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= Memcached Infrastructure SOP
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Our memcached setup is currently only used for wiki sessions. With
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mediawiki, sessions stored in files over NFS or in the DB are very slow.
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Memcached is a non-blocking solution for our session storage.
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== Contents
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2021-09-07 12:18:18 +02:00
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* <<_contact_information>>
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* <<_checking_status>>
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* <<_flushing_memcached>>
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* <<_restarting_memcached>>
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* <<_configuring_memcached>>
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== Contact Information
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Owner::
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Fedora Infrastructure Team
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Contact::
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#fedora-admin, sysadmin-main, sysadmin-web groups
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Servers::
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memcached03, memcached04
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Purpose::
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Provide caching for Fedora web applications.
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== Checking Status
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Our memcached instances are currently firewalled to only allow access
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from wiki application servers. To check the status of an instance, use:
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....
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echo stats | nc memcached0{3,4} 11211
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....
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from an allowed host.
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== Flushing Memcached
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Sometimes, wrong contents get cached, and the cache should be flushed.
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To do this, use:
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....
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echo flush_all | nc memcached0{3,4} 11211
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....
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from an allowed host.
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== Restarting Memcached
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Note that restarting an memcached instance will drop all sessions stored
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on that instance. As mediawiki uses hashing to distribute sessions
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across multiple instances, restarting one out of two instances will
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result in about half of the total sessions being dropped.
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To restart memcached:
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....
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sudo /etc/init.d/memcached restart
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....
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== Configuring Memcached
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2021-09-07 12:18:18 +02:00
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Memcached is currently setup as a
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https://pagure.io/fedora-infra/ansible/blob/main/f/roles/memcached[role in the ansible git repo].
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The main two tunables are the MAXCONN (the maximum number of concurrent
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connections) and CACHESIZE (the amount memory to use for storage). These
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variables can be set through $memcached_maxconn and $memcached_cachesize
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in ansible. Additionally, other options (as described in the memcached
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manpage) can be set via `$memcached_options`.
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