Best swap cylinders?

[mikebw at bilow.bilow.uu.ids.net (Mike Bilow)]

Chuck Young wrote in a message to Mike Bilow:

 CY> We have a typical beer-pub discussion going on here tonight
 CY> about which cylinders are the best to use for linux swap. 
 CY> All other things being equal (on an IDE drive) would most of
 CY> you rather have it in low numbered cyls (the beginning of
 CY> the drive) or the high ones (/dev/hda4)? 

 CY> I would assume one has a higher seek time on average and the 
 CY> other would be ideal for swap.  If true, which is it: hi or lo?

First, keep in mind that this may be a trade-off specific to any particular
machine.  For example, if you have 64 MB RAM, you probably will not hit the
swap file very often.

Second, IDE is an all-or-nothing I/O technology.  Whenever IDE I/O is actually
taking place, the CPU is fully occupied and no other IDE device (if any) can
use the bus.  This is very different from SCSI, where devices will "disconnect"
during I/O processing and leave the system and the device bus free for other
useful work.

Given these restrictions, the short answer is that there is effectively no
performance difference between putting the swap on the high or low cylinders. 
There may be non-peformance reasons for a preference, however.

The low cylinders are special because booting any operating system must be done
from the ROM BIOS, and there is always some point on a large drive which cannot
be reached from the ROM BIOS in real-mode.  In older IDE systems, that boundary
will be 512 MB, so your boot partition will have to be entirely below the 512
MB point of the drive.  In newer EIDE systems, that boundary should be 4 GB,
but often is really 2 GB due to bugs (related to using signed instead of
unsigned 32-bit numbers).  As a result, since swap is only accessed from
protected mode, it makes more sense to put swap on the high cylinders.

Modern drives all idle the head in the middle of the drive, since it is obvious
that average seek time is reduced by doing so.  All modern filesystems --
including Linux Second Extended -- take this into account, and put their most
frequently accessed areas, the directory bands, in the middle of the partition.
 Older IDE drives, such as the Seagate ST-157A, will idle the heads on the
highest numbered cylinder, which is usually unavailable for data and reserved
as the "customer engineering" cylinder.
 
-- Mike
   author of the BruteForce(TM) Disk Utilities