[geeks] recipe for Linux / iSCSI / thin provisioned space

[John Francini]

>> Imagine a sparse file as the backing store for a filesystem, and
>> you'll have the idea pretty close.
>
> Oh!
>
> Why is this useful?  I can see only two ways for it to be useful: (1)
> shorter setup time, with the difference amortized over future accesses
> (the ones which are first writes to the various blocks), and (2) disk
> space overcommit.

(2) is the reason our customers want it -- oftentimes, sysadmins  
create iSCSI volumes with wild overestimates of how much space they'll  
actually need. Thin provisioning allows them to continue to  
overestimate.  In the (relatively) infrequent case where they actually  
fill a volume to the fully provisioned size, the backing disk space  
will be there.

Apparently this is done frequently enough that we had to add it to our  
iSCSI array firmware in order to 'keep up with the Joneses'.

John Francini
(NOT speaking for my employer, EqualLogic (a division of Dell)).

p.s. For fairly-easy home iSCSI initiator set up, you could use the  
OpenFiler Linux distribution, which puts a nice Web gui wrapper around  
iSCSI, NFS, and SMB disk/file serving.