[linux-lvm] Sparse LVs, --virtualsize equal to --size
vkoukis at grnet.gr
Fri Jan 25 08:39:38 UTC 2013
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.
"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
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.
vkoukis at grnet.gr
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