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From Nicolas Spiegelberg <nspiegelb...@fb.com>
Subject Re: Major Compaction Concerns
Date Mon, 09 Jan 2012 19:42:44 GMT
Significant compaction JIRAs:
 - HBASE-2462 : original formulation of current compaction algorithm
 - HBASE-3209 : implementation
 - HBASE-1476 : multithreaded compactions
 - HBASE-3797 : storefile-based compaction selection

On 1/9/12 11:37 AM, "Ted Yu" <yuzhihong@gmail.com> wrote:

>Thanks for your insight.
>Can you point Mikael to a few of the JIRAs where algorithm mentioned in #1
>was implemented ?
>On Mon, Jan 9, 2012 at 10:55 AM, Nicolas Spiegelberg
>> Mikael,
>> Hi, I wrote the current compaction algorithm, so I should be able to
>> answer most questions that you have about the feature.  It sounds like
>> you're creating quite a task list of work to do, but I don't understand
>> what your use case is so a lot of that work may be not be critical and
>> can leverage existing functionality.  A better description of your
>> requirements is a must to getting a good solution.
>> 1. Major compactions are triggered by 3 methods: user issued, timed, and
>> size-based.  You are probably hitting size-based compactions where your
>> config is disabling time-based compactions.  Minor compactions are
>> on a size-based threshold.  The algorithm sees if sum(file[0:i] *
>>ratio) >
>> file[i+1] and includes file[0:i+1] if so.  This is a reverse iteration,
>> the highest 'i' value is used.  If all files match, then you can remove
>> delete markers [which is the difference between a major and minor
>> compaction].  Major compactions aren't a bad or time-intensive thing,
>> just delete marker removal.
>> As a note, we use timed majors in an OLTP production environment.  They
>> are less useful if you're doing bulk imports or have an OLAP environment
>> where you're either running a read-intensive test or the cluster is
>> In that case, it's definitely best to disable compactions and run them
>> when you're not using the cluster very much.
>> 2. See HBASE-4418 for showing all configuration options in the Web UI.
>> This is in 0.92 however.
>> 4. The compaction queue shows compactions that are waiting to happen.
>> you invoke a compaction and the queue is empty, the thread will
>> immediately pick up your request and the queue will remain empty.
>> 8. A patch for pluggable compactions had been thrown up in the past.  It
>> was not well-tested and the compaction algorithm was undergoing major
>> design changes at the time that clashed with the patch.  I think it's
>> a low priority because there are many other ways to get big performance
>> wins from HBase outside of pluggable compactions.  Most people don't
>> understand how to optimize the current algorithm, which is well-known
>> (very similar to BigTable's).  I think bigger wins can come from
>> laying out a good schema and understanding the config knobs currently at
>> our disposal.
>> On 1/8/12 7:25 AM, "Mikael Sitruk" <mikael.sitruk@gmail.com> wrote:
>> >Hi
>> >
>> >
>> >
>> >I have some concern regarding major compactions below...
>> >
>> >
>> >   1. According to best practices from the mailing list and from the
>> >   automatic major compaction should be disabled. This can be done by
>> >setting
>> >   the property Œhbase.hregion.majorcompaction¹ to Œ0¹. Neverhteless
>> >   after having doing this I STILL see ³major compaction² messages in
>> >logs.
>> >   therefore it is unclear how can I manage major compactions. (The
>> >system has
>> >   heavy insert - uniformly on the cluster, and major compaction affect
>> >the
>> >   performance of the system).
>> >   If I'm not wrong it seems from the code that: even if not requested
>> >   even if the indicator is set to '0' (no automatic major compaction),
>> >major
>> >   compaction can be triggered by the code in case all store files are
>> >   candidate for a compaction (from Store.compact(final boolean
>> >forceMajor)).
>> >   Shouldn't the code add a condition that automatic major compaction
>> >   disabled??
>> >
>> >   2. I tried to check the parameter  Œhbase.hregion.majorcompaction¹
>> >   runtime using several approaches - to validate that the server
>> >   loaded the parameter.
>> >
>> >a. Using a connection created from local config
>> >
>> >*conn = (HConnection) HConnectionManager.getConnection(m_hbConfig);*
>> >
>> >*conn.getConfiguration().getString(³hbase.hregion.majorcompaction²)*
>> >
>> >returns the parameter from local config and not from cluster. Is it a
>> >If I set the property via the configuration shouldn¹t all the cluster
>> >aware of? (supposing that the connection indeed connected to the
>> >
>> >b.  fetching the property from the table descriptor
>> >
>> >*HTableDescriptor hTableDescriptor =
>> >conn.getHTableDescriptor(Bytes.toBytes("my table"));*
>> >
>> >*hTableDescriptor.getValue("hbase.hregion.majorcompaction")*
>> >
>> >This will returns the default parameter value (1 day) not the parameter
>> >from the configuration (on the cluster). It seems to be a bug, isn¹t
>> >(the parameter from the config, should be the default if not set at the
>> >table level)
>> >
>> >c. The only way I could set the parameter to 0 and really see it is via
>> >the
>> >Admin API, updating the table descriptor or the column descriptor. Now
>> >could see the parameter on the web UI. So is it the only way to set
>> >correctly the parameter? If setting the parameter via the configuration
>> >file, shouldn¹t the webUI show this on any table created?
>> >
>> >d. I tried also to setup the parameter via hbase shell but setting such
>> >properties is not supported. (do you plan to add such support via the
>> >shell?)
>> >
>> >e. Generally is it possible to get via API the configuration used by
>> >servers? (at cluster/server level)
>> >
>> >    3.  I ran both major compaction  requests from the shell or from
>> >but since both are async there is no progress indication. Neither the
>> >nor the Web will help here since you don¹t know if a compaction task is
>> >running. Tailling the logs is not an efficient way to do this neither.
>> >point is that I would like to automate the process and avoid compaction
>> >storm. So I want to do that region, region, but if I don¹t know when a
>> >compaction started/ended I can¹t automate it.
>> >
>> >4.       In case there is no compaction files in queue (but still you
>> >more than 1 storefile per store e.g. minor compaction just finished)
>> >invoking major_compact will indeed decrease the number of store files,
>> >the compaction queue will remain to 0 during the compaction task
>> >(shouldn¹t
>> >the compaction queue increase by the number of file to compact and be
>> >reduced when the task ended?)
>> >
>> >
>> >5.       I saw already HBASE-3965 for getting status of major
>> >nevertheless it has be removed from 0.92, is it possible to put it
>> >Even sooner than 0.92?
>> >
>> >6.       In case a compaction (major) is running it seems there is no
>> >to stop-it. Do you plan to add such feature?
>> >
>> >7.       Do you plan to add functionality via JMX (starting/stopping
>> >compaction, splitting....)
>> >
>> >8.       Finally there were some request for allowing custom
>> >part of this was given via the RegionObserver in HBASE-2001,
>> >do you consider adding support for custom compaction (providing real
>> >pluggable compaction stategy not just observer)?
>> >
>> >
>> >Regards,
>> >Mikael.S

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