ken@turtlevax.UUCP (Ken Turkowski) (09/21/84)
> From: timos@ucbingres (Timos Sellis) > > I am looking for information on a software package which enables > UNIX users to work with ISAM structured files. What is ISAM? -- Ken Turkowski @ CADLINC, Palo Alto, CA UUCP: {amd,decwrl,dual,flairvax,nsc}!turtlevax!ken ARPA: turtlevax!ken@DECWRL.ARPA
dwight@timeb.UUCP (Dwight Ernest) (09/22/84)
ISAM="Indexed Sequential Access Method." -- --Dwight Ernest KA2CNN \ Usenet:...vax135!timeinc!dwight Time Inc. Editorial Technology Group, New York City Voice: (212) 554-5061 \ Compuserve: 70210,523 \ EIES: 1228 Telemail: EDPISG/TIMEINC \ MCI: DERNEST
glc@akgua.UUCP (G.L. Cleveland [Lindsay]) (09/24/84)
Re: >> From: timos@ucbingres (Timos Sellis) >> >> I am looking for information on a software package which enables >> UNIX users to work with ISAM structured files. > >What is ISAM? >-- >Ken Turkowski @ CADLINC, Palo Alto, CA Ken, As "One who was there" when IBM came out with ISAM in the late 60's, my comment on "What is ISAM" is "You *really* don't want to know!" ISAM stands for Indexed Sequential Access Method. I attended IBM classes on the description and usage of it. Also had to hand-hold a bunch of developers who were trying to use it for a random-access application. It could very often be a disk grinder! It had three different files per "data set". One was an index file which contained a (large) on-disk table of the main area. The second file was the actual data. It was primarily a sequentially- ordered file. The third file was the "overflow" area. To access a record, it would scan thru the index area until it got an "equal to or greater than" match on the search key. Since the IBM disk controller had that sort of search logic built into the hardware, that's why they designed this "serial scan". With the disk address of a block which might contain the desired record now known, that block was read. However, if the record had been added since the entire data set had been loaded, it wouldn't be there. So then a serial search of the "overflow" area was done to find the record (or report its absence). This technique meant that you had to do at least two access to the disk at two (often widely-separated) tracks, and a possibility of a third (or more) just to get one record. IBM later came out with some improvements in the techniques, such as maintaining an "index to the index" in memory. The kicker was if you had an active file! The overflow area would have too many records and the "hit" ratio in the main area would decrease. So then you would do a reorganization of the entire data. This was done by accessing it as if it were a sequential file and copy everything out to a tape. Next you would wipe out the file and recreate it from that tape, coming up with an empty overflow area. One fellow had a file which took up an entire disk pack. The processing (elapsed) time was painful. On my recommendation, he added a preliminary step to copy the "index" portion to a separate (temporary) disk. After the main processing step, he then copied the revised index back to the original pack. These two steps took 10 minutes. But the reduced disk arm movement changed the overall processing time from 12 hours to 3 hours! I think the idea behind ISAM was to make a random access method for a "dumb" programmer who only knew how to process seqential (tape) files. Fortunately the Data Base Manager packages started appearing in the mid 70's and got rid of the appeal of the ISAM beast. Now...aren't you sorry you asked! Cheers, Lindsay Lindsay Cleveland (...{ihnp4|mcnc|sdcsvax|clyde}!akgua!glc) AT&T Technologies/Bell Laboratories ... Atlanta, Ga (404) 447-3909 ... Cornet 583-3909
leon@hhb.UUCP (Leon Gordon) (09/24/84)
[ eat not this line..]
ISAM means indexed sequential acess method. It is one of the many approaches
used to improve the average speed for file access. Very popular with
inhabitants of the mainframe galaxies.
leon
{decvax,ihnp4,allegra}!philabs!hhb!leon