IBM TotalStorage Enterprise Storage Server (ESS) subsystem operating modes

The Linux FCP driver uses the default ESS subsystem operating mode Normal Cache Replacement. The mode cannot be changed for FCP/SCSI disks. This mode enables the ESS to switch between the standard algorithms depending on the actual I/O pattern. This is the preferred setting, also for DASD ECKD disks.

The subsystem operating mode can be set for every single DASD individually and lasts until the next reboot of the Linux system. After a reboot the operating mode reverts to the default. If you want to set the mode permanently for a specific DASD, include the mode switch in a Linux startup script (e.g. by adding the commands to the appropriate runlevel scripts in '/etc/init.d/rc3.d/').

Novell/SUSE SLES9

You can use the new Linux command tunedasd to switch between the subsystem operating modes.

# tunedasd -g /dev/dasdc
normal (2 cyl)
# tunedasd -c sequential -n 2 /dev/dasdc
Setting cache mode for device </dev/dasdc>...
Done.
# tunedasd -g /dev/dasdc
sequential (2 cyl)

Novell/SUSE SLES8

The DASD API (ioctl interface) can be used to switch between the subsystem operating modes. You can write your own tool with limited C programming language knowledge.

int ioctl (int fd, int command , xxx)

fd – descriptor of an open file
command – action requested
xxx – pointer to structure which is specific to the request

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/*                                          *
 *                                          *
 * (C) Copyright IBM Corp. 2003             *
 *                                          *
 *                                          *
 *                                          */

#include <sys/types.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>

#define DASD_NORMAL_CACHE  0x0
#define DASD_BYPASS_CACHE  0x1
#define DASD_INHIBIT_LOAD  0x2
#define DASD_SEQ_ACCESS    0x3
#define DASD_SEQ_PRESTAGE  0x4
#define DASD_REC_ACCESS    0x5

/*
 struct attrib_data_t
   Represents the operation (cache) bits for the device.
   Used to influence caching of the DASD.
*/
typedef struct attrib_data_t {
   unsigned char   operation:3;         /* cache operation mode */
   unsigned char   reserved:5;                      /* reserved */
   unsigned short  nr_cyl;         /* no of cyl. for read ahead */
   unsigned char   reserved2[29];             /* for future use */
} __attribute__ ((packed)) attrib_data_t;

/* DASD ioctl identification */
#define DASD_IOCTL_LETTER 'D'

/* set Attributes (cache operations) */
#define BIODASDSATTR _IOW (DASD_IOCTL_LETTER,2,attrib_data_t)

/*
 Set the caching algorithm used for the channel programs of this device.
 'cache' is the caching mode (see ESS docu for more info) and 'no_cyl'
 the number of cylinders to be cached.
*/
int main (int argc, char* argv[]) 
{
  int fd;
  attrib_data_t attrib_data;

  /* for example set normal caching mode and number of read ahead */
  /*cylinders (default 2; if nothing specified,only for seq. modes)*/
  attrib_data.operation = DASD_NORMAL_CACHE;

  /* attrib_data.nr_cyl = 2; -- only for  DASD_SEQ_ACCESS   */
  /*                                 and  DASD_SEQ_PRESTAGE */

  /* Open device file */
  fd = open ("/dev/dasdc", O_RDWR);
  if (fd == -1) {
     ; /* error handling */
   }

  /* Set the given caching attributes */
  if (ioctl (fd, BIODASDSATTR, &attrib_data))
   {
    ;   /* error handling */
   }

  close (fd);

  return 0;
}

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