PowerEdge HDD/SCSI/RAID

Last reply by 07-01-2017 Solved
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Interposers - what are they for?

Hello,

I've recently purchased a Dell 2950 and a Dell 2900, second hand. One server came with SAS drives, the other with SATA drives + interposers, all in the same 3.5" caddies.

There was some conflicting information around on the forum as to whether I needed interposers or a certain type of caddy to use SATA drives with a SATA/SAS backplane, so I had an experiment.

It appears that I'm able to mount a SATA drive in the SAS/uSATA caddy, lining it up to the SAS mounting holes and plug it in without an interposer...and it works. OS installed fine and data going to/from the discs happily.

Are the interposers that came with the server unnecessary? (They seem to be original according to the service tag.) There's quite a bit of electronics to an interposer, it's not just a pin converter or something simple, there's work gone into it - what are they for?

Cheers,
Chris

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I know that this is an old thread, but I was looking for this information and could not find the answer, so I though I would share what I found here to be of service to any others looking for the information.

SATA and SAS drive can work from the same controller, BUT there are distinct differences in how they work.

SATA drives are limited to 1 meter connection distance and they only use hlaf-duplex communication.

SAS drives are limited to 10 meter connection distance and they use full-duplex communication.

Additionally, SAS, because it has a SCSI base, began with a rich set of diagnostics and control not available in the base of ATA.  As a result, when mixing SATA and SAS, it is necessary to make the SATA drive appear to be a SAS drive.  The interposer chip provides that service.

SATA drives connected using the interposer should appear to the system as SAS drives.

The interposer also preserves signal quality in large backplane implementations while providing additional buffer and multiplexer services that enable active-active access (dual path controller redundancy).

Essentially, if you are using a SAS controller and you have the interposers, you get nothing but benefits from using them. 

Yes, it will work without them, but it makes it better to use them.  Theoretically. 

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Anonymous
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With what you described, you have either a GEN I or GEN II 2900 & 2950 Dell severs.  In the 3rd generation of the 2900/2950 (9th) Generation servers, the carrier was changed and interposer was no longer needed.  

The interposer provides the communication for SATA drives when connecting to a backplane that can run either SAS or SATA drives.  As you pointed out, the carrier is the same, but only the SATA drives have the interposer board within the carrier.  The interposer works as the interface to allow communication to the drive when the carrier is connected  to the backplane.

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Hi Todd,

Thanks for quick reply, you're right I do have 2 x Gen II servers.

Can you help me understand why I can use SATA drives without interposers? It appears that they are not necessary? The OS is running fine without them.

Cheers,
Chris

Anonymous
Not applicable

If thats the case, I would assume the systems are current with the latest firmware; BIOS, BMC (backplane) and so forth.  

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Hi Todd,

So to be very specific, if I'm using either a Generation 1 or 2 PowerEdge server with the latest firmware throughout, OR using a Generation 3, then interposers are not necessary to use SATA drives in a SATA/SAS backplane?

Kind regards,

Chris

Anonymous
Not applicable

I am attempting to find a specific link, but if I recall, it was used to fit the drives at the time into the carrier, but I know that by revision III of the unit, it has a universal carrier without use of interposer board.

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Thanks Todd, the official Dell shop on Amazon confirms this:

www.amazon.com/.../B0058F98O6

NOTE:Interposer is not required to use this tray with a SATA hard drive for PowerEdge servers. Just mount the SATA drive the same way as SAS drives. However, interposers are required if you want to use SATA drives in PowerVault MD1000.

Thanks for your help!

2 Bronze
2 Bronze

I know that this is an old thread, but I was looking for this information and could not find the answer, so I though I would share what I found here to be of service to any others looking for the information.

SATA and SAS drive can work from the same controller, BUT there are distinct differences in how they work.

SATA drives are limited to 1 meter connection distance and they only use hlaf-duplex communication.

SAS drives are limited to 10 meter connection distance and they use full-duplex communication.

Additionally, SAS, because it has a SCSI base, began with a rich set of diagnostics and control not available in the base of ATA.  As a result, when mixing SATA and SAS, it is necessary to make the SATA drive appear to be a SAS drive.  The interposer chip provides that service.

SATA drives connected using the interposer should appear to the system as SAS drives.

The interposer also preserves signal quality in large backplane implementations while providing additional buffer and multiplexer services that enable active-active access (dual path controller redundancy).

Essentially, if you are using a SAS controller and you have the interposers, you get nothing but benefits from using them. 

Yes, it will work without them, but it makes it better to use them.  Theoretically. 

Community Accepted Solution
2 Bronze
2 Bronze

Additionally, you cannot combine SAS and SATA drives on the same VD in those servers/storage arrays. 
If you want to use both SAS and SATA drives in a single array, you must use interposers. 

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This may no longer be true. I have an MD1000 running SATA drives without interposers. Whether this is firmware-dependent I have no idea. 

I ran for a while with some old SAS drives. While rotational speed is a factor, a modern SATA drive can be using far less power for the same total storage. I expect a modern SAS drive has the same advantage. I'm using a 2950/MD1000 combination as a file server, mostly video files, and they can saturate the network using SATA drive hardware without interposers.

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