Re: performance using versions as dimension

[Oliver Meyn]

Thanks for the quick reply Jonathan:

On 19-May-10, at 5:04 PM, Jonathan Gray wrote:

What you have is 100 columns in each row.  Each column has lots of  
versions.  You want values from all 100 columns within a specified  
TimeRange... correct?


This is the 2nd of my two gateway queries - this one being slow I  
understand and your explanation makes sense.  The first query is a  
simple "get me the latest version from every column for this row" and  
that is what, to me, is perplexingly slow.  To be clear, there's a  
good chance that each of those columns will have a different  
timestamp, but "the latest reading" is what I'm interested in.


I need to think on what the best way to implement this would be,  
perhaps with a better understanding now you can too :)


I know it's something of a religious topic, but as of 0.20.4, is using  
versions as a data dimension legitimate?  Because I could easily  
approach millions of versions per column, am I in danger of running  
into the elsewhere-mentioned row-split problem (each of my cell values  
is a double)?  I ask because if that's going to be a problem then I  
need to rethink my schema anyway, and then we don't need to waste  
cycles on the current problem.

Thanks again,
Oliver


-----Original Message-----
From: Oliver Meyn [mailto:oliver.m...@zerofootprint.net]
Sent: Wednesday, May 19, 2010 1:53 PM
To: user@hbase.apache.org
Subject: performance using versions as dimension

Hi All,

I'm new to hbase and columnar storage schemas, so any comments you
have on the schema or the actual problem at hand are very much
welcome.  I'm using 0.20.4, initially testing as standalone on my
development laptop (OS X), all settings default except for data
directory, and accessing hbase through the Java api.

In my initial testing I have 50 Gateways, each of which are
responsible for 100 unique Devices, each of which report their power
usage every second.  So that's 5000 total, unique Devices.  Here are
the queries I'd like to answer:

1) What is the current power consumption of Device X?
2) What is the average power consumption of Device X btw Date 1 and
Date 2?
3) What is the current power consumption at Gateway Y?
4) What is the average power consumption at Gateway Y btw Date 1 and
Date 2?

I'm imagining this as two tables - "devices" and "gateways".  The
devices table has a column family called "device_samples" which only
has one column "power" and 5000 rows (one for each device).  Every  
new
sample gets written to the power column of its device at the  
timestamp
from the original sample sent by the Device.  Now I can answer  
query 1
with a simple get, and I can answer query 2 using the api  
setTimeRange
call on another simple get (and do my own math to average the
results).  This works great so far - with 50k versions in each cell
query 1 is less than 50ms, and query 2 is only marginally more (on my
dev machine, remember).

The gateways table could just hold the list of its deviceids and then
I have to manually fetch its 100 device entries from the devices
table, but that proves to be quite slow.  So at the cost of disks I
tried a schema such that it has a cf "gateway_samples" where each row
is a gateway id (so exactly 50 rows), and it has a column for each of
its 100 devices (so each row has 100 columns, but the cf has 5000
columns).  Each sample is written to those cells in the same way as
the devices table.  Then I should be able to answer query 3 with a
"get latest versions from the whole row" and do my own sums, and
similarly query 4.  In practice though, this works as expected (50ms)
with very little data in the gateways table (50k total keyvalues),  
but
once I've run the devices for a bit (~1.5M total keyvalues) a single
row fetch takes 600ms.

Granted these are performance numbers from a dev machine with hbase
running in standalone mode, so have no bearing on reality.  But it
feels like I'm doing something wrong when the devices table responds
very quickly and the gateways doesn't.  I've tried moving hbase to an
old linux machine with the client still running from my dev machine
and got basically the same results with a bit extra time for the
network.

Any and all advice is appreciated.

Thanks,
Oliver