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Full Discussion: optimizing disk performance
Special Forums Hardware Filesystems, Disks and Memory optimizing disk performance Post 22907 by Perderabo on Wednesday 12th of June 2002 08:44:29 PM
Old 06-12-2002
I have to tell you that I don't really think this approach is a great idea. With unix filesystems, it's too hard to to keep a file precisely positioned in one spot. Unix wasn't meant to be used that way. But here are your answers...

In the olden days disks had a fixed geometry. The first track and the last track held the same amount of data. As disk manufacturers chased after greater data densities, they changed things so that the outer tracks now have more sectors than the inner tracks. Some disk optimization papers were written in those olden days. Every thing they say may no longer apply. This is the problem with exploiting disk geometry...it changes and suddenly your hack is now counterproductive.

But in the olden days, since each sector could be read with equal speed, your primary concern was getting the disk heads to your sector. This is why putting the data in the middle of the disk is a good idea. The heads cannot be more than half a disk away. So the mean seek time is as low as you can get it. But this assumes that the heads might be anywhere on the disk. If you can guarantee that the heads are positioned over your data, seek time becomes less of an issue. One way to do this is to use only the outer tracks of each disk drive and ignore the inner 90% of the disk.

If that's not possible, then it will depend on how much data is to be transferred. With large multi-sector transfers, the longer seek time may be compensated by the faster transfer time. The only way to be sure is to try it both ways and benchmark it.

And while you're at it, put the data on the inner tracks and benchmark that. That should be the worst case, longest mean seek time and longest transfer time. This will give you a feel for how little benefit you're reaping from a lot of work.
 

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creatediskbyname(3x)													      creatediskbyname(3x)

Name
       creatediskbyname - get the disk description associated with a file name

Syntax
       #include <disktab.h>

       struct disktab *
       creatediskbyname(name)
       char *name;

Description
       The subroutine takes the name of the character device special file representing a disk device (for example, and returns a structure pointer
       describing its geometry information and the default disk partition tables.  It obtains this information by  polling  the  controlling  disk
       device driver.  The subroutine returns information only for MSCP and SCSI disks.

       The file has the following form:
       #define DISKTAB	      "/etc/disktab"

       struct  disktab {
	     char   *d_name;	      /* drive name */
	     char   *d_type;	      /* drive type */
	     int    d_secsize;	      /* sector size in bytes */
	     int    d_ntracks;	      /* # tracks/cylinder */
	     int    d_nsectors;       /* # sectors/track */
	     int    d_ncylinders;     /* # cylinders */
	     int    d_rpm;	      /* revolutions/minute */
	     struct partition {
		     int     p_size;   /* #sectors in partition */
		     short   p_bsize;  /* block size in bytes */
		     short   p_fsize;  /* frag size in bytes */
	       } d_partitions[8];
       };

       struct  disktab *getdiskbyname();
       struct  disktab *creatediskbyname();

Diagnostics
       Successful completion of the subroutine returns a pointer to a valid disktab structure.	Failure of this subroutine returns a null pointer.
       The subroutine fails if it cannot obtain the necessary information from the device driver or disktab file.

       A check is done to ensure that the disktab file exists and is readable.	This check ensures that the subroutine is not being called because
       the disktab file was accidentally removed.  If there is no disktab file, the subroutine fails.

       The subroutine also fails if it cannot determine disk geometry attributes by polling the driver.  This can occur if the disk is not an MSCP
       or SCSI disk.  In some cases where the disk consists of removable media and the media is not loaded, the driver will be unable to determine
       disk attributes.

Restrictions
       The subroutine returns information only for MSCP and SCSI disks.

See Also
       getdiskbyname(3x), ra(4), rz(4), disktab(5)

															      creatediskbyname(3x)
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