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From Nicolas Spiegelberg <nspiegelb...@fb.com>
Subject Re: Major Compaction Concerns
Date Mon, 09 Jan 2012 18:55:58 GMT

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:

>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
>   the property Œhbase.hregion.majorcompaction¹ to Œ0¹. Neverhteless even
>   after having doing this I STILL see ³major compaction² messages in
>   therefore it is unclear how can I manage major compactions. (The
>system has
>   heavy insert - uniformly on the cluster, and major compaction affect
>   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),
>   compaction can be triggered by the code in case all store files are
>   candidate for a compaction (from Store.compact(final boolean
>   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);*
>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"));*
>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
>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
>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
>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)?

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