[linux-lvm] Sparse LVs, --virtualsize equal to --size

[Vangelis Koukis]

On Thu, Jan 24, 2013 at 01:35:56pm -0500, Stuart D Gathman wrote:
> Long ago, Nostradamus foresaw that on 01/24/2013 01:08 PM, Alasdair G
> Kergon would write:
> > If you've got the capacity to test, create one that's quite a bit
> > larger and see how much space it actually uses up when full, then use
> > that size in future.  There's a header, and then the blocks are written
> > sequentially with an index block inserted ahead of each group of
> > data blocks.
> >
> > Or try out thin LVs available in recent LVM releases/kernels for
> > an alternative 'sparse' approach.
> He doesn't actually want sparse, he wants "instant zero" via copy on
> write.  This is useful for virtual machines by preventing information
> leaks between customers.   Currently, you can create a "template" LV,
> and have a snapshot for each VM.  Since you (almost) never write to the
> template, you don't run into the performance issue.   He proposed a new
> type of LV that is a normal LV plus a small bitmap (1 bit per chunk)
> that tracks which chunks have been written to.  When a chunk is written
> to, the remainder of that chunk is set to zero.  Reads from unwritten
> chunks always return zeroes.
> 

Spot on!

"Guaranteed initialized to zero" is what I'm looking for.
So the LVM snapshot mechanism would work, but it may be a bit of an
overkill.

I'd rather not do it in a trial-and-error fashion, trying different
values of S just a bit larger than V, until it works. How large is the
overhead per chunk? Ideally, we could quantify the overhead as a
fraction of the chunk size, and use it as a safe margin for estimating
the needed physical size.

Thanks again,
Vangelis.

-- 
Vangelis Koukis
vkoukis at grnet.gr
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