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

[Paul Sture]

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