Administrator’s guide


Archipelago is a distinct storage layer that provides the logic and interfaces needed to integrate with a cloud platform, while in the same time being agnostic to the underlying storage technology, which is used to store the actual data.

In the figure below, we see Archipelago running in a number of physical nodes. It provides different endpoints for volume access by VMs or file access via HTTP:


It also provides the corresponding logic for thin volume cloning and snapshotting independently from the actual storage. Archipelago is deployed on each node where the volume will be used (e.g.: acting as a VM disk). The volume is exposed as an independent block device and accessed as such. The data of each volume can reside on any supported underlying storage type. The software stack will take care of any coordination or concurrency control needed between other nodes running Archipelago.

In the same way Archipelago may be deployed on a node that will act as an HTTP gateway for files. It will then provide the corresponding endpoint and the deduplication logic to export the data stored on the underlying storage as files.

Archipelago’s goal is to decouple the composition/snapshot/cloning/deduplicating logic from the storage backend used. Essentially, provide the necessary layer where the aforementioned logic and volume handling is implemented and also implement an interface with pluggable storage drivers, to operate over different storage types. Finally, export different access endpoints for use by the upper layers.


Every Volume inside a VM can be thought of as a linearly addressable set of fixed-size blocks. The storage of the actual blocks is orthogonal to the task of exposing a single block device for use by each VM. Bridging the gap between the VMs performing random access to Volumes and the storage of actual blocks is Archipelago: a custom storage handling layer which handles volumes as set of distinct blocks in the backend, a process we call volume composition.

For the actual storage of blocks, Archipelago is agnostic to the storage backend used. Through pluggable storage drivers, Archipelago can support multiple storage backends to suit the needs of each deployment. We currently provide two storage drivers. One for simple files, where each object is stored as a single file on the (shared) filesystem, and one for objects backed by RADOS. RADOS is the distributed object store which supports the Ceph parallel filesystem. With RADOS, we can solve the problem of reliable, fault-tolerant object storage through replication on multiple storage nodes.

As mentioned before, Archipelago composes the volume through individual blocks. This is accomplished by maintaining a map for each volume, to map offset in a volume with a single object. The exact offset inside the object, is calculated statically from the fixed object size and the offset in the volume. But having this map and the composition subsystems, allow us to do much more than simple volume composition. Archipelago offers Copy-On-Write snapshottable volumes. Furthermore, each snapshot can be hashed, to allow deduplication to play its part, reducing the storage cost of each hashed object. Furthermore, Archipelago can integrate with Pithos, and use Pithos images to provision a volume with Copy-On-Write semantics (i.e. a clone). Since Pithos images are already hashed, we can store Archipelago hashed volumes, which are indistinguishable from a Pithos image, along with the Pithos images, to enable further deduplication, or even registering an archipelago hashed snapshot as Pithos image file. The above process is illustrated on the following figure.


Archipelago is used by Cyclades and Ganeti for fast VM provisioning based on CoW volumes. Moreover, it enables live migration of thinly-provisioned VMs with no physically shared storage.

Bare in mind that currently Archipelago does not provide any garbage collection functionality. Should you decide to use it, make sure your storage capacity can meet your data needs.

Endpoint and Backend drivers


Archipelago allows users to manage and access the underlying data, which is backed by various storage types. In order to do that, Archipelago can provide multiple endpoints for the user or upper layers to interact. Some of them are:

  • block device driver
  • qemu driver
  • user provided process
  • command line tool
  • http gateway for files

It also implements different drivers to interact with different types of underlying storage technologies.

Internal Architecture

Archipelago consists of several components, both userspace and kernelspace, which communicate through a custom-built shared memory segment communication mechanism. This mechanism, which is called XSEG, also defines a common communication protocol between these components and is provided by the library libxseg. Each Archipelago component is an xseg peer. The segment provides ports, where each peer binds. The peer then uses the port to communicate with the other peers on the same segment. The communication consists of requests that are submitted to the receiver’s port request queue, and are responded to the submitter’s port reply queue.

This form of communication, allows us to develop distinct components for each operation of Archipelago, while being able to communicate with exactly the same protocol between these components.

Archipelago components

Each Archipelago component serves a distinct purpose and coordinates with the other components to provide the final service.

These components are described below.

The Volume composer (vlmcd)

Volume composer is responsible for the volume composition. Blktap devices direct I/O requests on the volume, to the volume composer. Volume composer then consults the mapper, to get the actual objects on which it will perform the appropriate I/O. It then directs I/O requests for each individual object to the blocker and wait for their completion. In the end, it composes the individual responses, to respond to the original volume request from blktap.

The Mapper (mapperd)

Mapper is responsible for keeping and updating the mappings from volume offsets to individual objects which actually hold the data. It is also responsible for creating new volumes, snapshotting existing ones and create new volume based on a previously captured snapshot (clones). It stores the mappings to the storage backend, from which it reads and/or updates them, keeping them cached when appropriate. It also ensure that each action on the volumes, does not happen unless the necessary volume locks are acquired.

The File blocker (filed)

File blocker is responsible for storing each object as a single file in a specified directory. It serves the requests for each objects as they come from the volume composer and the mapper components. Filed currently requires that all files are placed under one filesystem. The directory it operates on, must not contain symlinks or mountpoints to different filesystems. Also the permissions must be set to both read and write for the user Archipelago runs as (default to archipelago).

The RADOS blocker (radosd)

RADOS blocker is another form of blocker which stores each objects as a single object in a RADOS pool. It can be used instead of the file blocker, to create and use disks over RADOS storage.

The Block devices (blktap)

Each volume on Archipelago is exposed as a block device in the system /dev/xen/blktap-2/ directory. These special devices are nothing more than just another peer, which forwards the requests through the shared memory segment, to the volume composer for completion.

In a nutshell, in archipelago, each blktap device communicates through the shared memory segment with the volume composer. Then the volume composer requests the objects on which it should perform the I/O from the mapper. The mapper takes into account all the necessary logic (taking locks etc) and retrieves the mappings from the storage, by requesting the appropriate objects from the blocker responsible to hold the maps. It then performs any copy on write operations needed and returns the mapping to the volume composer. The volume composer then communicates with the blocker responsible for holding the objects where the actual data reside and composes the responses, to respond to the original request.

Archipelago mapfile architecture

Each Archipelago mapfile begins with a header, in big endian format, as follows:

struct ArchipelagoHeader {
    uint32_t signature;
    uint32_t version;
    uint64_t size;
    uint32_t blocksize;
    uint32_t flags;
    uint64_t epoch;
  • The first 4 bytes contain the characters ‘A’, ‘M’, ‘F’, ‘.’.
  • The next 4 bytes contain the format version used by the mapfile. Currently, there have been two versions of the format, version 1 and version 2. Pithos mapfiles weren’t following any specific mapfile header format until now.
  • The next 8 bytes contain the size, in bytes, of the file represented by the mapfile.
  • The blocksize field gives the block size used by the storage backend.
  • The value of the flags field is a mask of flags used to denote access permissions and properties of this mapfile.
  • The epoch field is an index number used as a reference counter.

Archipelago’s User/Group permissions

Archipelago runs by default as user archipelago and group archipelago. These groups are automatically created during installation.

Different Archipelago components use several “named” entities to communicate with each other (e.g. a shared memory segment, named pipes). By having all Archipelago components creating files (or directory entries in general) with read and write permissions for both the user and the group, we ensure that these distinct components can cooperate without permission problems. Furthermore, different services or components that want to integrate with Archipelago can run as their selective user, as long as this user belongs to the group archipelago.


Due to a bug in gunicorn handling of supplementary groups, Pithos does not fall into the above category. The relevant gunicorn worker must be executed as user or group archipelago. Please refer to the relevant Synnefo documentation.

Archipelago Integration with Synnefo and Ganeti

The following figure shows Archipelago, completely integrated with Synnefo and Ganeti in a real-world, large scale cloud environment:



There are three tools provided with Archipelago. The archipelago tool, the vlmc tool and the xseg tool.

Archipelago tool

archipelago tool provides the basic commands to control Archipelago (i.e., starting, restarting and shutting it down). It is meant to be called by the init scripts, but can also be called directly by the administrator.


# archipelago command

Currently it supports the commands described below. Each command supports an optional peer argument to affect only the specified peer. Available peers are defined in the configuration file. In the current version they are limited to one of the following: vlmcd, mapperd, blockerm, blockerb.

  • start [peer] Start archipelago.

    If peer is specified, only the specified peer is affected.

  • stop [-f] [peer] Stop archipelago unless mapped resources exist. In that case, use the -f option to stop Archipelago

    If peer is specified, only the specified peer is affected.

  • restart [peer] Restart archipelago or the specified peer.

    If peer is specified, only the specified peer is affected.

  • status Show the status of archipelago.

Archipelago volume commands

The vlmc tool provides a way to interact with Archipelago volumes. It is meant to be used by the Ganeti ExtStorage scripts, but also directly from the administrator to take actions on volumes.


$ vlmc command [args]

Available commands:

showmapped: Show the mapped volumes and the Archipelago devices on that node.

Usage: $ vlmc showmapped

map: Map the volume to a blktap device

Usage: $ vlmc map <volumename>

unmap: Unmap the specified device from the system.

Usage: $ vlmc unmap </dev/xen/blktap-2/tapdev[0-..]>

create: Create a new volume with an optional specified size from an optional specified snapshot.

Usage: $ vlmc create <volumename> --snap <snapname> --size <size>

Usage: $ vlmc create <volumename> --snap <snapname>

Usage: $ vlmc create <volumename> --size <size>

The --snap and --size are both optional, but at least one of them is mandatory. If snap is not specified, then a blank volume with the specified size is created. If size is not specified, the new volume inherits the size from the snapshot.

snapshot: Create a snapshot with the given name from the specified volume.

Usage: $ vlmc snapshot <volumename> <snapshotname>

remove: Remove the volume.

Usage: $ vlmc remove <volumename>

This may not actually delete all the blocks, but makes the volume inaccessible for usage. All the blocks are removed later, when a garbage collection is invoked.

info: Show volume information. Currently returns only the volume size.

Usage: $ vlmc info <volumename>

open: Open an Archipelago volume. That is, taking all the necessary locks and also make the rest of the infrastructure aware of the operation.

Usage: $ vlmc open <volumename>

This operation succeeds if the volume is already opened by the current host.

close: Close an Archipelago volume. That is, performing all the necessary functions in the insfrastrure to successfully release the volume. Also releases all the acquired locks.

Usage: $ vlmc close <volumename>

A explicit close command should be invoked an explicit open, to release the volume, unless another action triggered an implicit close.

lock: Lock a volume. This step allow the administrator to lock an Archipelago volume, independently from the rest of the infrastructure.

Usage: $ vlmc lock <volumename>

The locks are idempotent for the current owner of the lock. That is, a lock operation will succeed when the volume is already locked by the same blocker.

unlock: Unlock a volume. This allow the administrator to unlock a volume, independently from the rest of the infrastructure.

Usage: $ vlmc unlock [-f] <volumename>

The unlock option can be performed only by the blocker that acquired the lock in the first place. To unlock a volume from another blocker, -f option must be used to break the lock.

XSEG tool

xseg tool provides a way to interact in a very low-level manner with the shared memory segment. It is meant to be used as a last resort recovery tool. It is not recommended to use it under normal circumstances.

It is used as follows:

$ xseg <spec> [[[<src_port>]:[<dst_port>]] [<command> <arg>*] ]

where spec is: posix:<segmentname>:nr_ports:nr_dynports:segment_size:page_shift. Normally a shorter version of posix:<segmentname>: can be used to refer to the selected segment.

A few notable commands are:

create: Creates a new spec according to the spec.

Usage: xseg "posix:<segmentname>:nr_ports:nr_dynports:segment_size:page_shift" create

destroy: Destroys a previously created segment.

Usage: xseg "posix:<segmentname>:" destroy

reportall: Prints a report about the segment’s current status.

Usage: xseg "posix:<segmentname>:" reportall

recoverport: Used to recover all requests a peer had on the fly.

Usage: xseg "posix:<segmentname>:" recoverport <peer port>

recoverlocks: Used to recover all the locks a peer held when crashed.

Usage: xseg "posix:<segmentname>:" recoverlocks <peer port>

It also contains several other debugging and obsolete commands that will be removed on future versions.


archipelago peers and libxseg use the LOCAL0 facility of syslog for debug, info, warning and error logging. Depending on the system logger of your preference, you might want to change its configuration, in order to accommodate for archipelago logs. The log level of each archipelago peer can be changes at runtime, by using the SIGUSR1 and SIGUSR2 signals, to decrement or increment the log level accordingly.

Archipelago locking system

Currently, Archipelago uses a locking system consisting of storage-based locks to control concurrent accesses to the same volume and prevent its corruption. Each lock is held by the blockerm peer of a node. Concurrency control within the same node is handled by Archipelago, and more specifically by the mapper peer. In general, if a blockerm peer crashes, the lock owner is lost but the lock remains and needs to be broken.

Archipelago’s locks change with the storage backend used for blockerm. When using a RADOS backend, radosd uses the native librados locks. These are tied to the RADOS client id which is instantiated when the radosd is launched. When using a file backend, the filed uses files as locks. Each lockfile is tied to the hostname of the lock owner’s node.

Archipelago’s locking system forms the basis for the open/close functionality on volumes. Each volume is implicitly opened by the mapperd on the first I/O access, and explicitly closed by the top-level interface when the volume is unmapped. A successful open denotes that the corresponding volume lock is acquired and the mapperd has exclusive access on the volume. So, it can cache the map data in a write-through manner, along with the lock status. If the underlying lock status changes for any reason (e.g., the administrator breaks the lock), the mapperd is not notified which can result in potential volume corruption. So when the administrator breaks a lock after a blockerm crash, the mapperd still considers the volume locked. A userspace restart is required to reset the mapperd state. Bare in mind, that until this happens, there is a window for possible volume corruption.

Archipelago’s implicit open functionality has several caveats, since the implicit open may be triggered simultaneously from two or more nodes, normally when live-migrating a VM. So to clarify things:

a) for read-only I/O operations, when the volume is not already opened, the mapperd tries to open it by trying to acquire the corresponding volume lock. This lock operation does not block. If it succeeds, the volume is considered opened and the map data are cached by the mapperd. If it fails (e.g., the volume is locked from another node), the I/O is served and the mapperd does not cache anything.

b) for write I/O operations, when the volume is not already open, the mapperd tries to open it by acquiring the corresponding lock. This lock operation can block. If the lock succeeds, then the volume is considered opened by the mapperd and the map data are cached in write-through mode. Otherwise, if the lock is held by another node, the lock operation can either block until the lock is released if it is a volume I/O, or the I/O request will fail if it corresponds to a map modification action.

When closing a volume, the mapperd drops the cache and releases the volume lock. If the release operation fails, an error message is logged but the close operation proceeds. This is useful when unmapping erroneously locked volumes, or when restarting the mapperd with inconsistent locks.

Lock recovery

Due to the fact that the locks remain locked even if the owner goes away (e.g. crashes), a separate manual recovery procedure is needed when a lock owner crashes (either by a full node failure or a single process (blockerm) failure. Please refer to the Archipelago recovery section for the necessary steps.

Archipelago recovery

In the following sections, we describe the recovery procedure for the basic Archipelago failure scenarios. These are:

  • Node failure

    A whole that runs Archipelago node fails (e.g., crashes, reboots, looses power).

  • Peer failure

    A single Archipelago peer crashes (e.g., segfaults).

Node failure

When a node fails, the volumes that were locked by the blockerm of the crashed node, must be manually unlocked.

For each stale volume lock, the following command must be executed from any node that runs Archipelago.

# vlmc unlock -f <volumename>

The administrator must make sure that the volume is locked erroneously before unlocking it. With a RADOS backend, that means that the volume is locked by an older clientid and with a file backend, by a different node.

To get the current owner of a volume lock issue the following:

For a RADOS backend:

$ rados -p <maps pool> lock info <volumename>_lock RadosLock

The above command should print (among others) the (ip, clientid) tuple of the lock owner.

For a file backend:

# cat /path/to/lockdir/<volumename>_lock

The above command should print the hostname of the lock owner. Keep in mind, that since the locks are idempotent and for the file backend tied to the node running blockerm, mapping the volume on the same node does not require breaking any locks.

Peer failure

When a peer fails (this includes segfaults or and SIGKILL on misbehavior) follow these steps:

  1. Remove the pidfile. It is located under /var/run/archipelago/<peer>.pid.

    # rm /var/run/archipelago/<peer>.pid
  2. Recover all the locks the peer may have left stale:

    You must execute the following command for each port the peer was bound to. If the tool encounters an unrecoverable situation, you will be prompted to reboot the node.

    segmentname is the name of the segment configured in the configuration file. It defaults to archipelago.

    $ xseg "posix:<segmentname>:" recoverlocks <peer port>
  3. Fail all the requests that the peer had already accepted and processing.

    You must execute the following command for each port the peer was bound to.

    $ xseg "posix:<segmentname>:" recoverport <peer port>
  1. Recover volume locks.

    You can skip this step if the failed peer was not a blockerm backed by RADOS. Otherwise all the volume locks of the mapped volumes must be recovered manually.

    • Get a list of the currently mapped volumes.

      $ vlmc showmapped
    • For each mapped volume, make sure it is locked by the current node. Use the following command to get the current locker.

      $ rados -p <maps pool> lock info <volumename>_lock RadosLock

      If the volume is locked by the ip of the current node, it should be unlocked using the following command from another node that runs Archipelago.

      $ vlmc unlock -f <volumename>

    It is the Administrator’s responsibility to ensure that no other node is currently accessing the volume to be unlocked, and will not be accessed until this procedure completes.

  2. Start the peer

    # archipelago start <peer>
  3. Perform an Archipelago restart

    # archipelago restart

Calculating SEGMENT_SIZE

To calculate an appropriate value for SEGMENT_SIZE, one has to take into account several parameters. Peers bind to segment ports, allocate requests and shared buffers in order to communicate and transfer data among them. These operations occupy memory from the shared memory segment. The segment must have enough size to fulfil the above needs.

For example, when a VM wants to perform an I/O, the tapdisk process will eventually have to allocate a request describing this I/O, a buffer to hold the data transfered and a few extra space for bookkeeping purposes. The same goes for Pithos storage needs.

A good rule of thumb for one to calculate an appropriate segment size would be the following formula:

``256 + 128 * 0.5 * NR_VM_VOLUMES``


  • 256 is the base size.

  • 128 is the avergae max number of requests.

  • 0.5 is the max request size in MiB (512KiB)

    This is enforced by the tapdisk process.

  • NR_VM_VOLUMES is the expected number of volumes the host will have.

or for a Pithos node:



  • 256 is the base size

  • 4 is the max request size in MiB

    This is enforced by Pithos logic.


The Archipelago configuration file is /etc/archipelago/archipelago.conf.

A list of all configuration setting follows. Archipelago comes with sensible defaults for all of these parameters. You do not need to set any of these parameters to run it successfully. They are listed here for completeness, and may come handy for debugging

[Archipelago] section:


Description: Switch peer processes to run as this user.

Allowed values: Any username available on the node.


Description: Switch peer processes to run as this group.

Allowed values: Any groupname available on the node.


Description: Enable Archipelago’s blktap support

Allowed values: True, False


Description: Set umask for Archipelago processes and external tools (e.g. Ganeti’s ExtStorage scripts)

Allowed values: Any valid umask setting in any recognizable form by Python (e.g. ‘0o022’, ‘022’, ‘18’)

[XSEG] section:


Description: Max number of ports in the segment.

Allowed values: Any positive integer.


Description: The size of the shared memory segment in MiB.

Allowed values: Any positive integer.


Description: Start port of vlmc tool.

Allowed values: Any port in the range [0-SEGMENT_PORTS). It must not confict with any other port range.


Description: End port of vlmc tool.

Allowed values: Any port in the range (VTOOL_START-SEGMENT_PORTS). It must not confict with any other port range.

[PEERS] section:


Description: A list of roles that will be instantiated on the current node.

Allowed values: A list with space seperated values of the available peer roles (blockerm, blockerb, mapperd, vlmcd).


Description: The order that the roles will be started, or stopped (in reverse).

Allowed values: A list with space separated values of the available peer roles (blockerm, blockerb, mapperd, vlmcd).

Individual peer options

Each peer must have its own section in the config file to hold its configuration. The configuration is named after the role name.

Common options among all peers are:
Description: Type of the peer. Specifies the peer that will fulfil this role.
Description: Start port of the peer.
Description: End port of the peer.
Description: Loggging lever for the peer. Available logging levels 0-3.
Description: Number of ops, each peer can have flying.

Description: Set umask for peer.

Allowed values: Any valid umask setting in any recognizable form by Python (e.g. ‘0o022’, ‘022’, ‘18’)


Description: Number of threads of each peer.

Currently only blockers support threads with the following caveats:

  1. Threads in filed are I/O threads that perform blocking I/O.
  2. Threads in radosd are processing threads. For lock congestion reasons, avoid setting them to a value larger than 4.
filed-specific options:
Description: Number of I/O threads to server requests.

Description: Directory where the files will reside. This must be one filesystem and must not contain symlinks or mountpoints to different filesystems.

The user and group that Archipelago runs as (defaults to archipelago) must have both read and write permissions.


Description: Directory where the file based locks will reside. This must be one filesystem and must not contain symlinks or mountpoints to different filesystems.

The user and group that Archipelago runs as (defaults to archipelago) must have both read and write permissions.

Unless this option is provided, lock files are placed along with data files.

Description: Number of file descriptors to be kept open.
Description: Set filed to use the directIO option.
Description: Enable filed to lazily migrate Pithos objects from their old location, to their new one.
radosd-specific options:
Description: Number of threads to server requests.
Description: RADOS pool where the objects will be stored.
mapperd-specific options:
Description: Port for communication with the blocker responsible for the data blocks.
Description: Port for communication with the blocker responsible for the maps.
vlmcd-specific options:
Description: Port for communication with the blocker responsible for the data blocks.
Description: Port for communication with the mapper.