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From "Cardon, Tejay E" <>
Subject RE: EXTERNAL: Re: Accumulo and file locality
Date Mon, 25 Jun 2012 18:25:37 GMT
Great.  Thanks John.  That tells me what I need.

From: John Vines []
Sent: Monday, June 25, 2012 12:10 PM
Cc: Loveland, Jason C
Subject: Re: EXTERNAL: Re: Accumulo and file locality

Essentially it will right to the local disk. Technically, it is writing to HDFS, and when
it writes to HDFS that write goes to local disk among other places. When you read from that
tserver, it will read through hdfs. And if that datanode is a host for the block being read,
then there will be read locality. Once the tablet migrates to a tserver without locality,
there will be reads across the network until major compaction occurs to make the file local

On Mon, Jun 25, 2012 at 1:58 PM, Cardon, Tejay E <<>>
John, let me make sure I understand.  When a tserver is running on the same physical box as
a datanode, it will write to the local disk.  HDFS will then replicate that write across the
network.  When I read from that tserver, it will not need a network read (assuming no failures).
 Is that correct?


From: John Vines [<>]
Sent: Monday, June 25, 2012 11:46 AM
Subject: EXTERNAL: Re: Accumulo and file locality

When a tserver writes, it writes out to hdfs. When you utilize the hdfs api, data will be
written first to the local datanode. So it does go to local disk, but it's local disk as well
as others via datanodes. So each tserver should run a datanode so you actually get locality.
If there is no datanode where the tserver is, then all reads and writes go over the network,
which is suboptimal.

On Mon, Jun 25, 2012 at 1:21 PM, William Slacum <<>>
The loggers will write to local disk, however, the TabletServer will
write out files to HDFS during major and minor compactions.

I don't know how complex the tablet assignment algorithm is, but it's
safe to assume that if your tablet spans multiple HDFS blocks, a
TServer will, in all likelihood, only be hosting 1 HDFS block of a
given tablet at any given time, and do fetches for other HDFS (and
RFile) blocks as the need arises. There is a caching mechanism for
holding on to RFile blocks.

On Mon, Jun 25, 2012 at 10:13 AM, Cardon, Tejay E
<<>> wrote:
> All,
>                 If I understand things correctly, when an Accumulo tablet
> server writes data, things are organized such that those writes go to the
> local disk (ie each tablet server writes and reads data to/from the disk
> local to that server).  Is this correct?  And if so, then is it correct to
> assume that every tablet server should run on an HDFS data node?  Or am I
> completely off base here?
> Thanks,
> Tejay Cardon

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