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https://issues.apache.org/jira/browse/COUCHDB-1153?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13085491#comment-13085491
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Paul Joseph Davis commented on COUCHDB-1153:
--------------------------------------------
Couple notes so far.
I'm don't care much either way, but I would've just parsed proplists from
Erlang terms from the config file like we do for other various options instead
of creating the key=val syntax goop.
Never register anonymous config change functions. Always register functions
using the M:F/A pattern. This has to do with how functions are called and code
reloading. If module aren't calling exported functions it'll eventually cause
random processes to crash when the code they were referring to is purged.
I'm not a super huge fan of how os_mon is being started. There's a -args_file
command line switch that we might want to look into supporting for VM
configuration.
The compact_loop thing seems kinda weird. A pattern I've had luck with lately
is to use erlang:send_interval to replace loops like that. Not super concerned
about this, but on first skim it looks like it could clean that loop's logic up
a bit.
Also, I'm wondering if there should be some sort of throttling on how quickly
the scan for databases to compact runs. The concern is that for installs that
have non-trivial numbers of databases this could start doing mean things to
couch_server as well as start thrashing system resources by opening and closing
a large number of files.
> Database and view index compaction daemon
> -----------------------------------------
>
> Key: COUCHDB-1153
> URL: https://issues.apache.org/jira/browse/COUCHDB-1153
> Project: CouchDB
> Issue Type: New Feature
> Environment: trunk
> Reporter: Filipe Manana
> Assignee: Filipe Manana
> Priority: Minor
> Labels: compaction
>
> I've recently written an Erlang process to automatically compact databases
> and they're views based on some configurable parameters. These parameters can
> be global or per database and are: minimum database fragmentation, minimum
> view fragmentation, allowed period and "strict_window" (whether an ongoing
> compaction should be canceled if it doesn't finish within the allowed
> period). These fragmentation values are based on the recently added
> "data_size" parameter to the database and view group information URIs
> (COUCHDB-1132).
> I've documented the .ini configuration, as a comment in default.ini, which I
> paste here:
> [compaction_daemon]
> ; The delay, in seconds, between each check for which database and view
> indexes
> ; need to be compacted.
> check_interval = 60
> ; If a database or view index file is smaller then this value (in bytes),
> ; compaction will not happen. Very small files always have a very high
> ; fragmentation therefore it's not worth to compact them.
> min_file_size = 131072
> [compactions]
> ; List of compaction rules for the compaction daemon.
> ; The daemon compacts databases and they're respective view groups when all
> the
> ; condition parameters are satisfied. Configuration can be per database or
> ; global, and it has the following format:
> ;
> ; database_name = parameter=value [, parameter=value]*
> ; _default = parameter=value [, parameter=value]*
> ;
> ; Possible parameters:
> ;
> ; * db_fragmentation - If the ratio (as an integer percentage), of the amount
> ; of old data (and its supporting metadata) over the
> database
> ; file size is equal to or greater then this value, this
> ; database compaction condition is satisfied.
> ; This value is computed as:
> ;
> ; (file_size - data_size) / file_size * 100
> ;
> ; The data_size and file_size values can be obtained when
> ; querying a database's information URI (GET /dbname/).
> ;
> ; * view_fragmentation - If the ratio (as an integer percentage), of the
> amount
> ; of old data (and its supporting metadata) over the
> view
> ; index (view group) file size is equal to or greater
> then
> ; this value, then this view index compaction
> condition is
> ; satisfied. This value is computed as:
> ;
> ; (file_size - data_size) / file_size * 100
> ;
> ; The data_size and file_size values can be obtained
> when
> ; querying a view group's information URI
> ; (GET /dbname/_design/groupname/_info).
> ;
> ; * period - The period for which a database (and its view groups) compaction
> ; is allowed. This value must obey the following format:
> ;
> ; HH:MM - HH:MM (HH in [0..23], MM in [0..59])
> ;
> ; * strict_window - If a compaction is still running after the end of the
> allowed
> ; period, it will be canceled if this parameter is set to
> "yes".
> ; It defaults to "no" and it's meaningful only if the
> *period*
> ; parameter is also specified.
> ;
> ; * parallel_view_compaction - If set to "yes", the database and its views are
> ; compacted in parallel. This is only useful on
> ; certain setups, like for example when the
> database
> ; and view index directories point to different
> ; disks. It defaults to "no".
> ;
> ; Before a compaction is triggered, an estimation of how much free disk space
> is
> ; needed is computed. This estimation corresponds to 2 times the data size of
> ; the database or view index. When there's not enough free disk space to
> compact
> ; a particular database or view index, a warning message is logged.
> ;
> ; Examples:
> ;
> ; 1) foo = db_fragmentation = 70%, view_fragmentation = 60%
> ; The `foo` database is compacted if its fragmentation is 70% or more.
> ; Any view index of this database is compacted only if its fragmentation
> ; is 60% or more.
> ;
> ; 2) foo = db_fragmentation = 70%, view_fragmentation = 60%, period =
> 00:00-04:00
> ; Similar to the preceding example but a compaction (database or view
> index)
> ; is only triggered if the current time is between midnight and 4 AM.
> ;
> ; 3) foo = db_fragmentation = 70%, view_fragmentation = 60%, period =
> 00:00-04:00, strict_window = yes
> ; Similar to the preceding example - a compaction (database or view index)
> ; is only triggered if the current time is between midnight and 4 AM. If at
> ; 4 AM the database or one of its views is still compacting, the compaction
> ; process will be canceled.
> ;
> ;_default = db_fragmentation = 70%, view_fragmentation = 60%, period = 23:00
> - 04:00
> (from https://github.com/fdmanana/couchdb/compare/compaction_daemon#L0R195)
> The full patch is mostly a new module but also does some minimal changes and
> a small refactoring to the view compaction code, not changing the current
> behaviour.
> Patch is at:
> https://github.com/fdmanana/couchdb/compare/compaction_daemon.patch
> By default the daemon is idle, without any configuration enabled. I'm open to
> suggestions on additional parameters and a better configuration system.
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