Virtual machine drops ISCSI drives during Vmotion

celerra 1Recently during maintenance downtime early one Sunday morning, our VMware administrator was v-motioning numerous VM’s to patch the host environment blades on the Cisco UCS. One of the VM’s was a Windows 2008 R2 server that contained over 10 iSCSI connections to targets residing on an EMC Celerra NS-40G NAS. All migrations were going well until this particular VM was moved: all of the iSCSI drives disappeared from the Windows OS. A vMotion process is a transparent process and this should not have happened, but this time it had a negative effect on the VM: an unexpected glitch. This happens time to time. The iSCSI initiator in the Windows Server OS still registered the connections as connected and online, but the OS disk management would not see the drives.

celerra iSCSI targets

I disconnected and then re-connected the targets in the iSCSI client, but the Windows OS would still not see the drives. I restarted both the iSCSI initiator and Server service in the Windows OS with the same result. Re-scanning for storage in Windows disk management did not help. EMC tech support ran a check on the NAS end and came up with nothing. Eventually, I went into Celerra Manager and deleted the LUN masking for the target and then re-added it. The Windows Server OS was then able to see the drives. I was hesitant to do his at first, as I was unsure what effect it would have on the drive letter assignments on the server: it had no effect on that – all drives were reestablished as the previously were.

celerra iSCSI target LUN masking

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A review of IP traffic on its effect within SAN

This brings iSCSI to mind. According to Network Magazine, iSCSI makes SAN data accessible to everyone, as this enables a node to access a storage target and set it up for access so that it appears to the OS as a local drive. The advantage of an IP transport in comparison to Fiber Channel is that it is possible to scale a SAN over a longer distance. “TCP/IP can run over any physical medium, so it offers a choice of topologies… and IP packets can travel across alternative LAN topologies just as easily ” (Hall, 2004). This can happen only if the network hardware is sufficient to support the additional Gigabit traffic that iSCSI causes, as iSCSI produces large amounts of traffic bursts. As TCP contains flow control that is very sensitive to loss and delay of packets which could result in negative performance impact, it is essential to implement the fastest switching and routing protocols such as Gigabit switches and router RIP. Currently, there are new 10Gbs Ethernet switches on the market that can more than handle the increase traffic bursts of iSCSI such as Intransa’s 10G Ethernet storage appliance that is designed for the demands of video on-demand, IPTV, digital imaging, video surveillance, and data mining” (Messmer, 2009) which demand those higher speeds. Let’s not forget the backup of data within the IP SAN. Data backup is the cause of a large amount of SAN traffic both on the wire and on the storage targets. Using a LAN-free approach, such as having a backup server within the SAN would eliminate much (IP) backup traffic so that the backup data is not flowing between the client node and the backup server, but only between the backup server and the storage target. As stated in Arkeia’s article on its new backup software, Arkeia Network Backup version 6.0, the “…Server for SAN option allows LAN-free backup for SAN environments, thus easing network congestion. It allows direct SAN connectivity of servers, storage arrays and tape libraries” (Connor, 2006).

References:

iSCSI: Storage Networking Sans the SAN? — Farewell to Fibre Channel? iSCSI lets storage traffic run over a regular TCP/IP network.(Internet SCSI)(Storage Area Networks)(Transmission Control Protocol/Internet Protocol).Eric A. Hall. Network Magazine (May 1, 2004): p62. (3170 words) 

Messmer, E. (2009). Intransa debuts 10G Ethernet storage appliance; Latest StorStac appliance offers up to 180TB of storage capacity. Network World. Academic OneFile. Gale. BCR Regis University. 
http://find.galegroup.com.dml.regis.edu/itx/start.do?prodId=AONE

Connor, D. (2009). Arkeia rolls out new backup software. Network World. Academic OneFile. Gale. BCR Regis University.
http://find.galegroup.com.dml.regis.edu/itx/start.do?prodId=AONE

IP as a viable transport for storage networks

netwrokIP is a viable transport method within SAN’s, as this is a great method for attaching additional client nodes to the SAN without the need of purchasing and installing additional (and expensive) HBA’s for the nodes. iSCSI is one of the tools that enables this to happen. With the use of a software iSCSI initiator, the node can view an assigned iSCSI file system on the storage end and attach to it as a local disk.

Another advantage of IP within a storage network is the utilization of the spanning tree protocol within TCP. Spanning tree monitors redundant links and assigns alternative network paths on demand in the event of a port failure. It also prevents any packet stream from accessing more than one path at a time, preventing packet or broadcast flooding in the network. Think of it as a traffic cop. Spanning Tree is valuable for SAN traffic, as it will keep the data going in the event of a link failure. This is known as transparent bridging in the text (p.81). With Fiber Channel, there is usually a second fiber switch that will enable continuity of the connection if the other goes down. It seems to me that IP can provide more links to the target.

Spanning Tree is being replaced presently in a few organizations by the new Flex Link protocol:

“Flex Link is a Layer 2 availability feature that can co-exist with spanning tree. This enhancement allows a convergence time of less than 50 milliseconds. In addition, this convergence time remains consistent regardless of the number of VLANs or MAC addresses configured on switch uplink ports. It is a pair of a Layer 2 interfaces, either switchports or port channels, that are configured to act as a backup to another Layer 2 interface. The feature provides an alternative solution to the spanning tree protocol (STP), and it allows users to turn off STP and still provide basic link redundancy”

Reference:

Cisco. (2009). How to configure a Flex Link for link-level redundancy in Cisco Catalyst switches that run Cisco IOS. Retrieved July 16, 2009 from http://supportwiki.cisco.com/ViewWiki/index.php/How_to_configure_a_Flex_Link_for_link-level_redundancy_in_Cisco_Catalyst_switches_that_run_Cisco_IOS

iSCSI vrs Fiber Channel

From SearchSMBStorage: Data storage management is a multifaceted process that is always changing. In this tutorial on SMB storage management, learn about iSCSI versus Fibre Channel storage area networks (SANs), managing your data backups, and the biggest SAN storage management trends for the year ahead. Read this article at SearchSMBStorage:
http://searchsmbstorage.techtarget.com/generic/0,295582,sid188_gci1346117,00.html#

Creating and Connecting to an iSCSI LUN on an EMC Celerra

From UTube.

EMC on Using the iSCSI Wizard for Celerra

A short how-to on adding iSCSI storage with EMC Celerra.

http://www.youtube.com/watch?v=zPqWWxnTwdI