Maximum number of Logical Files?

[David Abramowitz]

I can remember working with a system where a CASE tool was used. The tool (which I shall not name. . . . . . . .O.K. - Synon) routinely generated well over a hundred LFs per PF. This was several years ago. Dave

[nycsusan@hotmail.com]

Maximum number of Logical Files?

I have found several PFs with a few dozen LFs over them in the system I am working in now. But is this a good idea?

[S.Mildenberger]

Maximum number of Logical Files?

I see no problem with it. We have some files with large numbers of logicals and I haven't noticed any performance problems. Scott Mildenberger

[David Abramowitz]

Maximum number of Logical Files?

A lot depends on your hardware, and available free disk space. I can pretty much guarantee, that anything over six LFs per PF, where there is no sharing of access paths can, and does affect performance. OTOH, if you throw enough memory, storage, and processor capacity at the situation, you will never notice the problem. Dave

[GlenKerner]

Maximum number of Logical Files?

I seem to remember the same think Susan, but I thought the number was a little higher. Now a days I don't think it matters as much. With the speed of the processors and I/O. I would limit the number of logicals over large files. Performance may be an issue but restore is even more of an issue. I remember a PF field attribute being changes (using SQL) on a file (S40 machine and dedicated) and taking over 3 days to finish building all the access paths (70 Million records). So, I would try to keep the number down (I like less than 10 when possible) for more than just performance.

[nycsusan@hotmail.com]

Maximum number of Logical Files?

I always thought that the maximum number of logical files that should be attached to any one physical file is 12. I know you can have many more than 12 LFs, but I thought this was not recommended because it might negatively affect performance? Am I hallucinating this? Has this recommendation changed? Does anyone have a link to IBM doco about this? We are on v5r2 in case that makes a difference. Thanks!

[Guest.Visitor]

Maximum number of Logical Files?

Susan, The number of LF's and the impact on performance, and even whether is it is a good idea to have a LOT of logicals, depends on how your logicals are created and maintained. In the CRTLF command, the access path maintenance (MAINT) parm determines whether and how the system will dynamically update your LF. The default is *IMMED which means the system will update the LF every time the physical file is updated. For a large PF with lots of transactions, this can lead to some significant system overhead, just to maintain a logical. Multiply that by a large number of logicals, and you WILL notice the impact on performance. A good rule of thumb is to limit the maintenance of the access path with *IMMED to only those LF's used frequently, as in interactive inquiry programs. If a logical is used only, say, once per month for a month-end report, then compile the LF with a MAINT parameter of *DLY so that the access path is rebuilt only when it is opened. The more LF's on a PF, the more important it is to limit those being maintained dynamically with *IMMED. So, it's not so much a case of the number of logicals that exist, but how they are maintained. Hope this helps.