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

[der Mouse]

>>> I have successfully provisioned a small iSCSI target that is
>>> "thick" (takes up 1GB disk space when provisioned as 1GB of space),
>>> but obviously this is not terribly useful.
>> Obviously?  I must be missing something; that's exactly how I'd
>> expect it to work.  How would it occupy any less space?
> Thin-provisioning means that the full gamut of blocks in the device
> are mapped-out ahead of time but only allocated on-demand through
> indirection.

> 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.

The reason I'm curious is that I wouldn't consider lacking either of
those basis for "not terribly useful" - (1) is annoying, but a one-time
time cost at setup, and (2) depends on the iSCSI initiator(s) to prefer
to write already-written blocks when available, which, while possible,
certainly isn't how any disk uses I know of work (or to just do less
total writing than the size of the target, which is an even less common
access pattern).

So there's clearly something I'm still missing.

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