[Info-vax] ISAM on disk layout. Was: Re: HP wins Oracle Itanium case

[abrsvc]

On Wednesday, August 29, 2012 11:45:02 AM UTC-4, Paul Sture wrote:
> On Tue, 28 Aug 2012 20:12:38 +0000, glen herrmannsfeldt wrote: > Stephen Hoffman <seaohveh at hoffmanlabs.invalid> wrote: >> On 2012-08-28 16:58:35 +0000, Bob Koehler said: > >>> Blows away folks when I access a keyed-indexed file "sequentially". > >> You're either being humored with that "blown away", or you're >> discussing this with folks that are ignorant of how databases work. >> What you're discussing goes back to the S in the IBM Indexed Sequential >> Access Method ISAM scheme, and which well predates RMS. > > And which uses IBM CKD (Count Key Data) disks where keys are written > before data blocks on the disk. You can ask the disk hardware to search > for a block with a given key, or with a key greater than or equal to the > given key. (The index is sorted to make the search easier.) > > ISAM files are also interesting in that the index contains disk block > addresses such that the files are unmovable. (There are special programs > that know how to backup and restore an ISAM file, and rewrite the index > as needed.) This highlights a problem with RMS. On mainframe systems I was used to placing files carefully on disk, aligning by cylinder etc. I cannot remember whether the compatibility mode RMS utilities (IFL - Initial File Load and friends) could do file placement, but once FDL came along you could. However, another black mark for BACKUP/IMAGE was that it doesn't honour FDL file placement. If you were going to use FDL's file placement functionality, you had better not use BACKUP/IMAGE. We chose to ignore file placement in favour of the convenience BACKUP/IMAGE brought us for defragmenting disks. -- Paul Sture

The "smart" controllers also made file placement less important.  The UDA50 was the first to "re-order" disk reads such that the heads moved in one direction to collect the data rather than the "normal" in-out movement.  I recall testing this with the 780 and confirmed that for small reads (1-3 blocks) the Unibus outperformed the Massbus even though the transfer rate for the Massbus was twice as fast.

Dan