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Dumping Lots of Data to Disk in Realtime?
AmiChris asks: "At work I need something that can dump sequential entries for several hundred thousand instruments in realtime. It also needs to be able to retrieve data for a single instrument relatively quickly. A standard relational database won't cut it. It has to keep up with 2000+ updates per second, mostly on a subset of a few hundred instruments active at a given time. I've got some ideas of how I would build such a beast, based on flat files and a system of caching entries in memory. I would like to know if: someone has already built something like this; and if not, would someone want to use it if I build it? I'm not sure what other applications there might be. I could see recording massive amounts of network traffic or scientific data with such a library. I'm guessing someone out there has done something like this before. I'm currently working with C++ on Windows. "
Have you considered a 2-stage approach? Stuff it to disk, and process/index it separately? A fast stream of data would let it all get recorded without loss, and then you could use whatever resources are necessary to index and search without impacting the data dump.
Cost... Are you going to go for local storage or NAS? Need SCSI and RAID or a less expensive hardware setup? Do you think gigabit ethernet will be sufficient for the transfer from the data dump hardware to the processing/indexing/search machines?
Sounds like you might want to run a test case using commodity hardware first.
Yeah, like it isn't obvious that this guy works for the government's TIA program and is looking for ways to maintain all of the data culled from the thousands of audio and video sensors they have planted around.
Suuuure.
Check out wonderware InSQL. We update roughly 50k points every 30 seconds without loading the server much at all. Pretty nice product, also has some custom extensions to SQL built in for querying the data (eg cyclic, resolution, delta storage, etc etc).
http://www.wonderware.com/
Of course, you'll need your data to come from an OPC/Suitelink/other supported protocol, but should work nicely for you.
- Joshua
Unless you really want to do a LOT of work. This sounds very much like a SCADA system. There are vendors of such systems. Most of the realtime databases are designed to stay in a large, proprietary, RAM database which is occasionally dumped to disk for backup purposes.
In order to process so many points realtime, it usually will have to be in RAM for performance reasons.
Zed's dead baby. Zed's dead.
I know your working with windows but when I read this I said yes.
I'm guessing someone out there has done something like this before.
Google has a cluster of machines far larger than you need but their approach was a Linux cluster. Plus, for the amount of writes going on your going to want not to have any burdens on the system that are not needed.
You can definitely use Oracle to write out 2000 updates per second if your hardware is up to it and your db skills are good.
I have a system that can record 32 streams of data 44,100 times per second. It's called a recording studio, and I make music with it.
If your data streams are continuous, and can be represented as audio data, then you are pretty much dealing with a solved problem, and your other problem of selecting from large number of possible 'instruments' is solved by an audio patchbay.
If this isn't feasible, then a number of solutions might be appropriate (spreading the load over a number of machines/huge ram caches/buffering/looking at the problem and thinking of a less intensive sampling strategy/etc.) but without more information on the sort of data you are collecting, and exactly how quickly you need to access it, it's very hard to be specific.
A pizza of radius z and thickness a has a volume of pi z z a
Sure, optimize single node performance first, but keep in mind that horizontal scaling is something to look for. Put N machines behind a load balancer, ingest gets scattered among 'n' machines, queries go to all simultaneously. Redundant Array of Inexpensive Databases :-)
Linux Virtual Server in front of several instances of your windows box will do, with some proxying stuff for queries. Probably cheaper than spending months trying to tweak single node to get to your scaling target, and will scale trivially much farther out.
I agree. In fact SQLite performs quite well on a reasonable sized machine. 3000+ SQL updates on an indexed table should be no problem.
Meme of the day: I browse "Disable Sigs: Checked". So should you.
the solution to your problem comes in the form of a little known software application from a vender called Microsoft.
:P
The program is called Microsoft Access 97
Here's a thought - just use a hard-RAM based database.
Either make a big ramdisk and put your database out there (see my Journal from a few months back, ramdisk throughput is pretty damn fast from the local machine, given certain constraints, and random access writing is hella fast), or use a database that runs entirely in memory (think Derby, aka Cloudscape that comes with WebSphere Application Developer.)
When you got your data, save it out to the hard drive.
Granted it helps to have a box with a ton of memory in it, but they are out there now, almost affordable. If you are collecting more than 4G of data in one session, well YMMV - but 4G is a LOT of data, perhaps consider your approach.
Glonoinha the MebiByte Slayer
2.000 items/sec means that you must do bulk updates. You cannot flush to disk 2.000 times per second. So you program will have to store the items temporarily in a buffer, which gets flushed by a secondary thread when a timer expires or when the buffer gets full. use a two-buffer approach so you can stil receive while committing to the database.
Depending on you application it may be beneficial to keep a cache of the most recent items for all instruments.
You also have to consider the disk setup. If you have to store all the items then any multi-disk setup will do. If you actually only store a few items per instrument and update them, then raid-5 will kill you because it performs poorly with tiny scattered updates.
Do you have to backup the items? How will you you handle backups while your program is running? This affects your choice of flat-file or database implementation.
The design of a data acquisition systems will of course differ, depending on how much data it records per sensor, how many sensors there are, how often to record the data, and if the data is to be available for online or offline processing.
In most of the "hard" cases, you will use a pipelined architecture, where data is received on one or more realtime boxes, and buffered for an appropriate (short) period. A second stage occurs when data is collected from these buffers, and buffered/reordered/processed to make writing the desired format to a file or DBMS easier. The last stage, is, of course, to write it. You might use zero or more computers at each stage, with a fast dedicated network in-between. You might even decide to split up some of the stages even further. Depending on how much you care about your data, you may also add redundancy. And make sure it's fault-tolerant, it's generally better to loose some data, as long as it's tagged as missing, than to loose it all. To check this in real-time you can also add data-monitoring anywhere it makes sense for your system.
In the simper cases, you simply remove things not needed, such as a soundcard instead of dedicated realtime-boxes, redundancy, monitoring, dedicated network, etc...
Some commercial off-the-shelf systems will surely do this. But the more advanced systems, you still build yourself, either from scratch, or by reusing code you find in other similar projects (I'm sure there are some scientific code available from people interested in medical science, biology, astrophysics, geophysics, meteorology, etc...).
Most of the "heavy" systems will not run on Windows, or even Intel, due to limitations of that platform for fast I/O. This has obviously changed a lot recently, so it's no longer the stupid choice it was, but don't expect too many projects of this kind to have noticed, as they probably have existed much longer.
I did some work on a DVD-Video authoring system that had some incredible file system requirments (obviously, when involving video data and the typical 4 GB data load for a single DVD disc).
The standard file API architechture just didn't hold up, so we (the development team I was working with) had to rewrite some of the file management routines ourselves and work directly with the memory mapped architechture directly. This does give you some other advantages beyond speed as well, as once you establish the file link and set it in a memory address range you can treat the data in the file as if it were RAM within your program, having fun with pointers and everything else you can imagine. Copying data to the file is simply a matter of a memory move operation, or copying from one pointer to another.
The thing to remember is that Windows (this is undocumented) won't allow you to open a memory-mapped file that is larger than 1 GB, and under FAT32 file systems (Windows 95/98/ME/and some low-end XP systems) the total of all memory mapped files on the entire operating system must be below 1 GB (this requirement really sucks the breath out of some applications).
Remember that if you are putting pointers into the file directly, that it works better if the pointers are relative offsets rather than direct memory pointers, even though direct memory pointers are in theory possible during a single session run.
This may be gross overkill, but there's specialized hardware specifically designed for sustained high-throughput disk storage. A company called Conduant makes specialized disk controllers that use on board microcontrollers to drive arrays of disks. When I last saw them demoed, they could sustain writes of 100MB/sec using direct card to card transfers across the PCI bus. They can configure a data acquisition card to directly store information into a shared buffer on the disk controller across the PCI bus. The disk controller then picks the data up and drives it across ten IDE channels. That was a few years ago, these days it looks like they can sustain 200MB/sec with a controller, and up to 600MB/sec and 6TB of capacity with custom box mounted in a rack.
I'm not so sure what their story is regarding reading or querying. My guess is you lose a lot of bandwidth, but not all. Anyway, it might be worth checking out.
http://www.conduant.com/products/overview.html
Another thing is that modern computers cam have lots innate capacity themselves. My hunch is that you could do a lot with a couple modern disks on seperate SATA channels and several GB of RAM. Maybe this is only a software problem...
Kdb+ by KX Systems (http://www.kx.com/ is by far and away the best thing for this. Its main use is to store tick data from financial markets, and is excellent at this (if expensive).
From how you descibed your needs, this would probably bit the bill..
NetCDF and HDF5 are optimized binary file formats for storing incredibly large amounts of data and quickly retrieving it.
I'm more familiar with NetCDF (because I use it) so let me tell you some of the things it can do. (HDF5 can also do these things, I'm sure).
With NetCDF, you can store +2 gigabyte files on a 32 bit machine (it supports Large File support). I've saved 12 gigabyte files with no problems. It supports both sequential and direct access, meaning you can read and write either starting from the beginning of the file or at any point in the middle of the file.
The format is array-based. You define dimensions of arrays and variables consisting of zero, one, or more dimensions. You can also define attributes that are used as metadata, information describing the data inside your variables.
You can read or write slices of your data, including strides and hyperslabs. This allows you to read/write only the data you're interested in and makes disk access much faster.
It's also easy to use with good APIs. They have APIs for C, Fortran95, C++, MATLAB, Python, Perl, Java, and Ruby.
Take a look at it. It might be what you're looking for.
-Howard Salis
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On the coding end, there are numerous (hell, hundreds) of commercial, F/OSS, and books on ISAM libraries for you to use for the actual storage and retrieval. It may even be included in your existing libraries given how old the technique is now. I was doing this back in the '80s for the US Navy using a 24 bit, very slow, mini-computer, so any normal box should be able to handle it today!
We use these techniques in electronic instrument monitoring, logistical systems, systems engineering, you get the idea. You may want to mosey over to the HP developer web site to see if there is a drop in solution, as I imagine there is (sorry, haven't looked).
I hope this helps.
"[I]t is a wise man who admits the limits of his knowledge or skill, and that pretending either causes harm." --Terry Go