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Accumulo >> mail # user >> Efficient Tablet Merging [SEC=UNOFFICIAL]

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RE: Efficient Tablet Merging [SEC=UNOFFICIAL]

Hi Eric,
Our answers are in blue. Just a note that we do have the write ahead log disabled for ingest performance.
We have a public holiday on Monday, so we may be delayed in our response.


From: Eric Newton [mailto:[EMAIL PROTECTED]]
Sent: Friday, 4 October 2013 11:20
Subject: Re: Efficient Tablet Merging [SEC=UNOFFICIAL]

Any errors on those servers?  Each server should be checking periodically for compactions, some crazy errors might escape error handling, though that is rare these days.
In the tserver debug log there is a repeating error of  "Internal error processing applyUpdates  org.apache.accumulo.server.tabletserver.HoldTimeoutException: Commits are held"

Also found in the tserver log:
ERROR: Failed to find midpoint Filesystem closed
WARN: Tablet .... has too many files, batch lookup cannont run

Are you experiencing any table level errors?  Unable to read or write files?
No table level errors or read errors
How full is HDFS?

If you scan the !METADATA table, are you seeing any trend in the tablets that have problems?
By getting the extent id of the tablets that are large and then finding the range of that tablet by using 'getsplits -v' I have scanned the !METADATA table and can see a massive number of *.rf files associated with the range.  Is there anything particular I should look at.

At this point, we're looking for logged anomalies, the earlier the better.  Anything red or yellow on the monitor pages.
I ran one of the scans that hang and then see the following:

Several "WARN Exception sying java.lang.reflect.InvocationTargetException"

Several "ERROR  Unexpected error writing to log, retrying attempt 1
    Caused by LeaseExpiredException: Lease mismatch on /accumulo/wal/... owned by DFSClient_NOMAPREDUCE_56390516_13 but is accessed by DFSClient_NOMAPREDUCE_1080760417_13"

"ERROR TTransportException: javav.net.SocketTimeoutException: ... while waiting for channel to be ready for write. ...."

Bunch of "WARN Tablet 234234234 has too many files..."

On Thu, Oct 3, 2013 at 8:43 PM, Dickson, Matt MR <[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>> wrote:


We have restarted the tablet servers that contain tablets with high volumes of files and did not see any majc's run.

Some more details are:
On 3 of our nodes we have 10-15 times the number of entries that are on the other nodes.  When I view the tablets for one of these nodes there are 2 tablets with almost 10 times the the number of entries as the others.

When we query on the date rowid's the queries are now hanging and there are several scans running on the 3 nodes that have higher entries and they are not completing, can I cancel these?

In the logs we are getting "tablet ..... has too many files, batch lookup can not run"

At this point I'm stuck for ideas, so any suggestions would be great.

From: Eric Newton [mailto:[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>]
Sent: Thursday, 3 October 2013 23:52

Subject: Re: Efficient Tablet Merging [SEC=UNOFFICIAL]

You should have a major compaction running if your tablet has too many files.  If you don't, something is wrong. It does take some time to re-write 10G of data.

If many merges occurred on a single tablet server, you may have these many-file tablets on the same server, and there are not enough major compaction threads to re-write those files right away.  If that's true, you may wish to restart the tablet server in order to get the tablets pushed to other idle servers.

Again, if you don't have major compactions running, you will want to start looking for other problems.


On Thu, Oct 3, 2013 at 2:29 AM, Dickson, Matt MR <[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>> wrote:


Hi Eric,

We have gone with the second more conservative option. We changed our split threshold to 10GB and then we ran a merge over a week worth of tablets which has resulted in one tablet with a massive number of files. We then ran a query over that range and it is returning an message saying:

Tablet has too many files (3n;20130914;20130907...) retrying...

We assumed that when the merge was done that a major compaction would be started, which would notice that the tablet is too large, split it into 10GB tablets. We assumed that we would not have to manually start any compaction but instead it would be scheduled at some point after the merge finished.

We have completed three separate merges of week long ranges and now have identified 3 tablet extents with too many files.

Can you please explain what is supposed to happen? And whether after the merge, compact command for those ranges needs to be run (or will it do it automatically, as we have not seen any started)?


From: Eric Newton [mailto:[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>]
Sent: Thursday, 3 October 2013 13:28
Subject: Re: Efficient Tablet Merging [SEC=UNOFFICIAL]

I'll use ASCII graphics to demonstrate the size of a tablet.

Small: []
Medium: [ ]
Large: [  ]

Think of it like this... if you are running age-off... you probably have lots of little buckets of rows at the beginning and larger buckets at the end:

[][][][][][][][][]...[ ][ ][ ][ ][ ][  ][  ][    ][    ][    ][    ][    ][    ]

What you probably want is something like this:

[               ][       ][       ][       ][       ][       ][       ][       ]

Some big bucket at the start, with old data, and some larger buckets for everything afterwards.  But... this would probably work:

[       ][       ][       ][       ][       ][       ][       ][       ][       ]

Just a bunch of larger tablets throughout.

So you need to set your merge size to "[      ]" (4G)