Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!rutgers!ames!amdcad!rpw3 From: rpw3@amdcad.AMD.COM (Rob Warnock) Newsgroups: comp.arch Subject: Re: Re: Disk Striping Message-ID: <17860@amdcad.AMD.COM> Date: Sun, 9-Aug-87 05:50:53 EDT Article-I.D.: amdcad.17860 Posted: Sun Aug 9 05:50:53 1987 Date-Received: Sun, 9-Aug-87 20:43:30 EDT References: <582@saturn.ucsc.edu> <310@cc4.bbn.com.BBN.COM> Reply-To: rpw3@amdcad.UUCP (Rob Warnock) Organization: [Consultant] San Mateo, CA Lines: 56 In article <310@cc4.bbn.com.BBN.COM> lfernand@cc4.bbn.com.BBN.COM (Louis F. Fernandez) writes: +--------------- | +--------------- | | Isn't this what IBM uses in their Airline Control Program? There was that | | article in CACM a while back about the TWA reservation system, and it said | | something about spreading files over a large number of spindles for greater | | throughput. | haynes@ucscc.ucsc.edu | +--------------- | What ACP does isn't what we are calling disk striping in this newsgroup. | ACP has an option to write each record to two different disks at the same | time. This doesn't increase throughput but does has several benefits... +--------------- Sorry, the original poster is correct (sort of). ACP *does* have disk striping, in addition to the redundant writing you mentioned, but still it isn't quite the same as we are talking about here. They spread a file across several disks, all right, but the allocation of records (all fixed length -- this is *old* database technology!) is such that the disk drive number is a direct-mapped hash of some key in the record! What this does is spread accesses to similar records (like adjacent seats on the same flight) across many disks (sometimes up to 100 packs!!!). Kind of special purpose, really... +--------------- | BTW, airline reservations systems have quite high performance. Large ones | (e.g., United and American) average over 1000 transactions per second, | where a transaction is a few characters of keyboard input, a half-dozen | disk access, and a few hundred characters of CRT output. | ...Lou | lfernandez@bbn.com | ...!decwrl!bbncc4!lfernandez +--------------- True. In fact, they are reaching the limits of that technology. The United Airlines system is approaching 2000 (!) transactions per second (as you defined them -- correctly) during the peak busy hour. Yikes! But note that perfect consistency is *not* a constraint of airline rez systems (as we all know ;-} ). They don't even have completely serialized transactions or even real record-locking. It is considered "acceptable" (for the sake of throughput) to drop or garble an occasional passenger record once in a while... as long as it is really once in a long while. On the other hand, crew flight records *must* be correct (because of FAA legal limits on flight crew work hours and recording thereof), so the crew scheduling sub-system *does* have "correct" behavior... but it pays a severe penalty in performance. Rob Warnock Systems Architecture Consultant UUCP: {amdcad,fortune,sun,attmail}!redwood!rpw3 ATTmail: !rpw3 DDD: (415)572-2607 USPS: 627 26th Ave, San Mateo, CA 94403