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with SAN - are you limited by the number of drive letters you are with DAS?

Posted on 2004-10-02
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With DAS i know i can have multiple SCSI RAID cards in my server and have as many drives attached to my server as there are drive letters (a - z) so 26.

With SAN - are you restricted in the same way?  i.e. do drives in a SAN just appear to the hosts as standard drives?  in that case - is a single host limited to recognising 26 drives?  using 300 GB drives thats only 7.8 TB.

How does SAN scale beyond that?
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Question by:kenshaw
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andyalder earned 2000 total points
ID: 12206817
You must have Windows since you mention drive letters. The limit is 26 logical drives as seen by the OS, you can have more than one physical disk per logical disk. Your RAID controller can take 7 * 300 GB dosks and make a RAID 5 logical disk of 1.8 TB out of them. This will only take up one drive letter since the OS sees it as just one disk.

But wait, you don't have to assign a drive letter to it, under disk manager you can then take two of these 1.8 TB disks that the OS sees as physical disks but the controller sees as logical and make a software RAID 0 stripe out of them, that gives you a 3.6 TB volume with only one drive letter. Of course disk manager is not limited to just 2 disks in a RAID 0 stripe, you could have 5 shelves full of disks and make 10 disks with your RAID controllers and stripe them into a single volume if you wanted, the only limits are that NTFS can't handle disks bigger than 2TB and the volume must be less than 16 exabytes. I doubt anyone would want a volume in the exabyte range, imagine running checkdisk on it, start today and come back next month.

The same applies with SAN except that the HBA in the server isn't a RAID controller, if you used JBOD shelves you would have to use software RAID which isn't efficient but if you use RAID boxes on the SAN you can do exactly the same thing, the RAID box presents an array as one LUN (one physical disk) to the HBA/OS.

Whether you chose to make the arrays RAID 5 or RAID 10 ius down to you, RAID 10 is faster but costs more for the same available amount of space.
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by:Lee W, MVP
ID: 12207225
I agree with Andy
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by:andyalder
ID: 12207431
Have you seen http://www.ntfs.com/ntfs_vs_fat.htm?

Because their domain name is NTFS.com Google puts this page first in the list if you type 'NTFS and 2TB' as your search string. Try to tell the webmaster that he's publishing garbage and what happens - you get a NDR!
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