SUMMARY: Dual homed RAID cabinets

Thanks to everyone who gave me a quick reply.  Many people gave me well
informed advice.

My original question involved how to use a dual headed SCSI cabinet such as
the D1000 (a Hot Swap SCSI cabinet) or the A1000 (a hardware RAID cabinet)
on multiple hosts at the same time.  I had heard that both connected servers
could simultaneously mount a filesystem from a single LUN.  I had further
heard that only one of the servers could mount the filesystem RW, while the
other server would have to mount RO.

Many people replied and I now have a better understanding about how a setup
like this works.  UFS in Read-Only mode heavily caches filesystem reads and
expects the underlying filesystem to be static.  If I need a filesystem
capable of doing this I would have to use VxFS (Veritas Proprietary File
System) but VxFS has a prohibitively high cost.

There is an exception.  Sun Cluster 3.0 software provides a solution for
multiple servers to both mount the same UFS filesystem in RW mode.  This
sounds like the true solution for anytime a mission critical service is
needed.  Veritas also has a product called Veritas Cluster Server that will
fit this situation.  Both products have 5 figure pricetags attached.

Many people have taken the cheaper approach.  They have an A1000 cabinet
attached to two servers but only one server has the filesystem mounted at
any time.  In the event of a server failure, the following steps take place:
   1) Failed server is powered off.
   2) Administrator runs an fsck of the LUN from the backup server.
   3) Administrator manually mounts the filesystem on the backup server.
   4) Any VIPs are added to the box and production is resumed.

This approach obviously has a window of downtime and requires human
intervention.  Many people have said that their service outage policy allows
them this window of time, and therefore cannot justify spending money on
clustering software.  This approach also means the two servers can't act as
a loadbalanced server pool.  Only one server is available to serve content
from the shared filesystem at one time.

Thanks to:
   David Evans
   Justin Stringfellow
   Tim Chipman
   Darren Dunham
   Steve Hammond
   Steve Camp




----- Original Message -----
From: "Terry Ewing" <sun@deadtrout.com>
To: <sunmanagers@sunmanagers.org>
Sent: Monday, November 19, 2001 2:30 PM
Subject: Dual homed RAID cabinets


> Hello,
>    We're looking to deploy a service where we have two servers running in
a
> redundant failover mode.  I know we can use clustering software to do this
> but... we're too cheap.  We're looking to use a Dual-headed RAID cabinet
> such as a D1000.  We've heard different things about the ability of both
> servers to read from the same box at the same time.  Can we have one box
> mount the FS read-write and the other mount it read-only?
>
>    I've been told that only on host can have control of the SCSI chain at
> any given time (for reading or writing) and that a hard SCSI reset has to
be
> issued to the cabinet before the possibility of a switch of the
controlling
> host.  I've also seen two Sun boxes using Veritas able to use differnt
disks
> on the same cabinet at the same time.
>
> Does anyone have a link to some information so I can dispell hte myths and
> start getting real info?
>
> I'd appreciate any help.
>
> -Terry
>
>
> _______________________________________________
> sunmanagers mailing list
> sunmanagers@sunmanagers.org
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>

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