Experimental Physics and Industrial Control System
On Mar 29, 2007, at 02:04 , Terry Cornall wrote:
Hi all.
I was wondering if anyone has a feeling (or a measured result) for
the maximum or typical number of values per second that can
reasonably be archived with a modern PC architecture (and if so, an
indication of the type of computer used)
I'm trying to dimension the archive requirements for all the 14
beamlines for the Australian Synchrotron and am wondering if I can
do it
with one PC or if I need many. Of course, sampling or monitoring
rates will play a big part, but also I need to know what is
possible per archive engine.
Thanks,
Terry Cornall M.Eng.Sc B.Sc.
Beamlines Control Systems Engineer
Australian Synchrotron Project
Hi:
This naive question doesn't have as simple an answer as you might hope.
One archive engine can usually collect about 10000 samples/second.
For example, monitor 1000 channels all updating at 10Hz.
Problem 1:
You won't know what your missing.
The channel access protocol can go into "flow control",
skipping a few samples, but it's really hard to find out
if/when/how that's happening.
Problem 2:
The amount of data. One 'double' sample uses a little over
20 bytes for the timestamp, status, sev, value.
For many values, the data file structure and index add
relatively little to that, but you'll still get about 20GB
per day. How do you intend to back that up?
At the SNS, we run about 70 different engines.
They are typically separated by subsystem,
and often restarted daily to limit the amount of
possible data loss in case a sub-archive gets corrupted.
Some store only a few values each second,
others store the value of the IOC clock each second
(don't ask me why), others store about
1000 values each second, including waveforms.
The total amount of data for Feb. 2007 is about 150G.
When you try to allow access to 'all' or as much as possible,
that means that somebody has to periodically create indices.
This is mostly automated, but in case there's a problem,
for example one index reaches the 2GB file size limit
and things need to get reorganized, that process takes a lot
of time. The index mechanism certainly needs some improvement,
but even after we eventually get that,
moving those amounts of data over the network takes many days.
If somebody tells you that disks are cheap, please ask
that person to take care of your archiving, then run
as fast as you can.
-Kay
- Replies:
- Re: Maximum archival rate Maren Purves
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