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A beginners guide to SAN

Posted on 2004-10-01
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Hi,

i'm trying to evaluation whether SAN would be an appropriate option for my company.  We need to set up a cluster with at least 2 TB of storage - and we'd like to be able to scale this.  I know that I can have multiple SCSI DAS boxes - and scale it that way by having multiple scsi cards in my servers.

But this is messy.

Problem is that I don't know anything about SAN.  Can someone give me an explanation of how it works and answer the following questions:

1. with SCSI DAS I'm limited to two webservers in a failover setup connected to a shared storage box - with SAN how many computers can be connected to one storage box?

2. does SQL Server 2000 support SAN?  our cluster is running SQL Server 2000

3. Could I setup a SAN to begin with using two servers and one storage box?  or is this too small to start a SAN?

4. where do switches come in?  in my two server system - do they connect to a switch and then the storage box connects to the switch as well?

5. in this model - with two servers - how many storage boxes can they be connected to?

6.  how do hard disk drives come up on the servers?  i.e. does each drive in the storage boxes come up as a separate drive with its own drive letter?

7. in terms of cost - is there a cheap way for me to get into SAN?  i.e. I can get two rack mounted IBM x345's for US 3K each roughly and a SCSI DAS IBM EXP400 for 2.5K US - is there any way for me to get into a two server and one storage box SAN for a similar price?

8.  Where can i go to read about SANs?  I obviously don't know much!
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Question by:kenshaw
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by:Duncan Meyers
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>1. with SCSI DAS I'm limited to two webservers in a failover setup connected to a shared storage box - with SAN how >many computers can be connected to one storage box?
 Depends on the SAN box. An EMC CX300 (entry level) can support up to a maximum of 128 hosts. A CX700 can support up to 512.

2. 2. does SQL Server 2000 support SAN?  our cluster is running SQL Server 2000
Yes.

3. Could I setup a SAN to begin with using two servers and one storage box?  or is this too small to start a SAN?
Yes. You can direct connect servers to the storage system. And its a good way to get into the technology.

4. 4. where do switches come in?  in my two server system - do they connect to a switch and then the storage box connects to the switch as well?
You need switches if you use "High Availabilty Attachment" (that is; have multiple I/O paths) or you have more than two hosts.

5. in this model - with two servers - how many storage boxes can they be connected to?
Just the one. You can connect to multiple storage boxes but you'd really need switches.

6.  how do hard disk drives come up on the servers?  i.e. does each drive in the storage boxes come up as a separate drive with its own drive letter?
Yes. The logical discs look to the server like a directly attched SCSI disc. The fibre channel adapters that you install show as SCSI adapters. All teh SAN hardware is totally transparent to a Windows server.

7. in terms of cost - is there a cheap way for me to get into SAN?  i.e. I can get two rack mounted IBM x345's for US 3K each roughly and a SCSI DAS IBM EXP400 for 2.5K US - is there any way for me to get into a two server and one storage box SAN for a similar price?
Hah! The $64 question! Yes, but you really, really get what you pay for. Entry level storage is avaialble from EMC (the AX100) and from Apple (the X-RAID). You're looking at $US5000 for 1 and a bit TB with FC adapters ready to go. Cheap as chips. The entry level stuff uses ATA discs instead of Fibre channel discs - you can expect *at least* one third to one half of the performance of a fibre channel array. You pay more for the Fibre channel stuff because it's faster, more expandable and has features that ensure better up-time. In a corporate environment I'd look very closely at my options before going one way or the other. You need to consider things like:
 - How much storage do I need?
 - How many users at once?
 - What are my availability expectations?
 - What is my backup window going to be?
 - For SQL - how many transactions per second and how many users?
And so on.

8.  Where can i go to read about SANs?  I obviously don't know much!
Some reading for you:

http://www.emc.com/techlib
http://www.emc.com/techlib/abstract.jsp?id=1444
http://publib-b.boulder.ibm.com/Redbooks.nsf/RedbookAbstracts/sg245470.html?Open
http://www.overclockers.com.au/techstuff/a_san/
www.pmc-sierra.com/cgi-bin/document.pl?docnum=2022178
http://www.microsoft.com/whdc/device/storage/stor-tech.mspx

I work with EMC storage a lot, and I think that their stuff is the cat's pyjamas.

You also need to be aware that there are some emerging technologies such as iSCSI (SCSI over IP) and iFCP (Fibre Channel over IP) that will change some of the SAN infrastructure hardware - presumably making it cheaper...
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by:kenshaw
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thanks for that - in response to 7. what are fibre channel disks?  i.e. i thought fiber channel was the internconnect between teh servers and teh boxes, and then i though inside the storage box it would use one of teh traditional buses of SCSI, ATA, or SATA.  Am I wrong on taht?  what are fiber channel drives?  

Also - if i'm going with two "hosts" (thats a server right?) and one storage box - this doesn't scale does it.  To scale I'd need to put in a switch... and then what?  as many boxes as I like?  how many storage boxes can you fit per switch?  How many drives can you fit in a storage box?  is it limited the way scsi is to a number (eg with SCSI its 15 id's so usually 14 drives)?
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by:andyalder
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Have you thought of the cost of clustering SQL Vs SQL replication? You need 2 windows Enterprise licenses and at least 1 SQL Enterprise license, that's more than your total hardware cost. You'll probably be able to get your SQL enterprise license for less than M$'s recommended price, http://www.microsoft.com/sql/howtobuy/default.asp.

If you use SQL replication and round robin DNS you just need 2 Windows web edition licenses and 2 SQL standard licenses.

Note that you can have a 4 node DAS cluster with HP's MSA500 G2 if you have the 4 port module in it, don't know if anyone else does a 4 port DAS cluster.
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Duncan Meyers earned 500 total points
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> what are fibre channel disks?

In an EMC Clariion storage system all the hard disc drives talk SCSI over fibre channel - so, yes, you're correct, FC is the interconnect. All the disc drives are connected to two abritrated loops (which is a fibre channel architecture -very broadly similar to token ring) which are in turn connected to two storage processors.

To clarify - fibre channel's sole purpose is to transport standard SCSI commands from the server to the storage. The benefit of using fibre channel discs is that you aren't operating with the limitation of 15 devices on the bus - you can have 125 discs on every fibre channel loop. (which is 127 less reserved addresses). EMC also make a Clariion expansion cabinet fileld with ATA discs. The cabinets and discs have a sub-board that handle SCSI - ATA and FC - ATA translation.

>Also - if i'm going with two "hosts" (thats a server right?) and one storage box - this doesn't scale does it.  To scale I'd >need to put in a switch... and then what?  as many boxes as I like?  how many storage boxes can you fit per switch?  >How many drives can you fit in a storage box?
Yep, Hosts = Servers
You can connect two hosts directly to an EMC Clariion storage box without going to the expense of switches.  Switches cost in the order of  $US1000 per port, so an 8 port switch from Brocade will costs you around $8000. Cisco also make a range of FC switches.
With appropriate switches, an EMC CX300 ("entry level") can connect to 128 servers. That's plenty for most people! There are limitations to cascading switches and so on that are really outside the scope of most installations so we can safely ignore them here, I think.

> is it limited the way scsi is to a number (eg with SCSI its 15 id's so >usually 14 drives)?
You can have up to 240 hard discs in a CX700 (top of the range) with a maximum of 240 RAID groups and 128 Logical discs within those RAID groups. There is a maximum of 2048 logical discs per CX700. Again,plenty for most people. There are some other limitations there - for example there is a maximum of 256 logical discs per initiator (that is; FC card in your server) - but again, we can safely ignore them for the purposes of this discussion.

Midrange CX500 has the following limits: maximum of 120 hard discs, 120 RAID groups, 128 logical discs per RAID group, maximum of 1024 logical discs per array. The comments above apply here too of course.

And finally, the entry level CX300: maximum of 60 hard discs, 60 RAID groups, 128 logical discs per RAID group, maximum of  512 logical discs per array.

The CX range support 36GB 10,000 or 15,000 RPM discs, 73GB 10,000 RPM discs, 146GB 10,000 RPM discs and 250GB/320GB 5400RPM ATA discs (in a special expansion cabinet). 240 * 146GB = 35TB raw. That's quite a bit.

I think I've addressed the questions you've posted. Let me know if I need to clarify anything.


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