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Tiny Deathstars of Foulness

Xsan 2 does remove some of the limitations that you encountered with Xsan 1. But not a ton of them.  There are still a few limitations, most of which can easily be worked around. The first is in the number of storage pools that can be in a single volume, which is 512. The second is the number of LUNs that can be in a storage pool, which is 32. This isn’t to say that you can actually put 512 X 32 LUNs (or 16384 LUNs) in a volume because you can’t.  A volume can actually only have 512 LUNs, whether or not you split them into multiple storage pools. This is important to consider, because if you think about it you’re going to be hard pressed to make a LUN greater than 16TB, especially if you’re using a Promise Vtrak.  At 512 LUNs you’re then seeing a 8,192 Terabyte maximum, which is only in place given the limitation of the 1TB drive modules in the RAIDs by the 512 LUNs.  

Of course, then there’s the fibre channel switching environment to consider…  You can stack 239 of those Qlogic 9xxx series swtiches, which is more than ample to meet the 512 LUNs even if you put 4 storage processors (you are correct if you were about to point out that they haven’t released the 9400 yet) in each SANbox and the ISLs required to interconnect the mesh, with only one blade worth of standard ports.  Either way, it’s possible and pretty easy to provide plenty of ports is the point.  And since the lowest common denominator is still the Xsan software (512 LUNs per volume) times the size of the LUNs (16 TB); the fabric isn’t the wall you might hit if trying to take Xsan as far as you can.

Another limitation to consider is the number of files that can be in a volume, which is 4,294,967,296. This is a limit many aren’t likely to encounter. Unless of course every frame of a feature film (and all the footage that doesn’t make it into the feature film) is a separate file. At 60 frames per second that’s what, about 20,000 hours of footage…  While difficult to imagine it’s still worth noting…

There are other limits defined as well, but these are specific to the naming of certain components of an Xsan. According to the Admin Guide:

Volume name length : 70 characters (A–Z, a–z, 0–9, and _ )
File or folder name length : 251 ASCII characters
SAN name length : 255 Unicode characters
Storage pool name length : 255 ASCII characters
Affinity name length : 8 ASCII characters
LUN name (label or disk name) : 242 ASCII characters

March 6th, 2009

Posted In: Xsan

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