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@grnet.gr
OpenPGP public key ID:
pub 1024D/1D038E97 2003-07-13 Vangelis Koukis <vkoukis@cslab.ece.ntua.gr>
Key fingerprint = C5CD E02E 2C78 7C10 8A00 53D8 FBFC 3799 1D03 8E97
Only those who will risk going too far
can possibly find out how far one can go.
-- T.S. Eliot
Attachment:
signature.asc
Description: Digital signature
_______________________________________________ linux-lvm mailing list linux-lvm@redhat.com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
