Creating a Repository (Stratum 0)¶
CernVM-FS is a file system with a single source of (new) data. This single source, the repository Stratum 0, is maintained by a dedicated release manager machine or installation box. A read-writable copy of the repository is accessible on the release manager machine. The CernVM-FS server tool kit is used to publish the current state of the repository on the release manager machine. Publishing is an atomic operation.
All data stored in CernVM-FS have to be converted into a CernVM-FS repository during the process of publishing. The CernVM-FS repository is a form of content-addressable storage. Conversion includes creating the file catalog(s), compressing new and updated files and calculating content hashes. Storing the data in a content-addressable format results in automatic file de-duplication. It furthermore simplifies data verification and it allows for file system snapshots.
In order to provide a writable CernVM-FS repository, CernVM-FS uses a union file system that combines a read-only CernVM-FS mount point with a writable scratch area [Wright04]. This figure below outlines the process of publishing a repository.
CernVM-FS Server Quick-Start Guide¶
Apache HTTP server or S3 compatible storage service
union file system in the kernel
- AUFS (see Installing the AUFS-enabled Kernel on Scientific Linux 6)
- OverlayFS (as of kernel version 4.2.x)
Officially supported platforms
Scientific Linux 5 (64 bit)
Scientific Linux 6 (64 bit - with custom AUFS enabled kernel - Appendix “Available RPMs”)
CentOS/SL >= 7.3, provided that /var/spool/cvmfs is served by an ext4 file system.
Fedora 22 and above (with kernel \(\ge\) 4.2.x)
Ubuntu 12.04 64 bit and above
- Ubuntu < 15.10: with installed AUFS kernel module (cf. linux-image-extra package)
- Ubuntu 15.10 and later (using upstream OverlayFS)
- Ensure enough disk space in
- For local storage: Ensure enough disk space in
- Create a repository with
cvmfs_server mkfs(See Repository Creation)
cvmfs_server transaction <repository name>
- Install content into
- Create nested catalogs at proper locations
cvmfs_server publish <repository name>
- Create backups of signing key files in
- Entire repository content
- For local storage:
- Stratum 1s can serve as last-ressort backup of repository content
- For local storage:
Installing the AUFS-enabled Kernel on Scientific Linux 6¶
CernVM-FS uses the union file-system aufs to efficiently determine file-system tree updates while publishing repository transactions on the server (see Figure below). Note that this is only required on a CernVM-FS server and not on the client machines.
Download the latest cvmfs-release package from the CernVM website
- Install the cvmfs-release package:
yum install cvmfs-release*.rpmThis adds the CernVM yum repositories to your machine’s configuration.
- Install the aufs enabled kernel from
yum --disablerepo=* --enablerepo=cernvm-kernel install kernel
- Install the aufs user utilities:
yum --enablerepo=cernvm-kernel install aufs2-util
Reboot the machine
Once a new kernel version is released
yum update will not pick the
upstream version but it will wait until the patched kernel with
aufs support is published by the CernVM team. We always try to follow
the kernel updates as quickly as possible.
Publishing a new Repository Revision¶
Since the repositories may contain many file system objects (i.e. ATLAS contains \(70 * 10^6\) file system objects – February 2016), we cannot afford to generate an entire repository from scratch for every update. Instead, we add a writable file system layer on top of a mounted read-only CernVM-FS repository using a union file system. This renders a read-only CernVM-FS mount point writable to the user, while all performed changes are stored in a special writable scratch area managed by the union file system. A similar approach is used by Linux Live Distributions that are shipped on read-only media, but allow virtual editing of files where changes are stored on a RAM disk.
If a file in the CernVM-FS repository gets changed, the union file system first copies it to the writable volume and applies any changes to this copy (copy-on-write semantics). Also newly created files or directories will be stored in the writable volume. Additionally the union file system creates special hidden files (called white-outs) to keep track of file deletions in the CernVM-FS repository.
Eventually, all changes applied to the repository are stored in this scratch area and can be merged into the actual CernVM-FS repository by a subsequent synchronization step. Up until the actual synchronization step takes place, no changes are applied to the CernVM-FS repository. Therefore, any unsuccessful updates to a repository can be rolled back by simply clearing the writable file system layer of the union file system.
Requirements for a new Repository¶
In order to create a repository, the server and client part of
CernVM-FS must be installed on the release manager machine. Furthermore
you will need a kernel containing a union file system implementation as
well as a running
Apache2 web server. Currently we support Scientific
Linux 6, Ubuntu 12.04+ and Fedora 22+ distributions. Please note, that
Scientific Linux 6 does not ship with an aufs enabled kernel, therefore
we provide a compatible patched kernel as RPMs (see
Installing the AUFS-enabled Kernel on Scientific Linux 6 for details).
Since CernVM-FS 2.2.0 we support the usage of both OverlayFS and aufs. Note however, that the first versions of OverlayFS were broken and will not work properly with CernVM-FS. At least a 4.2.x kernel is needed to use CernVM-FS with OverlayFS. (Red Hat) Enterprise Linux >= 7.3 works, too, provided that /var/spool/cvmfs is served by an ext4 file system. Furthermore note that OverlayFS cannot fully comply with POSIX semantics, in particular hard links must be broken into individual files. That is usually not a problem but should be kept in mind when installing certain software distributions into a CernVM-FS repository.
Notable CernVM-FS Server Locations and Files¶
There are a number of possible customisations in the CernVM-FS server installation. The following table provides an overview of important configuration files and intrinsical paths together with some customisation hints. For an exhaustive description of the CernVM-FS server infrastructure please consult Appendix “CernVM-FS Server Infrastructure”.
||Repository mount points Contains read-only union file system mountpoints that become writable during repository updates. Do not symlink or manually mount anything here.|
||Central repository storage location Can be mounted or symlinked to another location before creating the first repository.|
||Storage location of a repository
Can be symlinked to another location
before creating the repository
||Internal states of repositories Can be mounted or symlinked to another location before creating the first repository. Hosts the scratch area described here, thus might consume notable disk space during repository updates.|
||Configuration files and keychains Similar to the structure described in this table. Do not symlink this directory.|
||Customisable server behaviour See “Customizable Actions Using Server Hooks” for further details|
||Repository configuration location Contains repository server specific configuration files.|
CernVM-FS Repository Creation and Updating¶
The CernVM-FS server tool kit provides the
cvmfs_server utility in
order to perform all operations related to repository creation,
updating, deletion, replication and inspection. Without any parameters
it prints a short documentation of its commands.
A new repository is created by
cvmfs_server mkfs my.repo.name
The utility will ask for a user that should act as the owner of the
repository and afterwards create all the infrastructure for the new
CernVM-FS repository. Additionally it will create a reasonable default
configuration and generate a new release manager certificate and
software signing key. The public key in
/etc/cvmfs/keys/my.repo.name.pub needs to be distributed to all
cvmfs_server utility will use
/srv/cvmfs as storage location
by default. In case a separate hard disk should be used, a partition can
be mounted on /srv/cvmfs or /srv/cvmfs can be symlinked to another
location (see Notable CernVM-FS Server Locations and Files). Besides local storage it is
possible to use an S3 compatible storage service
as data backend.
Once created, the repository is mounted under
containing only a single file called
new_repository. The next steps
describe how to change the repository content.
The repository name resembles a DNS scheme but it does not need to
reflect any real server name. It is supposed to be a globally unique name that
indicates where/who the publishing of content takes place. A repository name
must only contain alphanumeric characters plus
. and it
is limited to a length of 60 characters.
Repositories for Volatile Files¶
Repositories can be flagged as containing volatile files using the
cvmfs_server mkfs -v my.repo.name
When CernVM-FS clients perform a cache cleanup, they treat files from volatile repositories with priority. Such volatile repositories can be useful, for instance, for experiment conditions data.
Repositories can be created with the
-V options or republished with the
-F option with a
membership requirement. Clients that mount
repositories with a membership requirement will grant or deny access to the
repository based on the decision made by an authorization helper. See
Section Authorization Helpers for details on authorization helpers.
For instance, a repository can be configured to grant access to a repository only to those users that have a X.509 certificate with a certain DN. Note that the corresponding client-side X.509 authorization helper is not part of CernVM-FS but is provided as a third-party plugin by the Open Science Grid.
A membership requirement makes most sense if the repository is served by an HTTPS server that requires client-side authentication. Note that such repositories cannot be replicated to Stratum 1 servers. Such repositories also cannot benefit from site proxies. Instead, such repositories are either part of a (non CernVM-FS) HTTPS content distribution network or they might be installed for a small number of users that, for example, require access to licensed software.
S3 Compatible Storage Systems¶
CernVM-FS can store files directly to S3 compatible storage systems,
such as Amazon S3, Huawei UDS and OpenStack SWIFT. The S3 storage
settings are given as parameters to
cvmfs_server mkfs or
cvmfs_server mkfs -s /etc/cvmfs/.../mys3.conf \ -w http://s3.amazonaws.com/mybucket-1-1 my.repo.name
The file “mys3.conf” contains the S3 settings (see :ref: table below
<tab_s3confparameters>). The “-w” option is used define the S3 server URL,
e.g. http://localhost:3128, which is used for accessing the repository’s
backend storage on S3. Note that this URL can be different than the S3 server
address that is used for uploads, e.g. if a proxy server is deployed in front
of the server. Note that the buckets need to exist before the repository is
created. In the example above, a single bucket
mybucket-1-1 needs to be
||Number of S3 accounts to be used, e.g. 1. With some S3 servers use of multiple accounts can increase the upload speed significantly|
||S3 account access key(s) separated with
||S3 account secret key(s) separated with
||S3 buckets used per account, e.g. 1. With some S3 servers use of multiple buckets can increase the upload speed significantly|
||S3 server hostname, e.g. s3.amazonaws.com|
||S3 bucket base name. Account and bucket
index are appended to the bucket base name.
If you use just one account and one bucket,
||Number of parallel uploads to the S3 server, e.g. 400|
In addition, if the S3 backend is configured to use multiple accounts or buckets, a proxy server is needed to map HTTP requests to correct buckets. This mapping is needed because CernVM-FS does not support buckets but assumes that all files are stored in a flat namespace. The recommendation is to use a Squid proxy server (version \(\geq 3.1.10\)). The squid.conf can look like this:
http_access allow all http_port 127.0.0.1:3128 intercept cache_peer swift.cern.ch parent 80 0 no-query originserver url_rewrite_program /usr/bin/s3_squid_rewrite.py cache deny all
The bucket mapping logic is implemented in
This script is not provided by CernVM-FS but needs to be written by the
repository owner (the CernVM-FS Git repository contains an example). The script
needs to read requests from stdin and write mapped URLs to stdout, for instance:
in: http://localhost:3128/data/.cvmfswhitelist out: http://swift.cern.ch/cernbucket-9-91/data/.cvmfswhitelist
Typically a repository publisher does the following steps in order to create a new revision of a repository:
cvmfs_server transactionto switch to a copy-on-write enabled CernVM-FS volume
- Make the necessary changes to the repository, add new directories, patch certain binaries, ...
- Test the software installation
- Do one of the following:
cvmfs_server publishto finalize the new repository revision or
cvmfs_server abortto clear all changes and start over again
CernVM-FS supports having more than one repository on a single server
machine. In case of a multi-repository host, the target repository of a
command needs to be given as a parameter when running the
cvmfs_server utility. The
cvmfs_server resign command should run
every 30 days to update the signatures of the repository. Most
cvmfs_server commands allow for wildcards to do manipulations on
more than one repository at once,
cvmfs_server migrate *.cern.ch
would migrate all present repositories ending with
The CernVM-FS server tools support the import of a CernVM-FS file storage together with its corresponding signing keychain. The import functionality is useful to bootstrap a release manager machine for a given file storage.
cvmfs_server import works similar to
cvmfs_server mkfs (described in
Repository Creation) except it uses the provided data storage instead of
creating a fresh (and empty) storage. In case of a CernVM-FS 2.0 file storage
cvmfs_server import also takes care of the file catalog migration into the
latest catalog schema (see Legacy Repository Import for details).
During the import it might be necessary to resign the repository’s whitelist.
Usually because the whitelist’s expiry date has exceeded. This operations
requires the corresponding masterkey to be available in /etc/cvmfs/keys.
Resigning is enabled by adding
An import can either use a provided repository keychain placed into
/etc/cvmfs/keys or generate a fresh repository key and certificate for the
imported repository. The latter case requires an update of the repository’s
whitelist to incorporate the newly generated repository key. To generate a fresh
repository key add
-t -r to
Refer to Section Repository Signature for a comprehensive description of the repository signature mechanics.
Legacy Repository Import¶
We strongly recommend to install CernVM-FS 2.1 on a fresh or at least a properly cleaned machine without any traces of the CernVM-FS 2.0 installation before installing CernVM-FS 2.1 server tools.
cvmfs_server import requires the full CernVM-FS 2.0 data
storage which is located at /srv/cvmfs by default as well as the
repository’s signing keys. Since the CernVM-FS 2.1 server backend
supports multiple repositories in contrast to its 2.0 counterpart, we
recommend to move the repository’s data storage to /srv/cvmfs/<FQRN>
upfront to avoid later inconsistencies.
The following steps describe the transformation of a repository from CernVM-FS 2.0 into 2.1. As an example we are using a repository called legacy.cern.ch.
Make sure that you have backups of both the repository’s backend storage and its signing keys
Install and test the CernVM-FS 2.1 server tools on the machine that is going to be used as new Stratum 0 maintenance machine
- Place the repository’s backend storage data in /srv/cvmfs/legacy.cern.ch(default storage location)
Transfer the repository’s signing keychain to the machine (f.e. to /legacy_keys/)
cvmfs_server importlike this:
cvmfs_server import -o <username of repo maintainer> \ -k ~/legacy_keys \ -l \ # for 2.0.x file catalog migration -s \ # for further repository statistics legacy.cern.ch
Check the imported repository with
cvmfs_server check legacy.cern.chfor integrity (see Integrity Check)
Customizable Actions Using Server Hooks¶
cvmfs_server utility allows release managers to trigger custom
actions before and after crucial repository manipulation steps. This can
be useful for example for logging purposes, establishing backend storage
connections automatically or other workflow triggers, depending on the
There are six designated server hooks that are potentially invoked during the repository update procedure:
- When running
- before the given repository is transitioned into transaction mode
- after the transition was successful
- When running
- before the publish procedure for the given repository is started
- after it was published and remounted successfully
- When running
- before the unpublished changes will be erased for the given repository
- after the repository was successfully reverted to the last published state
All server hooks must be defined in a single shell script file called:
cvmfs_server utility will check the existence of this script and
source it. To subscribe to the described hooks one needs to define one
or more of the following shell script functions:
The defined functions get called at the specified positions in the
repository update process and are provided with the fully qualified
repository name as their only parameter (
$1). Undefined functions
automatically default to a NO-OP. An example script is located at
cvmfs/cvmfs_server_hooks.sh.demo in the CernVM-FS sources.
Maintaining a CernVM-FS Repository¶
CernVM-FS is a versioning, snapshot-based file system. Similar to
versioning systems, changes to /cvmfs/...are temporary until they are
cvmfs_server publish) or discarded
cvmfs_server abort). That allows you to test and verify changes,
for instance to test a newly installed release before publishing it to
clients. Whenever changes are published (committed), a new file system
snapshot of the current state is created. These file system snapshots
can be tagged with a name, which makes them named snapshots. A named
snapshot is meant to stay in the file system. One can rollback to named
snapshots and it is possible, on the client side, to mount any of the
named snapshots in lieu of the newest available snapshot.
Two named snapshots are managed automatically by CernVM-FS,
trunk-previous. This allows for easy unpublishing of a mistake,
by rolling back to the
CernVM-FS provides an integrity checker for repositories. It is invoked by
The integrity checker verifies the sanity of file catalogs and verifies that referenced data chunks are present. Ideally, the integrity checker is used after every publish operation. Where this is not affordable due to the size of the repositories, the integrity checker should run regularly.
The checker can also run on a nested catalog subtree. This is useful to follow up a specific issue where a check on the full tree would take a lot of time:
cvmfs_server check -s <path to nested catalog mountpoint>
cvmfs_server check can also verify the data integrity
(command line flag
-i) of each data object in the repository. This
is a time consuming process and we recommend it only for diagnostic
Named snapshots or tags are an easy way to organise checkpoints in the file system history. CernVM-FS clients can explicitly mount a repository at a specific named snapshot to expose the file system content published with this tag. It also allows for rollbacks to previously created and tagged file system revisions. Tag names need to be unique for each repository and are not allowed to contain spaces or spacial characters. Besides the actual tag’s name they can also contain a free descriptive text and store a creation timestamp.
Named snapshots are best to use for larger modifications to the repository, for instance when a new major software release is installed. Named snapshots provide the ability to easily undo modifications and to preserve the state of the file system for the future. Nevertheless, named snapshots should not be used excessively. Less than 50 named snapshots are a good number of named snapshots in many cases.
Creating a Named Snapshot¶
Tags can be added while publishing a new file system revision. To do so,
the -a and -m options for
cvmfs_server publish are used. The
following command publishes a CernVM-FS revision with a new revision
that is tagged as “release-1.0”:
cvmfs_server transaction # Changes cvmfs_server publish -a release-1.0 -m "first stable release"
Managing Existing Named Snapshots¶
Management of existing tags is done by using the
command. Without any command line parameters, it will print all
currently available named snapshots. Snapshots can be inspected
-i <tag name>), removed (
-r <tag name>) or created
-a <tag name> -m <tag description> -h <catalog root hash>).
Furthermore machine readable modes for both listing (
-l -x) as well
as inspection (
-i <tag name> -x) is available.
A repository can be rolled back to any of the named snapshots. Rolling
back is achieved through the command
cvmfs_server rollback -t release-1.0 A rollback is, like restoring
from backups, not something one would do often. Use caution, a rollback
Managing Nested Catalogs¶
CernVM-FS stores meta-data (path names, file sizes, ...) in file catalogs. When a client accesses a repository, it has to download the file catalog first and then it downloads the files as they are opened. A single file catalog for an entire repository can quickly become large and impractical. Also, clients typically do not need all of the repository’s meta-data at the same time. For instance, clients using software release 1.0 do not need to know about the contents of software release 2.0.
With nested catalogs, CernVM-FS has a mechanism to partition the
directory tree of a repository into many catalogs. Repository
maintainers are responsible for sensible cutting of the directory trees
into nested catalogs. They can do so by creating and removing magic
For example, in order to create a nested catalog for software release 1.0 in the hypothetical repository experiment.cern.ch, one would invoke
cvmfs_server transaction touch /cvmfs/experiment.cern.ch/software/1.0/.cvmfscatalog cvmfs_server publish
In order to merge a nested catalog with its parent catalog, the
.cvmfscatalog file needs to be removed. Nested
catalogs can be nested on arbitrary many levels.
Recommendations for Nested Catalogs¶
Nested catalogs should be created having in mind which files and directories are accessed together. This is typically the case for software releases, but can be also on the directory level that separates platforms. For instance, for a directory layout like
/cvmfs/experiment.cern.ch |- /software | |- /i686 | | |- 1.0 | | |- 2.0 | ` |- common | |- /x86_64 | | |- 1.0 | ` |- common |- /grid-certificates |- /scripts
it makes sense to have nested catalogs at
/cvmfs/experiment.cern.ch/software/i686 /cvmfs/experiment.cern.ch/software/x86_64 /cvmfs/experiment.cern.ch/software/i686/1.0 /cvmfs/experiment.cern.ch/software/i686/2.0 /cvmfs/experiment.cern.ch/software/x86_64/1.0
A nested catalog at the top level of each software package release is generally the best approach because once package releases are installed they tend to never change, which reduces churn and garbage generated in the repository from old catalogs that have changed. In addition, each run only tends to access one version of any package so having a separate catalog per version avoids loading catalog information that will not be used. A nested catalog at the top level of each platform may make sense if there is a significant number of platform-specific files that aren’t included in other catalogs.
It could also make sense to have a nested catalog under grid-certificates, if the certificates are updated much more frequently than the other directories. It would not make sense to create a nested catalog under /cvmfs/experiment.cern.ch/software/i686/common, because this directory needs to be accessed anyway whenever its parent directory is needed. As a rule of thumb, a single file catalog should contain more than 1000 files and directories but not contain more than \(\approx\)200000 files. See Inspecting Nested Catalog Structure how to find catalogs that do not satisfy this recommendation.
Restructuring the repository’s directory tree is an expensive operation in CernVM-FS. Moreover, it can easily break client applications when they switch to a restructured file system snapshot. Therefore, the software directory tree layout should be relatively stable before filling the CernVM-FS repository.
Managing Nested Catalogs with
Rather than managing
.cvmfscatalog files by hand, a repository
administrator may create a file called
.cvmfsdirtab, in the top
directory of the repository, which contains a list of paths relative to
the top of the repository where
.cvmfscatalog files will be created.
Those paths may contain shell wildcards such as asterisk (
question mark (
?). This is useful for specifying patterns for
creating nested catalogs as new files are installed. A very good use of
the patterns is to identify directories where software releases will be
In addition, lines in
.cvmfsdirtab that begin with an exclamation
!) are shell patterns that will be excluded from those
matched by lines without an exclamation point. For example a
.cvmfsdirtab might contain these lines for the repository of the
/software/* /software/*/* ! */common /grid-certificates
This will create nested catalogs at
/cvmfs/experiment.cern.ch/software/i686 /cvmfs/experiment.cern.ch/software/i686/1.0 /cvmfs/experiment.cern.ch/software/i686/2.0 /cvmfs/experiment.cern.ch/software/x86_64 /cvmfs/experiment.cern.ch/software/x86_64/1.0 /cvmfs/experiment.cern.ch/grid-certificates
Note that unlike the regular lines that add catalogs, asterisks in the exclamation point exclusion lines can span the slashes separating directory levels.
Automatic Management of Nested Catalogs¶
An alternative to
.cvmfsdirtab is the automatic catalog generation.
This feature automatically generates nested catalogs based on their
weight (number of entries). It can be enabled by setting
CVMFS_AUTOCATALOGS=true in the server configuration file.
Catalogs are split when their weight is greater than a specified maximum
threshold, or removed if their weight is less than a minimum threshold.
Automatically generated catalogs contain a
(along with the
.cvmfscatalog file) in its root directory.
User-defined catalogs (containing only a
.cvmfscatalog file) always
remain untouched. Hence one can mix both manual and automatically
managed directory sub-trees.
The following conditions are applied when processing a nested catalog:
- If the weight is greater than
CVMFS_AUTOCATALOGS_MAX_WEIGHT, this catalog will be split in smaller catalogs that meet the maximum and minimum thresholds.
- If the weight is less than
CVMFS_AUTOCATALOGS_MIN_WEIGHT, this catalog will be merged into its parent.
CVMFS_AUTOCATALOGS_MIN_WEIGHT have reasonable defaults and usually
do not need to be defined by the user.
Inspecting Nested Catalog Structure¶
The following command visualizes the current nested file catalog layout of a repository.
Additionally this command allows to spot degenerated nested catalogs. As
stated here the recommended
maximal file entry count of a single catalog should not exceed
\(\approx\)200000. One can use the switch
list-catalogs -e to
inspect the current nested catalog entry counts in the repository.
list-catalgos -s will print the file sizes of the
catalogs in bytes.
Repository Mount Point Management¶
In rare occasions (for example at reboot of a release manager machine)
CernVM-FS might need to perform repair operations on those mount points.
As of CernVM-FS 2.2.0
those mount points are not automatically mounted on reboot of the release
manager machine anymore. Usually the mount point handling happens
automatically and transparently to the user when invoking arbitrary
cvmfs_server mount <repo name> allows users to explicitly
trigger this repair operation anytime for individual repositories. Mounting
all hosted repositories is possible with the
-a parameter but requires
root privileges. If you want to have all hosted repositories mounted after
reboot then put
cvmfs_server mount -a in a boot script, for example in
# properly mount a specific repository cvmfs_server mount test.cern.ch # properly mount all hosted repositories (as root) sudo cvmfs_server mount -a
Syncing files into a repository with cvmfs_rsync¶
A common method of publishing into CernVM-FS is to first install all the
files into a convenient shared filesystem, mount the shared filesystem
on the publishing machine, and then sync the files into the repository
during a transaction. The most common tool to do the syncing is
rsync by itself doesn’t have a convenient mechanism
for avoiding generated
in the CernVM-FS repository. Actually the
--exclude option is good
for avoiding the extra files, but the problem is that if a source
directory tree is removed, then
rsync will not remove the
corresponding copy of the directory tree in the repository if it
contains a catalog, because the extra file remains in the repository.
For this reason, a tool called
cvmfs_rsync is included in the
cvmfs-server package. This is a small wrapper around
--exclude options and removes
.cvmfsautocatalog files from a repository when the corresponding
source directory is removed. This is the usage:
cvmfs_rsync [rsync_options] srcdir /cvmfs/reponame[/destsubdir]
This is an example use case:
$ cvmfs_rsync -av --delete /data/lhapdf /cvmfs/cms.cern.ch
Migrate File Catalogs¶
In rare cases the further development of CernVM-FS makes it necessary to change the internal structure of file catalogs. Updating the CernVM-FS installation on a Stratum 0 machine might require a migration of the file catalogs.
It is recommended that
cvmfs_server list is issued after any
CernVM-FS update to review if any of the maintained repositories need a
migration. Outdated repositories will be marked as “INCOMPATIBLE” and
cvmfs_server refuses all actions on these repositories until the
file catalogs have been updated.
In order to run a file catalog migration use
for each of the outdated repositories. This will essentially create a
new repository revision that contains the exact same file structure as
the current revision. However, all file catalogs will be recreated from
scratch using the updated internal structure. Note that historic file
catalogs of all previous repository revisions stay untouched and are not
cvmfs_server migrate has successfully updated all file
catalogs repository maintenance can continue as usual.
Change File Ownership on File Catalog Level¶
CernVM-FS tracks the UID and GID of all contained files and exposes them through the client to all using machines. Repository maintainers should keep this in mind and plan their UID and GID assignments accordingly.
Repository operation might occasionally require to bulk-change many or all
UIDs/GIDs. While this is of course possible via
chmod -R in a normal
repository transaction, it is cumbersome for large repositories. We provide
a tool to quickly do such adaption on CernVM-FS catalog level using UID and GID mapping files:
cvmfs_server catalog-chown -u <uid map> -g <gid map> <repo name>
Both the UID and GID map contain a list of rules to apply to each file meta data record in the CernVM-FS catalogs. This is an example of such a rules list:
# map root UID/GID to 1001 0 1001 # swap UID/GID 1002 and 1003 1002 1003 1003 1002 # map everything else to 1004 * 1004
Note that running
cvmfs_server catalog-chown produces a new repository
revision containing CernVM-FS catalogs with updated
UIDs and GIDs according to the provided rules. Thus, previous revisions of
the CernVM-FS repository will not be affected by this update.
Repository Garbage Collection¶
Since CernVM-FS is a versioning file system it is following an insert-only policy regarding its backend storage. When files are deleted from a CernVM-FS repository, they are not automatically deleted from the underlying storage. Therefore legacy revisions stay intact and usable forever (cf. Named Snapshots) at the expense of an ever-growing storage volume both on the Stratum 0 and the Stratum 1s.
For this reason, applications that frequently install files into a repository and delete older ones - for example the output from nightly software builds - might quickly fill up the repository’s backend storage. Furthermore these applications might actually never make use of the aforementioned long-term revision preservation rendering most of the stored objects “garbage”.
CernVM-FS supports garbage-collected repositories that automatically remove unreferenced data objects and free storage space. This feature needs to be enabled on the Stratum 0 and automatically scans the repository’s catalog structure for unreferenced objects both on the Stratum 0 and the Stratum 1 installations on every publish respectively snapshot operation.
Garbage Sweeping Policy¶
The garbage collector of CernVM-FS is using a mark-and-sweep algorithm
to detect unused files in the internal catalog graph. Revisions that are
referenced by named snapshots (cf. Named Snapshots) or that
are recent enough are preserved while all other revisions are condemned
to be removed. By default this time-based threshold is three days but
can be changed using the configuration variable
CVMFS_AUTO_GC_TIMESPAN both on Stratum 0 and Stratum 1. The value of
this variable is expected to be parseable by the
date command, for
3 days ago or
1 week ago.
Enabling Garbage Collection¶
Creating a Garbage Collectable Repository¶
Repositories can be created as garbage-collectable from the start by adding
-z to the
cvmfs_server mkfs command (cf. Repository Creation). It
is generally recommended to also add
-g to switch off automatic tagging in
a garbage collectable repository.
For debugging or bookkeeping it is possible to log deleted objects into a file
CVMFS_GC_DELETION_LOG to a writable file path.
Enabling Garbage Collection on an Existing Repository (Stratum 0)¶
server.confof the repository. Furthermore it is recommended to switch off automatic tagging by setting
CVMFS_AUTO_TAG=falsefor a garbage collectable repository. The garbage collection will be enabled with the next published transaction.
Enabling Garbage Collection on an Existing Replication (Stratum 1)¶
In order to use automatic garbage collection on a stratum 1 replica
CVMFS_AUTO_GC=true needs to be added in the
server.conf file of
the stratum 1 installation. This will only work if the upstream stratum
0 repository has garbage collection enabled.
Limitations on Repository Content¶
Because CernVM-FS provides what appears to be a POSIX filesystem to clients, it is easy to think that it is a general purpose filesystem and that it will work well with all kinds of files. That is not the case, however, because CernVM-FS is optimized for particular types of files and usage. This section contains guidelines for limitations on the content of repositories for best operation.
First and foremost, CernVM-FS is designed to distribute executable code that is shared between a large number of jobs that run together at grid sites, clouds, or clusters. Worker node cache sizes and web proxy bandwidth are generally engineered to accommodate that application. The total amount read per job is expected to be roughly limited by the amount of RAM per job slot. The same files are also expected to be read from the worker node cache multiple times for the same type of job, and read from a caching web proxy by multiple worker nodes.
If there are data files distributed by CernVM-FS that follow similar access patterns and size limits as executable code, it will probably work fine. In addition, if there are files that are larger but read slowly throughout long jobs, as opposed to all at once at the beginning, that can also work well if the same files are read by many jobs. That is because web proxies have to be engineered for handling bursts at the beginning of jobs and so they tend to be lightly loaded a majority of the time.
In general, a good rule of thumb is to calculate the maximum rate at which jobs typically start and limit the amount of data that might be read from a web proxy to per thousand jobs, assuming a reasonable amount of overlap of jobs onto the same worker nodes. Also, limit the amount of data that will be put into any one worker node cache to . Of course, if you have a special arrangement with particular sites to have large caches and bandwidths available, these limits can be made higher at those sites. Web proxies may also need to be engineered with faster disks if the data causes their cache hit ratios to be reduced.
Also, keep in mind that the total amount of data distributed is not unlimited. The files are stored and distributed compressed, and files with the same content stored in multiple places in the same repository are collapsed to the same file in storage, but the storage space is used not only on the original repository server, it is also replicated onto multiple Stratum 1 servers. Generally if only executable code is distributed, there is no problem with the space taken on Stratum 1s, but if many large data files are distributed they may exceed the Stratum 1 storage capacity. Data files also tend to not compress as well, and that is especially the case of course if they are already compressed before installation.
Tarballs, zip files, and other archive files¶
If the contents of a tarball, zip file, or some other type of archive file is desired to be distributed by CernVM-FS, it is usually better to first unpack it into its separate pieces first. This is because it allows better sharing of content between multiple releases of the file; some pieces inside the archive file might change and other pieces might not in the next release, and pieces that don’t change will be stored as the same file in the repository. CernVM-FS will compress the content of the individual pieces, so even if there’s no sharing between releases it shouldn’t take much more space.
Care should be taken to make all the files in a repository readable by “other”. This is because permissions on files in the original repository are generally the same as those seen by end clients, except the files are owned by the “cvmfs” user and group. The write permissions are ignored by the client since it is a read-only filesystem. However, unless the client has set
(and most do not), unprivileged users will not be able to read files unless they are readable by “other” and all their parent directories have at least “execute” permissions. It makes little sense to publish files in CernVM-FS if they won’t be able to be read by anyone.