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From Keith Turner <>
Subject Re: Recovering Tables from HDFS
Date Thu, 05 Jul 2012 14:30:30 GMT

In the comments of ACCUMULO-456, I outlined out a procedure for doing
this for 1.4.

By default cloning a table will flush anything in memory.


On Thu, Jul 5, 2012 at 10:13 AM, Adam Fuchs <> wrote:
> Hi Patrick,
> The short answer is yes, but there are a few caveats:
> 1. As you said, information that is sitting in the in-memory map and in the
> write-ahead log will not be in those files. You can periodically call flush
> (Connector.getTableOperations().flush(...)) to guarantee that your data has
> made it into the RFiles.
> 2. Old data that has been deleted may reappear. RFiles can span multiple
> tablets, which happens when tablets split. Often, one of the tablets
> compacts, getting rid of delete keys. However, the file that holds the
> original data is still in HDFS because it is referenced by another tablet
> (or because it has not yet been garbage collected). If you're using Accumulo
> in an append-only fashion, then this will not be a problem.
> 3. For the same reasons as #2, if you're doing any aggregation you might run
> into counts being incorrect.
> You might also check out the table cloning feature introduced in 1.4 as a
> means for backing up a table:
> Cheers,
> Adam
> On Thu, Jul 5, 2012 at 9:52 AM, <> wrote:
>> users@accumulo,
>> I need help understanding if one could recover or backup tables by taking
>> their files stored in HDFS and reattaching them to tablet servers, even
>> though this would mean the loss of information from recent mutations and
>> write ahead logs. The documentation on recovery is focused on the failure of
>> a tablet server, but, in the event of a failure of the master or other
>> situation where the tablet servers cannot be utilized, it would be
>> beneficial to know whether the files in HDFS can be used for recovery.
>> Thanks,
>> Patrick Lynch

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