Multipath iSCSI storage

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Hello,

I am trying to configure multipath for a client over iSCSI using the stgt 
package in Red Hat Enterprise Linux 6.1 (tgt-1.0.14-2).

Here is a very rough sketch of what I am trying to do here: 
http://i51.tinypic.com/6jir1v.jpg

I have a two node Red Hat Cluster where both nodes share a LUN off our 
SAN.  I then encapsulate the shared LUN and create a volume group with the 
cluster bit set so CLVMD will track metadata changes between cluster 
members.  I finally carve up a 64GB volume out of the volume group and 
export it as an iSCSI target on both cluster members (targets.conf below).

I log into the iSCSI service on both cluster nodes with the client and 
enumerate the targets as /dev/sda and /dev/sdb.

I then start the multipath service on my client with an active/passive 
configuration.  It detects that /dev/sda and /dev/sdb are paths to the 
same disk which in turn creates a pseudo disk called /dev/mpatha.

I am able to format /dev/mpatha and perform work on it.

If I change the multipath configuration to active/active where IO is 
uniformly sent down each iSCSI target at the same time I end up corrupting 
/dev/mpatha.

Why does this occur when my client communicates with iSCSI targets via 
stgt and not when my client communicates with SAN storage directly using 
the same native multipath software (dm-multipath)?

Could this problem be a result of IO being split between paths not making 
it to the disk in order/correctly because stgt is using bufered IO rather 
than direct IO?

Any help would be GREATLY appreciated.

Cheers,


-- SNIP /etc/tgt/targets.conf --
default-driver iscsi

<target iqn.1970-01.com.example:netclient-0.disk>
  incominguser netclient ed86543cf2a15600c93a7c52c0f61b2bae710230
  backing-store /dev/cvg00/netclient-0
  write-cache off
</target>
-- SNIP /etc/tgt/targets.conf --

On the client I have the following Active/Passive multipath configuration:

-- SNIP /etc/multipath.conf --
defaults {
        user_friendly_names yes
}
blacklist {
        devnode "^(ram|raw|loop|fd|md|dm-|sr|scd|st)[0-9]*"
        devnode "^hd[a-z]"
        devnode "^dcssblk[0-9]*"
        device {
                vendor "DGC"
                product "LUNZ"
        }
        device {
                vendor "IBM"
                product "S/390.*"
        }
        # don't count normal SATA devices as multipaths
        device {
                vendor  "ATA"
        }
        # don't count 3ware devices as multipaths
        device {
                vendor  "3ware"
        }
        device {
                vendor  "AMCC"
        }
        # nor highpoint devices
        device {
                vendor  "HPT"
        }
        device {
                vendor QEMU
                product QEMU_DVD-ROM
        }
        wwid "*"
}
blacklist_exceptions {
        wwid "1IET     00010001"
}
multipaths {
        multipath {
                uid 0
                alias mpatha
                gid 0
                wwid "1IET     00010001"
                mode 0600
        }
}
-- SNIP /etc/multipath.conf --
--

Greg Procunier
UNIX Administrator III - Enterprise Servers and Storage
1 Robert Speck Parkway, Suite 400, Mississauga, Ontario L4Z 4E7
Office: 416-673-3320 
Mobile: 647-895-2977
Email: gprocunier@xxxxxxxxxx

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