Re: [PATCH 2.6.13 5/14] sas-class: sas_discover.c Discover process (end devices)

[Sergey Panov <sipan@sipan.org>]

On Tue, 2005-09-13 at 14:36 -0600, Matthew Wilcox wrote: 
> 
> Could you explain the difference please?  Why is it preferable to keep
> the LUN as an array of bytes instead of a single large integer?
> 

Because set of valid LUN id represented by 8 byte combinations is not
isomorphic to the set of unsigned int values from 0 to UINT64_MAX. While
scsilun_to_int() will convert legal LUN id into some integer, the
int_to_scsilun() function will not produce legal  LUN id for any
arbitrary integer lun value. 

For example, sequential LUN scanning should be stopped at int lun = 255
because result of converting value 256 by int_to_scsilun() will be
either illegal(best case) or equivalent to int lun  = 0.

LUN id should be presented to the management layers in a way similar to
MAC addresses or FC/SAS/... WWN . E.g. the usual LUN 4  on some FC
device will be identified by something like (in 00b, or "Peripheral
device addressing"):

WWPN = 22:00:00:0c:50:05:df:6d
LUN  = 00:04:00:00:00:00:00:00


Interestingly enough, the following is also LUN = 4 device, but in a
different addressing mode (01b, AKA "Logical unit addressing"):

WWPN = 22:00:00:0c:50:05:df:6d
LUN  = 40:04:00:00:00:00:00:00


Sergey Panov

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