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From Donald Miner <dmi...@clearedgeit.com>
Subject Re: ISAM file location vs. read performance
Date Mon, 13 Jan 2014 00:26:32 GMT
HDFS-385 ( https://issues.apache.org/jira/plugins/servlet/mobile#issue/HDFS-385 ) is for custom
pluggable block placement policies and there has been some talk (i think) about improving
mean time to recovering and data locality in hbase.

Basically this would allow accumulo to have a policy for its blocks and control its own destiny...
Instead of things like the rebalancer screwing things up.

I honestly don't know much else about this. Just thought it might be relevant to the conversation.


> On Jan 12, 2014, at 6:42 PM, Josh Elser <josh.elser@gmail.com> wrote:
> 
> 
> 
>> On 1/12/14, 6:17 PM, Sean Busbey wrote:
>> On Sun, Jan 12, 2014 at 4:42 PM, William Slacum
>> <wilhelm.von.cloud@accumulo.net <mailto:wilhelm.von.cloud@accumulo.net>>
>> wrote:
>> 
>>    Some data on short circuit reads would be great to have.
>> 
>> 
>> What kind of data are you looking for? Just HDFS read rates? or
>> specifically Accumulo when set up to make use of it?
> 
> I believe what Bill means, and what I'm also curious about, is specifically the impact
on performance for Accumulo's workload: a merged read over multiple files. An easy test might
be to create multiple RFiles (1 to 10 files?) which contain interspersed data. Test some sort
of random-read and random-seek+sequential-read workloads, from 1 to 10 RFiles, and with shortcircuit
reads on an off.
> 
> Perhaps a slightly more accurate test would be to up the compaction ratio on a table,
and then bulk import them to a single table, and then just use the regular client API.
> 
>>    I'm unsure of how correct the "compaction leading to eventual
>>    locality" postulation is. It seems, to me at least, that in the case
>>    of a multi-block file, the file system would eventually try to
>>    distribute those blocks rather than leave them all on a single host.
>> 
>> 
>> 
>> 
>> I know in HBase set ups, it's common to either disable the HDFS Balancer
>> or just disable for a namespace containing the part of the filesystem
>> that handles HBase. Otherwise, when the blocks are moved off to other
>> hosts you get performance degradation until compaction can happen again.
>> I would expect the same thing ought to be done for Accumulo.
> 
> AFAIK, HBase also does a lot more in regards to assigning Tablets in regards to the blocks
that serve them, no? To my knowledge, Accumulo doesn't do anything like this. I don't want
users to think that disabling the HDFS balancer is a good idea for Accumulo unless we have
actual evidence.

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