topic Re: Logical and phisical queue .... in Operating System - HP-UX

Logical and phisical queue ....

Manuales — Fri, 03 Feb 2006 11:49:35 GMT

Hi all !!!

:0)

Can you explain me what is a logical and a
phisical queue?

thanks, Manuales.

Re: Logical and phisical queue ....

Fabio Ettore — Fri, 03 Feb 2006 12:07:39 GMT

Hi Manuales,

sorry, can you explain better your question?

Thanks and regards,
Fabio

Re: Logical and phisical queue ....

Pete Randall — Fri, 03 Feb 2006 12:12:44 GMT

If you are referring to I/O queueing, a logical queue is buffered in RAM, while physical is written directly to the disk.

Pete

Re: Logical and phisical queue ....

Manuales — Fri, 03 Feb 2006 12:58:55 GMT

Thanks Pete ..
exactly .. what is a queue?
could be a message?

I have listened about queues' Tuxedo for example ...

exactly .. what is a queue?

what kind of queues does exist?

Thanks ...

Re: Logical and phisical queue ....

Manuales — Fri, 03 Feb 2006 12:59:37 GMT

Thanks Pete ..
exactly .. what is a queue?
could be a message?

I have listened about queues' Tuxedo for example ...

exactly .. what is a queue?

what kind of queues does exist?

Can you give me examples about physical and logicals queues?

Thanks ...

Re: Logical and phisical queue ....

A. Clay Stephenson — Fri, 03 Feb 2006 13:10:09 GMT

A queue is simply a group of things lined up sequentially waiting to be serviced. A queue might be a line of people at a store waiting to pay for their purchases. It could also be a number of i/o requests waiting to be serviced. Normally a disk i/o request in UNIX is serviced in a two-phase manner: 1) First the buffer cache is examined to see if the data are already there (logical i/o) 2) Next, a physical i/o is done to access the data --- and usually also copies this data to the buffer cache so that subsequents requests can be satisfied as a logical request.

A message queue is an IPCS (Inter-Process Communication System) facility that allows a group of processes to communicate with each other. One process pushes a message on the queue and another process pops the message. IPCS message queues might not be serviced in the order that they were pushed on the queue because there is also a priority associated with each message.

Re: Logical and phisical queue ....

Manuales — Fri, 03 Feb 2006 13:41:08 GMT

can you tell me what is the buffer cache?

what is its function?

Thanks, Manuales. ...

Re: Logical and phisical queue ....

Manuales — Fri, 03 Feb 2006 13:42:16 GMT

and ...

what is i/o ?

Manuales.

Re: Logical and phisical queue ....

Pete Randall — Fri, 03 Feb 2006 13:43:51 GMT

The buffer cache is a portion or memory set aside for I/O requests to be queued into in order to speed up access. Disks being relatively slow, the theory is that you can read relatively large amounts of data into memory and then satisfy I/O request out of the much quicker memory.

Pete

Re: Logical and phisical queue ....

Manuales — Fri, 03 Feb 2006 13:47:52 GMT

thanks ..
and that portion of memory is internal or external, is physics memory or RAM?

Manuela.

Re: Logical and phisical queue ....

Pete Randall — Fri, 03 Feb 2006 13:49:52 GMT

Physical memory.

Pete

Re: Logical and phisical queue ....

Chan 007 — Fri, 03 Feb 2006 13:50:05 GMT

Man,

I/O is Input & Output.

What Buffercache is related your the system using the swap system but in MEM cache and swap.

Chan

Re: Logical and phisical queue ....

James R. Ferguson — Fri, 03 Feb 2006 13:52:37 GMT

Hi Manuales:

The Unix buffer cache is a portion of physical memory that holds file buffers.

When an I/O is requested, a "logical" or a "phsical" transfer occurs. If the requested data resides in memory; that is, in the buffer cache, then the I/O was a "logical" one. If a disk access is required to retrieve the data, then a "physical" I/O occured.

Similarly, when a write is performed, data may not be written immediately to the physical medium (e.g. disk), but rather kept in the buffer cache. A read request may then be satisfied from this buffer.

Clearly, keeping some buffer cache generally improves performance since reading and writing memory is much, much faster than doing I/O to electromechanical devices.

The job of the 'syncer' daemon, which runs every 30-seconds, is to flush modified buffers from memory to disk. Too big a buffer cache can cause peaks in processor and I/O which otherwise can impede server performance.

With the latest HP-UX releases, the 'syncer' process has been improved.

The amount of memory you define for the buffer cache is controled by the kernel parameters 'cbc_max_pct', 'dbc_min_pct', 'bufpages' and 'nbuf'.

If you are running a database like Oracle with its own internal buffer cache (SGA) you generally want to keep a small Unix buffer cache. To do otherwise is to "double buffer" which gains nothing.

For more information on configuring the Unix buffer cache, see here:

http://docs.hp.com/en/TKP-90202/index.html

Regards!

...JRF...

Re: Logical and phisical queue ....

Manuales — Fri, 03 Feb 2006 15:41:23 GMT

Friends !!!

Thanks all !!!!

Manuales.