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What would be the advantages of using a External SCSI Storage Array over "Onboard" Storage on a server?

My guesses are:

  1. Two servers can access the logical drives installed to the Array like they were onboard, sharing the drive resources.
  2. More space for drives over a standard server

Is this all there is? Looks like back in the day you could cluster two Windows 2003 servers together to make a HA pair but that functionality has been discontinued in Server 2008.

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You can still do clustering in Windows Server 2008 and Server 2008 R2. It simply is not available for all versions (i.e. it is not available in Server 2008(R2) Standard Edition). – Eli Apr 13 '11 at 0:40
Eli: Eventhough clustering is available, using a External SCSI Storage Array as a shared storage option was taken out of Windows 2008 in favor of SAS solution. – Frank Owen Apr 13 '11 at 1:18
up vote 3 down vote accepted

SCSI as we knew and loved it is slowly going away in favor of SAS. SAS, unlike SCSI, is more Fibre Channel like in that more than two servers can talk to the same devices. Because of this, if you have a large SAS-based disk-pack, you can share that disk to a bunch of servers in the same rack without having to bust out the Fibre Channel cards.

But, you asked about SCSI.

The chief reason for External SCSI arrays is simply that they can be expanded farther than the internal kind. Internal SCSI is limited by space in the server, whereas external SCSI is limited by space in the rack.

Back in 2000 I put together a 4-shelf SCSI array for an Oracle database. It involved 48 disks in 24 Raid-1 pairs. We needed both space and speed for this system, and the DBA liked this design. It was dedicated to this one DB server. Looking back, it was horribly over-speced for what it was doing, but hey, I was new back then and the DBA was a grizzled veteran.

A brand new system will use SAS for much the same thing, only it's easier to get multiple servers (known as initiators) to talk to the same storage (known as targets). SCSI was limited to no more than two initiators on a SCSI bus.

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This doesn't answer the question which was about internal vs external storage rather than SCSI vs SAS (which incidently is Serial Attached SCSI). Implementations of parallel SCSI have always dealt with sharing/redundancy by simply having multiple busses. So while your point that it is easier to connect multiple machines is true, it is not such a huge jump as you suggest. – JamesRyan Apr 13 '11 at 10:18

The main advantage that you have not mentioned is that if you have a problem with a physical server it makes it much easier to attach the storage to another as either a temporary or permanent replacement.

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Storage shelves and servers have different life span. Some people are still using storage shelves bought 10-15 years but it is unlikely you find a server in use from that era. So, it is not uncommon that storage survives 3 generations of server hardware. It also makes IT management a little less complicated (i.e. when you can deal with storage and processing power independently) and it helps with budgeting too. I would even say it may even be cheaper in a long run (if you play your cards right :-).

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