SCSI Drives question

[Rick Stevens]

On Sun, 2006-10-01 at 13:02 -0700, Bret Stern wrote:
> On my server, i have two 18gb scsi drives which are
> part of a Logical Volume (LVM), and are internal
> drives to the Dell 2450.
> 
> They are /dev/sda, and /dev/sdb
> 
> 
> I'm adding a Dell Powervault connected to an Adaptec
> scsi card (not the onboard scsi) and the two 18gb drives
> above now become /dev/sdh and /dev/sdi.
> 
> It seems like the server drives should stay /dev/sda and /dev/sdb,
> and the new drives in the powervault should start their
> scsi id's after after those in the server??
> 
> I need some scsi tips.

You have two issues here...the sequence in which Linux scans the PCI
and internal busses and the sequence in which the SCSI bus is scanned.
This affects not only SCSI, but also NICs and any other items in the PCI
bus.

The kernel, when it boots up, must scan the hardware config of the
machine.  When scanning the PCI bus, it starts with slot 0, then 1, and
so on.  Now, which slot is slot 0.  By convention, it's normally the one
closest to the CPU, however that may not be true depending on how the
hardware and BIOS of your machine was configured.

For example, we use Dell 1850s and 2850s.  We have two glass NICs (fiber
channel) in the PCI buses in addition to the two copper NICS on the
motherboard.  On the 1850s, the glass NICs in the PCI bus are eth0 and
eth1, and the copper ones on the mobo are eth2 and eth3.  On the 2850s,
it's EXACTLY opposite! (the copper ports are eth0 and eth1, the glass
NICs are eth2 and eth3).

Now, when you get to SCSI, you also have to add the additional SCSI
bus scanning.  Normally, the SCSI bus IDs are scanned 0 to 7, then 8-15
(SCSI-2 and SCSI-3).  The controller is normally at SCSI ID 7.  Change
the buses and you change the layout of the machine and the names of the
devices may change.  There are tools to equate SCSI IDs to /dev
entries. 

If you use LVM however, it's somewhat irrelevant as LVM uses the PV IDs
written to the disks to put itself back together at reboot time.  It
doesn't matter what gets added or deleted from the bus, the IDs are on
the actual media of the disk and can be found.

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- Rick Stevens, Senior Systems Engineer     rstevens at vitalstream.com -
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