Mailbox Creation Distribution¶
By default, when creating a mailbox in Cyrus IMAP:
the backend with the most free disk space is selected on the Murder frontend
the partition with the most free disk space is selected on the backend
This may not be the most appropriate backend or partition to create the new mailbox on, and Cyrus IMAP therefor allows for a variety of additional modes of calculating and selecting the most appropriate backend and partition. The exact mode for the selection is controlled with the imapd.conf setting serverlist_select_mode on the frontend and partition_select_mode on the backend.
Alternatively, a default backend can be configured with the defaultserver setting on a frontend, and a default partition can be configured with the defaultpartition on a backend.
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, orthe server that is found to have the most free disk space, and
the configured
defaultpartition, orthe most fitting partition if
defaultpartitionis 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-mostselection on backendSuppose that the configured partitions are:
part1which has a 1000GiB capacity, 400GiB being free (that is, 40% free space)part2which has a 1000GiB capacity, 600GiB being free (that is, 60% free space)part3which has a 100GiB capacity, 30GiB being free (that is, 30% free space)part4which has a 100GiB capacity, 70GiB being free (that is, 70% free space)
In that case
part2will 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-mostselection on backendIn the same conditions,
part4would 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-weightedselection on backendIn the same conditions, the weight of each partition would be:
40 for
part160 for
part230 for
part370 for
part4
The sum of each weight being 200, the probability for each partition to be selected as most fitting would be:
20% for
part130% for
part215% for
part335% 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
part160 - 30 + 0.5 = 30.5 for
part230 - 30 + 0.5 = 0.5 for
part370 - 30 + 0.5 = 40.5 for
part4
Then the probability for each partition to be selected as most fitting would be:
12.8% for
part137.2% for
part20.6% for
part349.4% 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,
partaandpartbwould 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 thepartition_mode_soft_usage_limitinteger 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_limitIn 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_limitto50would exclude partitionspart1andpart3since their disk usage is respectively 60% and 70%.But setting the option to
20would 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
defaultserverbackendthe most fitting backend among the servers listed in the
serverlistsetting, ifdefaultserveris 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-mostSelection on FrontendSuppose that the configured backends are:
backend1which has a 2000GiB capacity, 1000GiB being free (that is, 50% free space), composed of 2 partitionspart1which has a 1000GiB capacity, 500GiB being free (that is, 50% free space)part2which has a 1000GiB capacity, 500GiB being free (that is, 50% free space)backend2which has a 2000GiB capacity, 900GiB being free (that is, 45% free space), composed of 2 partitionspart1which has a 1000GiB capacity, 200GiB being free (that is, 20% free space)part2which has a 1000GiB capacity, 700GiB being free (that is, 70% free space)backend3which has a 200GiB capacity, 110GiB being free (that is, 55% free space), composed of 2 partitionspart1which has a 100GiB capacity, 30GiB being free (that is, 30% free space)part2which has a 100GiB capacity, 80GiB being free (that is, 80% free space)
In that case
backend1will 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-mostSelection on FrontendIn the same conditions than <xref linkend="exam-Deployment_Guide-Available_Selection_Modes_on_Frontend-Example_of_freespace_most_Selection_on_Frontend" />
backend3would 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-weightedSelection on FrontendIn 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
backend170 for
backend280 for
backend3
The sum of each weight being 200, the probability for each backend to be selected as most fitting would be:
25% for
backend135% for
backend240% 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
backend170 - 50 + 0.5 = 20.5 for
backend280 - 50 + 0.5 = 30.5 for
backend3Then the probability for each backend to be selected as most fitting would be:
1.0% for
backend139.8% for
backend259.2% 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_limitIn the same conditions than <xref linkend="exam-Deployment_Guide-Available_Selection_Modes_on_Frontend-Example_of_freespace_most_Selection_on_Frontend" />, using
freespace-mostselection mode, settingserverlist_mode_soft_usage_limitto49would excludebackend1andbackend2since in that mode they have a disk usage of 50% and 55%. In other modes it would however only excludebackend1whose considered partition has a disk usage of 50%, while onbackend2the 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.