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Say Goodbye to LUN Ownership and Hello to SAS

by Hu Yoshida on Oct 13, 2008

The value of  modular storage is that it is modular, you build it out by adding drawers into a 19 “ rack frame, and it is low cost since it only has two controllers which act as a fail over for each other. But there are also disadvantages when compared to monolithic arrays that have 32 to 128 controllers that share a large common cache.

One is LUN ownership which requires LUNs to be assigned to one controller or the other, but not both. Modular storage controllers do not share a common cache; each controller has its own cache. If access to LUNs were allowed to be done across both controllers, you would have thrashing, as the I/O request would ping-pong between controllers trying to find the cache image of the data it is requesting, and tying up both controllers trying to load data into its separate caches. LUN ownership means that I/O requests can not be load balanced across controllers, and operations becomes complicated with the need to keep track of these assignments.

You can say goodbye to LUN ownership with the new AMS 2000 modular storage systems which were announced today from Hitachi. LUNs can now be assigned without specifying controller ownership, and I/Os can be automatically load balanced between controllers, and provide automatic fail over in the event of a controller failure.

The other disadvantage with modular storage is the limited paths to the back end disks. Each controller has, at most 2 paths to the disk, which gives a maximum total of 4 back end paths. These paths are over FC arbitrated loops, which means only one request is serviced on each loop once the arbitration is resolved. When modular storage was designed over 20 years ago, they were designed to address 0-15 targets, or 16 disks. Today, modular storage systems can connect up to 240 disks or more! Two paths are no longer enough to service the back end disks!

Its time to say hello to SAS, Serial attached SCSI, which enables us to attach 32 concurrent paths behind the two controllers, for improved performance and improved fault isolation in the event of a disk failure which takes a loop down. SAS is available in our new AMS 2000.

Say Hello to the AMS 2000 which was announced today. To get more information of these new features, read the following analyst reports on our website.


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Comments (2 )

Richard D. Davis on 04 Nov 2008 at 2:46 pm

Will this work with our current HDS AMS controllers. I am looking at storage Naviagtor and it says we have DF700M_75040028 configuration. The Controllers are not ID’d. We are on Maintenance and an HDS customer. Is this a software upgrade or hardware upgrade or both ?

Keng Seng on 20 Nov 2008 at 3:56 am

I’m currently using a HDS 9570V and is considering to upgrade my storage and hence, the AMS 2000 is a natural successor candidate. I was however, on inspecting the specifications, disturbed that HDS include only either ISCSI OR FC configuration(AMS 2100 & 2300) for their front end connectivity as compared to their competitor(CX-4) where their front end can be easily upgraded to a mixture of ISCSI and FC connectivity.

Being a SME shop, having the ability to upgrade my storage and add capability for my other (I/O unintensive) server to use it thru ISCSI is too great a point to overlook. The company can save a lot of money turning to the ISCSI solution than that expensive FC fabric. HDS enginneers really dug a hole for themselves now by excluding this much sought after function. Tell me.. which SME shop looking at lowering their TCO is not looking at ISCSI connectivity to their servers and maximizing their existing FC fabric?

U can say that we can add 2 iscsi routers to the fabric. But having said that, U will burn 2 more FC switch ports and incur additional cost of 2 iSCSI routers.(Much more expensive than if u have it already on the storage.) I really think HDS engineers should really be storage administrators for a SME before they go to the drawing board to built a Modular storage that a SME would want.(One that maximises performance and reduces TCO)

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