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Can a DAS based Windows cluster be fault tolerant?

Posted on 2016-10-01
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Last Modified: 2016-10-06
I have seen several posts recently about the use of DAS only Windows cluster!
I have real difficulty envisioning how these can be fault tolerant like you would expect with a SAN based cluster.
Can the experts enlightening me?
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Question by:Gerald Connolly
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16 Comments
 
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Expert Comment

by:Aaron Tomosky
ID: 41825054
Most DAS just means SAS connections from the major vendors and they have multiple connections. So you can run 2-4 hosts via SAS cables and it functions just like a iscsi/nfs san.
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Expert Comment

by:arnold
ID: 41825064
DAS means Direct Attach Storage from SCSI, to SAS, TO ESATA, etc.
SAN Storage Array Network, is network attached (FC or ISCSI)

Usually as long as the DAS remains functional and the controller it has does not fail, it will provide tolerance for NODE failure.
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Expert Comment

by:Roman khan
ID: 41825070
DAS Storage can work as SAN and provide fault tolerance, if you connect it with any storage virtualization appliance, For example we can connect IBM DAS3500 Series with IBM SAN volume controller or IBM storwize V7000,

DAS also provide redundancy as it has two controller and dual ports each.
We can also connect DAS with FC if available
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Expert Comment

by:dlethe
ID: 41825113
You're getting hung up on terminology and making assumptions about fault tolerance and requirements of just one method of interface.  Application software; operating systems; failover/failback; load balancing vs load distribution;  distributed processing capability; whether active/passive vs active/active; heartbeat mechanisms ..

All contribute and affect redundancy availability, and fault tolerance.  Heck, google uses DAS storage, not SANS, and they manage to stay online. That alone should tell you that SANs aren't inherently fault tolerant and DAS is fault tolerant.
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Author Comment

by:Gerald Connolly
ID: 41825787
@Aaron - I think i would consider shared or clustered SAS storage as a SAN, rather than DAS!

@Arnold - I know what DAS is thanks, but wanting to understand how a DAS Cluster works?

@Roman - I think you are missing the point.

@dlethe - David, no i am just trying to understand what people mean by DAS clusters, i am looking for some technical advice on how they are connected and how they work, Aaron & Roman both talk about shared SAS storage (ie a SAN) but not about how DAS in a cluster works?
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Assisted Solution

by:Aaron Tomosky
Aaron Tomosky earned 250 total points
ID: 41825821
The world calls a multiple SAS connected device a DAS because it's SAS and not a network. But since you can connect multiple hosts it's still got the SAN benefits, just no network.
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arnold earned 250 total points
ID: 41825841
it is the same scenari


node 1 ==================
                                                      \
                                                         == DAS
                                                       /
node 2 ==================

The DAS usually have differential access on ports such that only one node can read/write into the allocated set of storage.
The storage similar to a SAN in the cluster config, application the storage is a resources that shifts access based on the active node.

DAS would act as the common storage.

Scaling and functionality, as SAN has its own operating system and built-in redundancies i.e. processors, controllers.
Often as DAS consists of a RAID controller to deals with the data within with multiple external connectors SAS, SCSI, etc. The nodes create the volumes that are served UP
SAN can provide access to storage via multiple modes, FC network, iscsi network ....using FC switches or network switches. Within its own OS/software the storage is allocated to nodes ,.......
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Author Comment

by:Gerald Connolly
ID: 41826067
What you describe is not Directly Attached Storage, at best it's shared storage which qualifies it as a SAN!

And if this config was connected via FC you would call this a SAN, so why isn't this a SAN?
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Expert Comment

by:arnold
ID: 41826113
How would you define a direct Attached Storage?
SAN is a self contained System meaning it runs an OS, it has an interface through which the LUNS can be defined and allocated.

DAS is configured from and by the NODE/s to which it is connected and usually is limited to two nodes connecting at the same time.

No, if the connection was via FC it will not necesasrily make it a SAN.

Look at the SAN, it has the brain (processor unit with or without storage beyond the OS) and then it has DAS (Storage expansion modules) connected via FC.

Difference between SAN and DAS can be explained in the following manner, DAS is a self serve repository of data with two doors to the two adjacent offices. Only one door can be used at a time. Each adjacent room has one person authorized to be there.
A SAN has an attendant through whom all requests go and has a list of of who is authorized for access to which documents. SAN FC/ISCSI allocation while there can be multiple-nodes defined as allowed, to make sure there is no data corruption in both cases, only one node can access the data.
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by:Gerald Connolly
ID: 41827411
When the term Storage Area Network was coined in the Nineties, a SAN was usually a collection of devices (aka JBOD)  connected on a FCAL loop and connected to 2 or more servers!
This quickly transformed into a RAID controller connected via FCAL or via Fabric, with FCAL disks attached. It didn't take long for the server attachment to be all Fabric.

We went down that route for a long time, with a SAN basically a remotely connected multi-user RAID controller.

Eventually iSCSI came along and changed people's view on what a SAN is, but adding to the definition not replacing it.

So a SAN is network of storage devices, it doesn't have to have intelligence, or nodes that are gatekeepers, it can have them but it's a broad term (now) , 2 or more servers sharing storage constitutes a SAN, the interconnect is irrelevant - we have already used ESCON, FICON, FCAL, FC, Infiniband, Ethernet amongst others

Direct Attached Storage (DAS) is classicly defined as storage attached directly to a server and not connected on a shared bus and dedicated to that server
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Expert Comment

by:arnold
ID: 41828235
I think we are running in circles SAYING similar things hung up on the terminology used.  I.e. was the encounter with Technical/ Engineers or Sales people who might have combined/conflated the terminology.
DAS was the original The attach part is what is important. No matter the interface it is limited to TWO.
JBOD enclosure required it to be connected to a RAID controller in the host to have redundancy.
The PowerVault (scsi always comes with two Ports per SCSI standard active termination, ) included a switch when it was configured as a shared storage between two nodes.
 I do not believe a chain of storage devices connected to nodes with a differential BUS was ever termed a SAN (where is the network for that matter where is the Array),
NAS and SAN were developed sequentially (netapp, EMC)
One is a self contained system that provides network based access to storage
The other deals with multiple connection/hosts and avails a different set of storage resources to different nodes for different purpose.
The difference is in scale.
In the SCSI days, you were limited to the number of devices you can chain.. (6 or 15 depending on SCSI I/II or SCSi360)

lets try it this way. You take JBOD units. You attach the storage units to a windows storage server or any linux server.
What you have here is a DAS as far as the Windows Storage server or linux OS is concerned.
Now you configure you windows storage server/Linux server allocating portions of the storage available to it as resources to other hosts accessing the data via any mode you want, FC, ISCSI, NFS, CIFS, etc.

If you enclose the storage plus the windows storage server or linux server, defining the combination as the system, your new system is now a SAN/NAS for the purpose of the hosts/nodes that access the storage resources presented to them.
Hosts that use iscsi/FC see it as a SAN (resources here are seen as raw devices)
Hosts that use NFS/CIFS see it as a NAS (resource here are seen as block devices)

SAS has Attached as part of the protocol definition.
FC is a carrier that can carry Ethernet or SCSI or other commands.
iSCSI SCSI over Ethernet effectively.
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Expert Comment

by:arnold
ID: 41828264
SAN, A is area not array. array locked in from the beginning and can not switch it out.
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Author Comment

by:Gerald Connolly
ID: 41829764
Arnold, i have always know the A in SAN stood for Area.

I also know that as far as Windows is concerned any LUN presented to it from whatever source  - Parallel SCSI, Serial SCSI, Fibre Channel, Infiniband, iSCSI etc, all look like locally attached SCSI disk and therefore in your definition are DAS.

Yes i agree we didnt term SCSI clusters ( even those done with Standard and/or Fast SCSI) as a SAN maybe that was because the term wasnt in general use by then, we certainly designed and built quite a few when UltraSCSI came along courtesy of Bill Ham at Digital. I note his 1997 paper doesnt mention SAN's at all.
An early SAN product from Compaq was the MSA1000 which if i remember correctly was basically a SmartArray Backplane Array, with a Fibre Channel front end rather than PCI and was designed to be shared between only a couple of systems, in fact there might even have been a SCSI attached version in the early days!
I understand the point you are trying to make, its just another blurring of the line between really locally attached storage and a SAN, which is why i posed the question in the first place.
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Expert Comment

by:arnold
ID: 41830268
a DAS with a differential interface can be used as the common storage for a cluster and will provide redundancy.
Dell PowerVault is such.
The dell equalogic, etc are the SAN types.

Try this, look at write ups to build your own SAN etc. and you will see that a SAN is basically made up of a System that acts as the brain, processor, and a storage that it presents to clients.
A dumb box is as DAS no matter how it connects SAN includes DAS units that expand the available space.

So the answer, a the correct DAS can be used in a cluster. However, if reading it different, the DAS itself can not be clustered.
I am not sure wether the RAID controller can use disks from each two chained DAS units in a RAID grouping.
Might that be your question if you have two DAS units, can they be configured to provide fault tolerance for a das failure?
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Expert Comment

by:arnold
ID: 41830308
To clear up and answer to your question, Yes a Windows Cluster can be made up of two windows Hosts with a single DAS (connected to each via a differential module SAS, UltraSCSI) can be fault tolerant (deals with tolerating the loss/failure of a node with the presumption the storage is comprised of RAID volumes with the DAS).
The Cluster will usually not have tolerance for the DAS itself or its controller failing.
i.e. unplug the das from power, the cluster stops functioning.
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Author Closing Comment

by:Gerald Connolly
ID: 41831470
My nomenclature/definitions updated
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