Cyrus Backups

Introduction

Cyrus Backups are a replication-based backup service for Cyrus IMAP servers. This is currently an experimental feature. If you have the resources to try it out alongside your existing backup solutions, feedback would be appreciated.

This document is intended to be a guide to the configuration and administration of Cyrus Backups.

This document is a work in progress and at this point is incomplete.

This document assumes that you are familiar with compiling, installing, configuring and maintaining Cyrus IMAP servers generally, and will only discuss backup-related portions in detail.

This document assumes a passing familiarity with Cyrus Replication.

Limitations

Cyrus Backups are experimental and incomplete.

The following components exist and appear to work:

  • backupd, and therefore inbound replication
  • autovivification of backup storage for new users, with automatic partition selection
  • rebuilding of backup indexes from backup data files
  • compaction of backup files to remove stale data and combine chunks for better compression
  • deep verification of backup file/index state
  • examination of backup data
  • locking tool, for safe non-cyrus operations on backup files
  • recovery of data back into a Cyrus IMAP server

The following components don’t yet exist in a workable state – these tasks must be massaged through manually (with care):

  • reconstruct of backups.db from backup files

The following types of information are currently backed up and recoverable

  • mailbox state and annotations
  • messages
  • mailbox message records, flags, and annotations

The following types of information are currently backed up, but tools to recover them don’t yet exist:

  • sieve scripts (but not active script status)
  • subscriptions
  • seen data

The following types of information are not currently backed up

  • quota information

Architecture

Cyrus Backups are designed to run on one or more standalone, dedicated backup servers, with suitably-sized storage partitions. These servers generally do not run an IMAP daemon, nor do they have conventional mailbox storage.

Your Cyrus IMAP servers synchronise mailbox state to the Cyrus Backup server(s) using the Cyrus replication (aka sync, aka csync) protocol.

Backup data is stored in two files per user: a data file, containing gzipped chunks of replication commands; and an SQLite database, which indexes the current state of the backed up data. User backup files are stored in a hashed subdirectory of their containing partition.

A twoskip database, backups.db, stores mappings of users to their backup file locations

Installation

Requirements

  • At least one Cyrus IMAP server, serving and storing user data.
  • At least one machine which will become the first backup server.

Cyrus Backups server

  1. Compile cyrus with the –enable-backup configure option and install it.

  2. Set up an imapd.conf(5) file for it with the following options (default values shown):

    backup_db: twoskip

    The twoskip database format is recommended for backups.db

    backup_db_path: {configdirectory}/backups.db

    The backups db contains a mapping of user ids to their backup locations

    backup_staging_path: {temp_path}/backup

    Directory to use for staging message files during backup operations. The replication protocol will transfer as many as 1024 messages in a single sync operation, so, conservatively, this directory needs to contain enough storage for 1024 * your maximum message size * number of running backupd’s, plus some wiggle room.

    backup_retention_days: 7

    Number of days for which backup data (messages etc) should be kept within the backup storage after the corresponding item has been deleted/expunged from the Cyrus IMAP server.

    backuppartition-name: /path/to/this/partition

    You need at least one backuppartition-name to store backup data. These work similarly to regular/archive IMAP partitions, but note that there is no relationship between backup partition names and regular/archive partition names. New users will be have their backup storage provisioned according to the usual partition selection rules.

    backup_compact_minsize: 0

    The ideal minimum data chunk size within backup files, in kB. The compact tool will try to combine chunks that are smaller than this into neighbouring chunks. Larger values tend to yield better compression ratios, but if the data is corrupted on disk, the entire chunk will become unreadable. Zero turns this behaviour off.

    backup_compact_maxsize: 0

    The ideal maximum data chunk size within backup files, in kB. The compact tool will try to split chunks that are larger than this into multiple smaller chunks. Zero turns this behaviour off.

    backup_compact_work_threshold: 1

    The number of chunks within a backup file that must obviously need compaction before the compact tool will attempt to compact the file. Larger values are expected to reduce compaction I/O load at the expense of delayed recovery of storage space.

  3. Create a user for authenticating to the backup system, and add it to the admins setting in imapd.conf(5)

  4. Add appropriate sasl_* settings for your authentication method to imapd.conf(5)

  5. Set up a cyrus.conf(5) file for it:

    SERVICES {
        # backupd is probably the only service entry your backup server needs
        backupd cmd="backupd" listen="csync" prefork=0
    }
    
    EVENTS {
        # this is required
        checkpoint cmd="ctl_cyrusdb -c" period=30
    
        # arrange for compact to run at some interval
        compact cmd="ctl_backups compact -A" at=0400
    }
    
  6. Start up the server, and use synctest(1) to verify that you can authenticate to backupd

Cyrus IMAP servers

Your Cyrus IMAP servers must be running version 3 or later of Cyrus, and must have been compiled with the --enable-replication configure option. It does not need to be recompiled with the --enable-backup option.

It’s recommended to set up a dedicated replication channel for backups, so that your backup replication can coexist independently of your other replication configurations

Add settings to imapd.conf(5) like (default values shown):

channel_sync_host: backup-server.example.com
The host name of your Cyrus Backup server
channel_sync_port: csync
The port on which your Cyrus Backup server’s backupd process listens
channel_sync_authname: ...
Credentials for authenticating to the Cyrus Backup server
channel_sync_password: ...
Credentials for authenticating to the Cyrus Backup server

Using rolling replication

You can configure backups to use rolling replication. Depending on the sync repeat interval you configure, this can be used to keep your backups very current – potentially as current as your other replicas.

To configure rolling replication, add additional settings to imapd.conf(5) like:

sync_log: 1
Enable sync log if it wasn’t already.
sync_log_channels: channel
Add a new channel “channel” to whatever was already here. Suggest calling this “backup”
channel_sync_repeat_interval: 1
Minimum time in seconds between rolling replication runs. Smaller value means livelier backups but more network I/O. Larger value reduces I/O.

Update cyrus.conf(5) to add a sync_client(8) invocation to the SERVICES section specifying (at least) the -r and -n channel options.

See imapd.conf(5) for additional sync_ settings that can be used to affect the replication behaviour. Many can be prefixed with a channel to limit their affect to only backups, if necessary.

Using scheduled replication (push)

You can configure backups to occur on a schedule determined by the IMAP server.

To do this, add sync_client(8) invocations to the EVENTS section of cyrus.conf(5) (or cron, etc), specifying at least the -n channel option (to use the channel-specific configuration), plus whatever other options you need for selecting users to back up. See the sync_client(8) manpage for details.

You could also invoke sync_client(8) in a similar way from a custom script running on the IMAP server.

Using scheduled replication (pull)

You can configure backups to occur on a schedule determined by the backup server. For example, you may have a custom script that examines the existing backups, and provokes fresh backups to occur if they are determined to be out of date.

To to this, enable XBACKUP on your IMAP server by adding the following setting to imapd.conf(5):

xbackup_enabled: yes
Enables the XBACKUP command in imapd.

Your custom script can then authenticate to the IMAP server as an admin user, and invoke the command XBACKUP pattern [channel]. A replication of the users or shared mailboxes matching the specified pattern will occur to the backup server defined by the named channel. If no channel is specified, default sync configuration will be used.

For example:

C: 1 XBACKUP user.* backup
S: * OK USER anne
S: * OK USER bethany
S: * NO USER cassandane (Operation is not supported on mailbox)
S: * OK USER demi
S: * OK USER ellie
S: 1 OK Completed

This replicates all users to the channel backup.

Administration

Storage requirements

It’s not really known yet how to predict the storage requirements for a backup server. Experimentation in dev environment suggests around 20-40% compressed backup file size relative to the backed up data, depending on compact settings, but this is with relatively tiny mailboxes and non-pathological data.

The backup staging spool conservatively needs to be large enough to hold an entire sync’s worth of message files at once. Which is your maximum message size * 1024 messages * the number of backupd processes you’re running, plus some wiggle room probably. In practice it’ll probably not hit this limit unless someone is trying to. (Most users, I suspect, don’t have 1024 maximum-sized messages in their account, or don’t receive them all at once anyway.)

Certain invocations of ctl_backups and cyr_backup also require staging spool space, due to the way replication protocol (and thus backup data) parsing handles messages. So keep this in mind I suppose.

Initial backups

Once a Cyrus Backup system is configured and running, new users that are created on the IMAP servers will be backed up seamlessly without administrator intervention.

The very first backup taken of a pre-existing mailbox will be big – the entire mailbox in one hit. It’s suggested that, when initially provisioning a Cyrus Backup server for an existing Cyrus IMAP environment, that the sync_client(8) commands be run carefully, for a small group of mailboxes at a time, until all/most of your mailboxes have been backed up at least once. Also run the ctl_backups(8) compact command on the backups, to break up big chunks, if you wish. Only then should you enable rolling/scheduled replication.

Restoring from backups

The restore(8) tool will restore mailboxes and messages from a specified backup to a specified destination server. The destination server must be running a replication-capable imapd(8) or sync_server(8). The restore tool should be run from the backup server containing the specified backup.

File locking

All backupd(8)/ctl_backups(8)/cyr_backup(8) operations first obtain a lock on the relevant backup file. ctl_backups and cyr_backup will try to do this without blocking (unless told otherwise), whereas backupd will never block.

Moving backup files to different backup partitions

There’s no tool for this (yet). To do it manually, stop backupd, copy the files to the new partition, then use cyr_dbtool(8) to update the user’s backups.db entry to point to the new location. Run the ctl_backups(8) verify command on both the new filename (-f mode) and the user’s userid (-u mode) to ensure everything is okay, then restart backupd.

Provoking a backup for a particular user/user group/everyone/etc right now

Just run sync_client(8) by hand with appropriate options (as cyrus user, of course). See its man page for ways of specifying items to replicate.

If the IMAP server with the user’s mail has been configured with the xbackup_enabled: yes option in imapd.conf(5), then an admin user can cause a backup to occur by sending the IMAP server an XBACKUP command.

What about tape backups?

As long as backupd, ctl_backups and cyr_backup are not currently running (and assuming no-one’s poking around in things otherwise), it’s safe to take/restore a filesystem snapshot of backup partitions. So to schedule, say, a nightly tape dump of your Cyrus Backup server, make your cron job shut down Cyrus, make the copy, then restart Cyrus.

Meanwhile, your Cyrus IMAP servers are still online and available. Regular backups will resume once your backupd is running again.

If you can work at a finer granularity than file system, you don’t need to shut down backupd. Just use the ctl_backups(8) lock command to hold a lock on each backup while you work with its files, and the rest of the backup system will work around that.

Restorating is more complicated, depending on what you actually need to do: when you restart the backupd after restoring a filesystem snapshot, the next time your Cyrus IMAP server replicates to it, the restored backups will be brought up to date. Probably not what you wanted – so don’t restart backupd until you’ve done whatever you were doing.

Multiple IMAP servers, one backup server

This is fine, as long as each user being backed up is only being backed up by one server (or they are otherwise synchronised). If IMAP servers have different ideas about the state of a user’s mailboxes, one of those will be in sync with the backup server and the other will get a lot of replication failures.

Multiple IMAP servers, multiple backup servers

Make sure your sync_client(8) configuration(s) on each IMAP server knows which users are being backed up to which backup servers, and selects them appropriately. See the sync_client(8) man page for options for specifying users, and run it as an event (rather than rolling).

Or just distribute it at server granularity, such that backup server A serves IMAP servers A, B and C, and backup server B serves IMAP servers D, E, F, etc.

One IMAP server, multiple backup servers

Configure one channel plus one rolling sync_client(8) per backup server, and your IMAP server can be more or less simultaneously backed up to multiple backup destinations.

Reducing load

To reduce load on your client-facing IMAP servers, configure sync log chaining on their replicas and let those take the load of replicating to the backup servers.

To reduce network traffic, do the same thing, specifically using replicas that are already co-located with the backup server.

Other setups

The use of the replication protocol and sync_client(8) allows a lot of interesting configuration possibilities to shake out. Have a rummage in the sync_client(8) man page for inspiration.

Tools

ctl_backups

This tool is generally for mass operations that require few/fixed arguments across multiple/all backups

Supported operations:

compact

Reduce backups’ disk usage by:

  • combining small chunks for better gzip compression – especially important for hot backups, which produce many tiny chunks
  • removing deleted content that has passed its retention period
list
List known backups.
lock
Lock a single backup, so you can safely work on it with non-cyrus tools.
reindex
Regenerate indexes for backups from their data files. Useful if index becomes corrupted by some bug, or invalidated by working on data with non-cyrus tools.
stat
Show statistics about backups – disk usage, compression ratio, etc.
verify

Deep verification of backups. Verifies that:

  • Checksums for each chunk in index match data
  • Mailbox states are in the chunk that the index says they’re in
  • Mailbox states match indexed states
  • Messages are in the chunk the index says they’re in
  • Message data checksum matches indexed checksums

See the ctl_backups(8) man page for more information.

cyr_backup

This tool is generally for operations on a single mailbox that require multiple additional arguments

Supported operations

list [ chunks | mailboxes | messages | all ]
Line-per-item listing of information stored in a backup.
show [ chunks | mailboxes | messages ] items...
Paragraph-per-item listing of information for specified items. Chunk items are specified by id, mailboxes by mboxname or uniqueid, messages by guid.
dump [ chunk | message ] item
Full dump of one item. chunk dumps the uncompressed content of a chunk (i.e. a bunch of sync protocol commands). message dumps a raw rfc822 message (useful for manually restoring)

See the cyr_backup(8) man page for more information.

restore

This tool is for restoring mail from backup files.

Required arguments are a destination server (in ip:port or host:port format), a backup file, and mboxnames, uniqueids or guids specifying the mailboxes or messages to be restored.

If the target mailbox does not already exist on the destination server, options are available to preserve the mailbox and message properties as they existed in the backup. This is useful for rebuilding a lost server from backups, such that client state remains consistent.

If the target mailbox already exists on the destination server, restored messages will be assigned new, unused uids and will appear to the client as new messages.

See the restore(8) man page for more information.