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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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LEARN ABOUT OSF1

disklabel

disklabel(4)						     Kernel Interfaces Manual						      disklabel(4)

NAME

       disklabel - Disk pack label

SYNOPSIS

       #include <sys/disklabel.h>

DESCRIPTION

       Each disk or disk pack on a system may contain a disk label which provides detailed information about the geometry of the disk and the par-
       titions into which the disk is divided.	It should be initialized when the disk is formatted, and may be changed later with  the  disklabel
       program.   This	information is used by the system disk driver and by the bootstrap program to determine how to program the drive and where
       to find the file systems on the disk partitions.  Additional information is used by the file system in order to use  the  disk  most  effi-
       ciently	and  to locate important file system information.  The description of each partition contains an identifier for the partition type
       (standard file system, swap area, etc.).  The file system updates the in-core copy of the label if it contains incomplete information about
       the file system.

       The label is located in sector number LABELSECTOR of the drive, usually sector 0 (zero) where it may be found without any information about
       the disk geometry.  It is at an offset LABELOFFSET from the beginning of the sector, to allow room for the  initial  bootstrap.	 The  disk
       sector  containing  the	label  is normally made read-only so that it is not accidentally overwritten by pack-to-pack copies or swap opera-
       tions; the DIOCWLABEL ioctl, which is done as needed by the disklabel program, allows modification of the label sector.

       A copy of the in-core label for a disk can be obtained with the DIOCGDINFO ioctl; this works with a file descriptor for a block or  charac-
       ter  (raw)  device for any partition of the disk.  The in-core copy of the label is set by the DIOCSDINFO ioctl.  The offset of a partition
       cannot generally be changed, nor made smaller while it is open.	One exception is that any change is allowed if no label was found  on  the
       disk,  and  the	driver was able to construct only a skeletal label without partition information.  Finally, the DIOCWDINFO ioctl operation
       sets the in-core label and then updates the on-disk label; there must be an existing label on the  disk	for  this  operation  to  succeed.
       Thus,  the  initial  label for a disk or disk pack must be installed by writing to the raw disk.  All of these operations are normally done
       using the disklabel program.

RELATED INFORMATION

       Files: disktab(4)

       Commands: disklabel(8) delim off

																      disklabel(4)
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