At 3:24 PM +1000 2000/3/29, Carl Makin wrote:
> There doesn't seem much point in using VINUM with an external RAID5 box
> (although we use VINUM a lot on other machines).
Speaking only for myself and the Comparex D1400 (Hitachi DF400)
unit that I've been playing with so far, there is *very* good reason
to use vinum on top of the external drive array.
In particular, the unit has five SCSI busses internally, and when
you configure LUNs you typically configure one or more rows of five
disks per row. However, when you configure more than one row for a
LUN, it simply appends one row to the next, and doesn't stripe or
RAID-5 across all disks simultaneously. This really kills your
performance.
If I instead create one LUN for each row of disks (RAID-5), I can
then stripe across them in software using vinum, and start
approaching the levels of performance that I could get if I instead
were using vinum exclusively and striping across twenty high-speed
disks on five separate SCSI busses that were directly connected to
the host.
Unfortunately, this device doesn't have any concept of creating
LUNs composed of other LUNs, so that I could keep all this
bizarreness strictly within the cabinet.
> The box to which I'm hooking this up has sufficient performance to handle
> 16 U2W SCSI links running hard. Being able to utilise multiple paths
> could be a big performance win.
Theoretically, the box I'm connecting to can handle that level of
performance, too. However, I've had problems with it when I push it
hard with just two Adaptec 2940U2W host adaptors, each connecting to
one interface on separate device controllers.
In theory, practice and theory are the same. In practice, they
are frequently radically different.
> I'm getting a little beyond my depth in CAM internals here. But it is
> *very* interesting. :)
I'm out of my depth in CAM SCSI internals, too but I'm also
willing to provide whatever assistance I can, with regards to testing.
--
These are my opinions -- not to be taken as official Skynet policy
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