Mailbox Creation Distribution

Prior to Cyrus IMAP version 2.5, when creating a mailbox, should no target partition have been specified, the mailbox is either created on:

  • the configured defaultserver, or
  • the server that is found to have the most free disk space, and
  • the configured defaultpartition, or
  • the most fitting partition if defaultpartition is not configured.

New configuration options are available since Cyrus IMAP 2.5, that allow more weighted and better balanced backend and partition selection.

Selection Mode

Among the partitions, how the most fitting one is selected depends on the configured selection mode: partition_mode setting in /etc/imapd.conf

Available Selection Modes

  • random

    Choice is (pseudo-)random. Each partition has the same probability of being selected.

  • freespace-most

    The partition which has the most free space (counted in KiB units) is selected.

    Note

    Example of freespace-most selection on backend

    Suppose that the configured partitions are:

    • part1 which has a 1000GiB capacity, 400GiB being free (that is, 40% free space)
    • part2 which has a 1000GiB capacity, 600GiB being free (that is, 60% free space)
    • part3 which has a 100GiB capacity, 30GiB being free (that is, 30% free space)
    • part4 which has a 100GiB capacity, 70GiB being free (that is, 70% free space)

    In that case part2 will be selected as most fitting, since 600GiB of free space is the biggest of all partitions.

  • freespace-percent-most

    The partition which has the most percentage of free space is selected.

    Note

    Example of freespace-percent-most selection on backend

    In the same conditions, part4 would be selected as most fitting, since 70% of free space is the biggest of all partitions.

  • freespace-percent-weighted

    For each partition, the percentage of free space is its weight. Then a weighted choice is performed to select one of those partitions.

    As such, the more free space the partition has, the more chances it has to be selected.

    Note

    Example of freespace-percent-weighted selection on backend

    In the same conditions, the weight of each partition would be:

    • 40 for part1
    • 60 for part2
    • 30 for part3
    • 70 for part4

    The sum of each weight being 200, the probability for each partition to be selected as most fitting would be:

    • 20% for part1
    • 30% for part2
    • 15% for part3
    • 35% for part4

Usage convergence

In freespace-percent-weighted mode, partitions percentage usages converge towards 100%. So if they have different usages, those differences will stay and only really diminish upon reaching 100% of usage.

You may also observe growing differences between partitions usage when those partitions do not have the same total disk space.

  • freespace-percent-weighted-delta

As for freespace-percent-weighted, a weight is associated to each partition. It is computed as follows: (percentage of freespace of partition) - (lowest percentage of freespace of all partitions) + 0.5

Then a weighted choice is performed to select one of those partitions.

As such, considering the percentages of usage, the more the partition is lagging behind the most used partition (which is the one with the lowest percentage of free space), the more chances it has to be selected.

Computed weight

The added 0.5 in partitions weight is so that selection gets smoother when all partitions get close to each other.

Note

Example of ``freespace-percent-weighted-delta`` Selection on Backend

In the same conditions, the weight of each partition would be:

  • 40 - 30 + 0.5 = 10.5 for part1
  • 60 - 30 + 0.5 = 30.5 for part2
  • 30 - 30 + 0.5 = 0.5 for part3
  • 70 - 30 + 0.5 = 40.5 for part4

Then the probability for each partition to be selected as most fitting would be:

  • 12.8% for part1
  • 37.2% for part2
  • 0.6% for part3
  • 49.4% for part4

Usage convergence

In freespace-percent-weighted-delta mode, partitions percentage usages converge towards the most used one. And then partitions usages grow equally.

Special cases

What happens when two partitions are equal as most fitting?

Suppose you are using the freespace-most selection mode, that two (or more) partitions have the same free size, and that this freespace happens to be the biggest one of all configured partitions.

In that case, only one of those partitions will be selected. You may not know in advance which one will be: it depends of the order in which configured partitions are stored in memory (hashtable). In particular, it may not be the first one that appears listed in your /etc/imapd.conf configuration file.

Also note that since the selected partition will now have less free space, it shall not be seen as most fitting next time.

What happens when two partitions point to the same device?

Suppose you are using the freespace-most or freespace-percent-most selection mode, and that two (or more) partitions actually point to the same device (that is the device id is the same).

In that case, only one of those partitions will be checked, as if the others were not configured. You may not know in advance which one will be: it depends of the order in which configured partitions are stored in memory (hashtable). In particular, it may not be the first one that appears listed in your /etc/imapd.conf configuration file.

Partitions Exclusion

Partitions listed in the partition_mode_exclude setting are permanently excluded from being selected.

  • partition_mode_exclude

    Listed partitions names are separated by space or comma. Only configured partition names (and not paths) are expected.

    Note

    Example of permanent partition exclusion

    With the following configuration in /etc/imapd.conf, parta and partb would be permanently excluded from being selected:

    partition-part1: /path/to/part1 partition-part2: /path/to/part2 partition-part3: /path/to/part3 partition-part4: /path/to/part4 partition-parta: /path/to/parta partition-partb: /path/to/partb

    partition_mode_exclude: parta partb

    When using a selection mode other than random, partitions are automatically excluded if their usage percentage is beyond the partition_mode_soft_usage_limit integer setting.

  • partition_mode_soft_usage_limit

    If all partitions are beyond the configured value, this feature is automatically deactivated. A partition is thus selected as if the setting was not set.

    Note

    Example of partition exclusion using partition_mode_soft_usage_limit

    In the same conditions than <xref linkend=”exam-Deployment_Guide-Available_Selection_Modes_on_Backend-Example_of_freespace_most_Selection_on_Backend” />, setting partition_mode_soft_usage_limit to 50 would exclude partitions part1 and part3 since their disk usage is respectively 60% and 70%.

    But setting the option to 20 would have no effect, since the usage of all partitions is beyond 20%.

    Warning

    Exclusion is not absolute

    Partitions are only excluded when creating a new user mailbox according to the configured selection mode.

    If you explicitly specify an excluded partition, the mailbox will be created on that partition.

    Mailboxes previously created on such partitions are of course still accessible, and subfolders are by default still created on the same partition as the parent folder.

Partitions Usage Data Reset

By default partitions usage data are retrieved only once upon service initialization. This only concerns selection modes other than random.

If you tend to use the same service instance for a long lapse of time and performs a large amount of mailboxes creation, it may be useful to configure the partition_mode_usage_reinit so that partitions usage data are refreshed after the configured number of creation requests.

Mailbox Creation Distribution Through murder frontend

Upon creating a user mailbox, if the target server is not given as extra parameter, the mailbox is either created on

  • the configured defaultserver backend
  • the most fitting backend among the servers listed in the serverlist setting, if defaultserver is not configured

Selection Mode

Among the backends, how the most fitting one is selected depends on the configured selection mode: serverlist_mode setting in /etc/imapd.conf.

The principle is similar to the mailbox creation distribution on backend (see <xref linkend=”sect-Deployment_Guide-Mailbox_Creation_Distribution-On_backend” />).

Available Selection Modes on Frontend

  • random

    Choice is (pseudo-)random. Each backend has the same probability of being selected.

  • freespace-most

    The backend which has the most free space (counted in KiB units) is selected. The considered free space is the sum of all available partitions free space on the backend.

    Note

    Example of freespace-most Selection on Frontend

    Suppose that the configured backends are:

    • backend1 which has a 2000GiB capacity, 1000GiB being free (that is, 50% free space), composed of 2 partitions

    • part1 which has a 1000GiB capacity, 500GiB being free (that is, 50% free space)

    • part2 which has a 1000GiB capacity, 500GiB being free (that is, 50% free space)

      • backend2 which has a 2000GiB capacity, 900GiB being free (that is, 45% free space), composed of 2 partitions

      • part1 which has a 1000GiB capacity, 200GiB being free (that is, 20% free space)

      • part2 which has a 1000GiB capacity, 700GiB being free (that is, 70% free space)

        • backend3 which has a 200GiB capacity, 110GiB being free (that is, 55% free space), composed of 2 partitions
        • part1 which has a 100GiB capacity, 30GiB being free (that is, 30% free space)
        • part2 which has a 100GiB capacity, 80GiB being free (that is, 80% free space)
    • In that case backend1 will be selected as most fitting, since 1000GiB of free space is the biggest of all backends.

  • freespace-percent-most

    On each backend, the partition with the most percentage of free space is considered. The selected backend is the one whose partition has the most percentage of free space.

    Note

    Example of freespace-percent-most Selection on Frontend

    In the same conditions than <xref linkend=”exam-Deployment_Guide-Available_Selection_Modes_on_Frontend-Example_of_freespace_most_Selection_on_Frontend” /> backend3 would be selected as most fitting, since it has a partition with 80% of free space which is the biggest of all backends.

  • freespace-percent-weighted

    On each backend, the partition with the most percentage of free space is considered: it is the backend weight. Then a weighted choice is performed to select one of the backends.

    Note

    Example of freespace-percent-weighted Selection on Frontend

    In the same conditions than <xref linkend=”exam-Deployment_Guide-Available_Selection_Modes_on_Frontend-Example_of_freespace_most_Selection_on_Frontend” />, the weight of each backend would be:

    • 50 for backend1
    • 70 for backend2
    • 80 for backend3

    The sum of each weight being 200, the probability for each backend to be selected as most fitting would be:

    • 25% for backend1
    • 35% for backend2
    • 40% for backend3
  • freespace-percent-weighted-delta

On each backend, the partition with the most percentage of free space is considered. As for freespace-percent-weighted, a weight is associated to each backend. It is computed as follows: (percentage of freespace on backend) - (lowest percentage of freespace of all backends) + 0.5

Then a weighted choice is performed to select one of the backends.

Note

Example of ``freespace-percent-weighted-delta`` Selection on Frontend

In the same conditions than <xref linkend=”exam-Deployment_Guide-Available_Selection_Modes_on_Frontend-Example_of_freespace_most_Selection_on_Frontend” />, the weight of each backend would be:

  • 50 - 50 + 0.5 = 0.5 for backend1
  • 70 - 50 + 0.5 = 20.5 for backend2
  • 80 - 50 + 0.5 = 30.5 for backend3

Then the probability for each backend to be selected as most fitting would be:

  • 1.0% for backend1
  • 39.8% for backend2
  • 59.2% for backend3

Backends Exclusion

When using a selection mode other than random, backends are automatically excluded if their considered usage percentage is beyond the serverlist_mode_soft_usage_limit integer setting.

  • partition_mode_soft_usage_limit

If all backends are beyond the configured value, this feature is automatically deactivated. A backend is thus selected as if the setting was not set.

Note

Example of partition exclusion using serverlist_mode_soft_usage_limit

In the same conditions than <xref linkend=”exam-Deployment_Guide-Available_Selection_Modes_on_Frontend-Example_of_freespace_most_Selection_on_Frontend” />, using freespace-most selection mode, setting serverlist_mode_soft_usage_limit to 49 would exclude backend1 and backend2 since in that mode they have a disk usage of 50% and 55%. In other modes it would however only exclude backend1 whose considered partition has a disk usage of 50%, while on backend2 the considered partition has a disk usage of 30%.

Warning

Exclusion is not absolute

Backends are only excluded when creating a new user mailbox according to the configured selection mode.

If you explicitly specify an excluded backend, the mailbox will be created on that backend.

Mailboxes previously created on such backends are of course still accessible.

Backends Usage Data Reset

By default backends usage data are retrieved only once upon service initialization. This only concerns selection modes other than random.

If you tend to use the same service instance for a long lapse of time and performs a large amount of mailboxes creation, it may be useful to configure the serverlist_mode_usage_reinit so that backends usage data are refreshed after the configured number of creation requests.