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SAN vs DAS

Posted on 2006-03-30
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We are out of space.  We have 3 seperate servers running SQLServer, 2 on Server2k3, 1 on Server2k.  All boxes are Dell PowerEdge variants.

I am looking for a single solution that will expand capacity on all 3 servers.  I am a newb in this realm, and I'm wondering if I can use a SAN solution, or would it make more sense to use a DAS solution for each box?

Areas of consideration:

1. Performance
2. Compatibility with SQL Server (it's my understanding that it requires block addressable storage)
3. Price
4. Growth
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Question by:darrennelson
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by:kkrans
ID: 16337320
SAN if you are loaded with $$

iSCSI if you need an affordable solution for a fewer number of servers (3 is few servers)

If you need loads of performance the local disk is almost always faster than a remote disk

I see iSCSI growing fast on few app servers, you pay a fraction to get a good solution. Remember to buy some iSCSI adapters for your servers as well because IP trafic needs some offloding and processor power.



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Duncan Meyers earned 252 total points
ID: 16337648
Depending on the nature of the workload, how many users and so on, an entry level SAN could be just what you need - for example, an EMC CX300 or CX300i

1. A good SAN system uses fibre channel disc drives and also a large chuck of write cache. Your servers actually end up writing into cache, then the storage system takes care of staging the data out to disc, so you get the fastest possible write performance.
 
2. Most (if not all) storage vendors support SQL server. Certainly, all EMC's Clariion and Symmetrix range do. EMC also has replication and snapshotting software that sepcifically integrates with SQL Server and Exchange

3. An entry level EMC Clariion CX300 is not that expensive. It really depends on  how much disc space you need. Purely indicative cost - around the US$22K mark.

4. An EMC Clariion CX300 scales to around 16TB (ish) raw using only fibre channel disc, or 19TB using a combination of FC and ATA disc.

More information here: http://www.emc.com/products/systems/clariion_cx300.jsp

>If you need loads of performance the local disk is almost always faster than a remote disk
Sorry, I can't possibly agree with that.. One of teh points of a good SAN system is that it is considerably faster than internal disc.

Just a not on iSCSI -

It's well nifty stuff - but... And there is always a 'but'

Currently, FC is a faster connection protocol than iSCSI, and it will remain so until 10GB Ethernet becomes available. The MS iSCSI initiators also put a heavy processing overhead on the CPU - remember that all the TCP traffic processing has to be handled by the server CPU. You can get around this problem by using TCP Offload Engines, but they cost about the same as Fibre Channel HBAs. You should also run your iSCSI traffic on a seperate LAN - for performance reasons mostly.  

Since the price of (particularly entry level) Fibre Channel switches is dropping, the price differential between iSCSI and Fibre Channel is nowhere near as great as it was even 12 months ago. Given the performance considerations, the balance is still in favour of Fibre Channel - it doesn't cost a lot more that a proper iSCSI set up, and it is faster, more robust and more secure.
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by:IPKON_Networks
IPKON_Networks earned 248 total points
ID: 16337735
DAS will give you faster performance generally but you will be limited to physical space, sharing the spare space between servers (you can't) and as you grow, you need to add more and have down time (usually lots of it).

SAN allows you to make most of the available space between your servers. Add a single disk, and all 3 can benefit. Simpler backup/recovery (although it could be worse in some cases!). Higher cost, but these are coming down significantly.

iSCSI is still in development with competing technologies and standards. Expect lots of change.

NAS is an alternative to SAN. However, the file sharing overhead is generally too much for SQL/transactional servers so best avoided.

If you have some money and can take this financial pain, I would look to utilise a SAN. You could start by moving the LOG files, Transactional files, Backup Dumps, then Data and finally system files over in a migration.

If you are brave then you can also boot from the SAN and then your file servers become simply processor units, and hot-swapable (in theory). In practice this is harder to achieve than the salesman will let you believe. :-)

A small all-in-one SAN with using copper CAT-5 cabling is easily available. LeftHand, Equallogic Dell all do a cheap set of all-in-one SAN's depending on price willing to pay !

Hope this helps
Barny
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Author Comment

by:darrennelson
ID: 16337841
great information, thank you for you time.  I was looking into the SAN option after I posted this and realized, even at entry level, its WAY out of budget.  It looks like the solution will either be a DAS or simply upgrading the existing drives.

Thanks All
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by:Duncan Meyers
ID: 16338227
Thanks!
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by:Duncan Meyers
ID: 16338270
I can't let these go unchallenged...

>iSCSI is still in development with competing technologies and standards. Expect lots of change.
iSCSI is no longer in development, and it is being heavily adopted. There is one standard - iSCSI
http://www.ietf.org/html.charters/ips-charter.html
http://www.ietf.org/rfc/rfc3347.txt
Don't expect lots of change - its an industrry-accepted RFC.
What may happen is the Serial-Attached SCSI may come along and steam-roller all over the top of iSCSI. We'll just have to wait and see on that.

>DAS will give you faster performance
No, and that is one reason why you buy a SAN in the first place. A properly configured SAN is orders of magnitude faster than DAS - because of things like write cache, RAID optimistaions, read-ahead optimisations and so on.
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