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RE: linux vs SAN



Hi Tom,
Thank you very much for the detail, it is really quite usefull but in
our situation there are strange things happening. Just 10 minmutes ago
when I type fdsik -l from Host B I have seen that 2 disks are active
because the other disks cannot be read. I have created a partition  on
sdd (sdd1) and make fs and label it. Afterwards I typed fdisk -l again
to see the second active disk but I have been suprized that 4 disks seem
to be accessiable now.. 
Isn't it strange ? 
Ps. :Do you have some documents ?

-----Original Message-----
From: redhat-list-bounces redhat com
[mailto:redhat-list-bounces redhat com] On Behalf Of Tom Callahan
Sent: Wednesday, February 01, 2006 3:46 PM
To: General Red Hat Linux discussion list
Subject: Re: linux vs SAN

Welcome to Linux vs. the SAN. It's sometimes a tough road, but it's
wonderful once it's working.

We currently have several RHEL3 and RHEL4 systems booting and running
exclusively off our SAN's, with no internal storage. We use Qlogic 2340
HBA's, 2 in each system for High Availability. Our setups utilize two
SAN cabinets, with MD software mirroring between the two SANs.

Linux has multiple ways of dealing with the Active/Passive pathing
issues of seeing 4 disks, when only two are presented. We currently use
Qlogic's drivers which include "failover" support, this way the driver
loads and all "passive" devices are not presented to the OS, only the
actives. The card handles the occurrence of a path failure, and fails to
the passive path, without the OS ever knowing (short of some entries in
/var/log/messages).

Multipathing is another option, but I am unsure of support in RHEL 2.1.
There are tons of documents around the net on how to setup multipathing,
including redhat's own documentation in their knowledgebase.

Behavior you should see in an Active/Passive setup is below:
1 - You have 2 disks presented from the SAN, one 10Gig and one 20Gig
2 - Upon boot, you should see /dev/sda as 10Gig, /dev/sdb as 20Gig,
/dev/sdc as 10Gig, and /dev/sdd as 20Gig (this could be reversed, with
the 20Gig disk showing up first. it depends on the LUNs you have
assigned the disks.)
3 - if you `fdisk -l` each disk, you "should" have two disks come up
with "unable to read from /dev/sdx", those are the passive disks.
4 - When you are formatting/partitioning the disks, you will want to use
the set of disks (set being /dev/sda,b and /dev/sdc,d) that you have
determined are on the active controller path.
5 - Make sure that when you format a disk, you are prepending a Label
onto the disk(mkfs.ext3 -L/labelyouchose /dev/sdx or e2label /dev/sdx
"/labelyouchose"), and use that Label for mounting in /etc/fstab
(LABEL=/labelyouchose   /mountpoint   ext3   defaults   0 0)

Again, without a multipathing or failover mechanism, things get very
tricky when determining disks to use. I suggest using one of the above
options, or if multipathing is not available, to upgrade to a newer
release of RHEL.

Thanks,

Tom Callahan
TESSCO Technologies
Desk: (410)-229-1361
Cell: (443)-506-6216
Email: callahant tessco com

A real engineer only resorts to documentation when the keyboard dents on
the forehead get too noticeable.



baran yurdagul turkcell com tr wrote:

>Hi All,
>
> 
>
>I have connected my linux box to Hitachi SAN with 2 HBA. I have two
>nodes and I want to use Veritas as cluster sw. When I wanted to make
>partitioning and create filesystem on the disks I get a bit confused.
>Because of 2 HBA, I have seen 4 disks (Infact I have two). I have
>created a partition and make mkfs.ext3 on to it. But after mkfs.ext3 It
>is strange that partitionin info has been flushed. Did you experience
>such problem before or  is there any documentation or howto etc ?? 
>
> 
>
>BR,
>
> 
>
>
>
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