Re: [Scst-devel] Two Instances of SCST on different Server

Hi Steve,

On Jun 10, 2010, at 9:18 AM, Support WVNET wrote:

> Hello ,
> 
> I'm new to SCST and i have a question regarding SCST and VMWARE ( vSphere 4
> )
> 
> My setup look like this
> 
> Two Storage-Servers with Adaptec 5244 Raidcontroller ( 12 x SAS Raid 50 )
> and
> Mellanox MT 26418 Infiniband Card ( two ports ) which are connected
> to two Voltaire ISR9024DM Infiniband Switches .
> 
> My VMWARE Nodes are also connected via Infiniband to the Switches .
> 
> The Storage-Servers are running Centos 5.5 ( x86_64 ) with a vanila 
> 2.6.33.5 Kernel ( patched with SCST ) .
> No OFED drivers .
> SCST version: SCST version 2.0.0-rc2-procfs
> DRBD version : 8.3.7
> 
> The Raidvolume is mirrored by DRBD in primary/primary mode between the two 
> Storage-servers. ( replication via IPoIB with Bonding )
> DRBD Resource is the base for my Volumegroups ( LVM2, clvmd) .
> 
> The LVM Volumes are exported by SCST ( FILEIO with WRITE_TROUGH ) via
> IB_SRPT
> SCST is running on both Storage-Servers with the same Config .
> 
> On the VMWARE Hosts i can see the SRP LUNs via multiple pathes.
> I can add the LUNs to my datastorage and can create VMFS , but only
> with VMWARE Multipathing policy "Fixed"
> 
> 
> And now my question
> 
> - is it possible to use VMWARE RoundRobin with this Setup and which
> Multipath-Module should i use ?
> 

While VMware recognizes the multiple paths to the same LUNs and may appear to work just fine, you should be aware that running with multiple VMware hosts in this configuration is not safe (if you were to run the vmware storage certification tests, your setup would fail the reserve/release/reset tests).  VMware uses SPC-2 reservations when operating with multiple hosts, and those reservations must be consistent across all paths to the same LUN.  For example, if a RESERVE is executed along one path, a subsequent WRITE from a different host along a different path should fail with a reservation conflict.  With multiple instances of SCST running on different machines, this will not be the case (the WRITE would succeed, and would modify data while another host is holding a reservation).

For safety, you should use either a single VMware host with multiple SCST instances, or a single SCST instance with multiple VMware hosts.

With regards to your specific question re path policies, I haven't seen the issue you describe (where the Fixed policy works but the Round-robin does not).  IIRC, the round robin policy simply sends 1000 CDBs down each path in turn.  You might need to dig around for initiator logs in the vmware console.

--Phil


> 
> 
> Kind regards
> Steve
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
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