expiry rules remain unchanged.
The following lines should appear accordingly in /etc/squid/squid.conf:
http_port 80 accel http_port 8000 accel http_access allow all cache_peer <APACHE_HOSTNAME> parent <APACHE_PORT> 0 no-query originserver cache_mem <MEM_CACHE_SIZE> MB cache_dir ufs /var/spool/squid <DISK_CACHE_SIZE in MB> 16 256 maximum_object_size 1024 MB maximum_object_size_in_memory 128 KB
http_access allow allhas to be inserted before (or instead of) the line
http_access deny all. If Apache is running on the same host, the
localhost. Also, in that case there is not a performance advantage for squid to cache files that came from the same machine, so you can configure squid to not cache files. Do that with the following lines:
acl CVMFSAPI urlpath_regex ^/cvmfs/[^/]*/api/ cache deny !CVMFSAPI
Then the squid will only cache API calls. You can then set
DISK_CACHE_SIZE quite small.
Check the configuration syntax by
squid -k parse. Create the hard
disk cache area with
squid -z. In order to make the increased number
of file descriptors effective for Squid, execute
ulimit -n 8192
prior to starting the squid service.
cvmfs_server utility reports status and problems to
For the web server infrastructure, we recommend standard Nagios HTTP checks. They should be configured with the URL http://$replica-server/cvmfs/$repository_name/.cvmfspublished. This file can also be used to monitor if the same repository revision is served by the Stratum 0 server and all the Stratum 1 servers. In order to tune the hardware and cache sizes, keep an eye on the Squid server’s CPU and I/O load.
Keep an eye on HTTP 404 errors. For normal CernVM-FS traffic, such
failures should not occur. Traffic from CernVM-FS clients is marked by