Wednesday, September 28, 2011

Troubleshooting OSI PI compression

I just got back from a client where I was getting a copy of their PI server configuration. My customer offhandedly asked me about the size of his archives- "Is it normal to use 600 megabytes every 2 days?" Off-the-bat, I could tell there was something wrong with the data compression of this system. This PI server was < 5000 points and it collects data from about 20 production units.

Customers with similarly sized-plants and run-rates burn through 600 megabytes a MONTH. The largest cell culture facility west of the Mississippi goes through 1000 megabytes a month, so this particular client was definitely looking at something obvious and something that is statistically outside of normal.

Here's how I troubleshot it:

Look for compressing = 0

The PI Point attribute that determines if the data to a point is to be compressed is the compressing attribute. This value ought to be 1. A lot of people like turning this off for low-frequency tags but it's like unprotected copulation - you're not necessarily going to get pregnant, but there's a chance that an errant configuration runs your system down.

Look for compdev = 0

The compdev point attribute determines what data makes it into the archive. Compdev settings ought to be 0.5 instrument accuracy according to solid recommendations on PI data compression for biotech manufacturers. If you find yourself loathe to define this number, I'd make compdevpercent = 0.1. What this does is it eliminates repeats from the archive.

Use PI DataLink to look for diskspace hogs

The easiest way to identify which tags are the culprit is to pull it up in PI DataLink and use the Calculated Tag feature to find tags with high event count. Start by looking in the last hour... then in the last 12-hours, then last day, then last week. The blatant offenders should be obvious within even 1-minute.

In the case of my customer, he had 7 tags out of about 5000 that was uncompressed. Each of these 7 tags was collecting 64 events/sec. 3840 events/minute. 5.5 million events per day. All told, these 7 tags were recording 39 million zeros into the archive per day... burning through diskspace faster than Nancy Pelosi likes to spend your income.

Modern hardware has made these problems insignificant, but burning through diskspace is a latent problem that rears its head at the most inopportune moment.


and learn about OSIsoft's data compression settings and what they ought to be.

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