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From "Paul Joseph Davis (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (COUCHDB-1153) Database and view index compaction daemon
Date Tue, 16 Aug 2011 04:32:27 GMT

    [ https://issues.apache.org/jira/browse/COUCHDB-1153?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13085523#comment-13085523
] 

Paul Joseph Davis commented on COUCHDB-1153:
--------------------------------------------

My thoughts on the loop were based on my day dreaming that it's entirely possible that there's
going to be feature requests to handle multiple simultaneous compactions. I tend to have better
luck reacting to messages to maintain the state of a set of long running process directly
from the gen_server rather than have this middleman process looping around accepting messages.
Also, the more I look at this compact_loop the more things I see wrong with it:

    * You have a "Pid = spawn_link/1, MonRef = erlang:monitor(process, Pid)" sequence for
the parallel
      view compactor. One of these is redundant. You want a link if you want the compactor_loop
to
      exit when the view compactor crashes, or you want the monitor if you just want to know
when it dies.
    * When you wait for the view compaction process to end there's no timeout. That means
that the
      compactor loop could never move depending on whether the view compactor process exits
or not.
    * You never flush monitor messages. This means the compact_loop process mailbox
      will slowly fill with messages over time causing hard to track memory leaks.
    * Views don't seem to be checked to see if they need to be compacted if their database
doesn't
      need to be.
    * View compaction holds open a reference to the database its compacting views for. What
happens if
      views haven't finished compacting before the main database compaction gets swapped out?


I'd prefer to either have os_mon in an app file or started as an app when the VM boots. If
we're going to talk about moving towards being more OTP compliant we should be trying to avoid
adding more non-OTP bits when possible.

The important part to trigger the couch_server issues you need to have a lot of active databases
as well as a lot of load so that try_close_lru turns into a table scan of that ets table.
Adam rewrote couch_server quite a long time ago to replace this so that requests for open
databases turned into a single ets lookup on a public table which helped quite a bit. Though
it introduces the possibility of a race condition when opening a database that's just about
to be shut. Since then other things have been fixed and couch_server has become a bottleneck
again. I looked at it the other day and the only thing I came up with would require some non-trivial
changes to the close semantics of databases.

I think the general approach here is quite good and I'm quite fine with leaving room for improvement.
On the flip side, we need to avoid just pushing features into trunk without considering how
we might be asked to improve them or what sort of maintenance cost they'll incur.




> 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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