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From Ted Yu <yuzhih...@gmail.com>
Subject Re: Major Compaction Concerns
Date Mon, 09 Jan 2012 19:37:32 GMT
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 <nspiegelberg@fb.com>wrote:

> 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 you
> can leverage existing functionality.  A better description of your system
> 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 issued
> 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, so
> 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, it's
> 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 idle.
> 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.  If
> 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 been
> 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 correctly
> 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 book,
> >   automatic major compaction should be disabled. This can be done by
> >setting
> >   the property Œhbase.hregion.majorcompaction¹ to Œ0¹. Neverhteless even
> >   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 and
> >   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 is
> >   disabled??
> >
> >   2. I tried to check the parameter  Œhbase.hregion.majorcompaction¹  at
> >   runtime using several approaches - to validate that the server indeed
> >   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 bug?
> >If I set the property via the configuration shouldn¹t all the cluster be
> >aware of? (supposing that the connection indeed connected to the cluster)
> >
> >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 it?
> >(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 I
> >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 the
> >servers? (at cluster/server level)
> >
> >    3.  I ran both major compaction  requests from the shell or from API
> >but since both are async there is no progress indication. Neither the JMX
> >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. The
> >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 have
> >more than 1 storefile per store e.g. minor compaction just finished) then
> >invoking major_compact will indeed decrease the number of store files, but
> >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 compaction,
> >nevertheless it has be removed from 0.92, is it possible to put it back?
> >Even sooner than 0.92?
> >
> >6.       In case a compaction (major) is running it seems there is no way
> >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 compaction,
> >part of this was given via the RegionObserver in HBASE-2001, nevertheless
> >do you consider adding support for custom compaction (providing real
> >pluggable compaction stategy not just observer)?
> >
> >
> >Regards,
> >Mikael.S

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