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Re: [Qemu-devel] [PATCH] LUKS: support preallocation in qemu-img

From: Daniel P . Berrangé
Subject: Re: [Qemu-devel] [PATCH] LUKS: support preallocation in qemu-img
Date: Thu, 11 Jul 2019 10:20:31 +0100
User-agent: Mutt/1.12.0 (2019-05-25)

On Wed, Jul 10, 2019 at 11:24:46PM +0200, Max Reitz wrote:
> On 10.07.19 19:03, Maxim Levitsky wrote:
> > preallocation=off and preallocation=metadata
> > both allocate luks header only, and preallocation=falloc/full
> > is passed to underlying file, with the given image size.
> > 
> > Note that the actual preallocated size is a bit smaller due
> > to luks header.
> Couldn’t you just preallocate it after creating the crypto header so
> qcrypto_block_get_payload_offset(crypto->block) + size is the actual
> file size?

Yeah that would be preferrable. If that's really not possible, we
could likely provide some API to query the expected hreader size for
a given set of creation options. 

> > Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1534951
> > 
> > Signed-off-by: Maxim Levitsky <address@hidden>
> > ---
> >  block/crypto.c | 28 ++++++++++++++++++++++++++--
> >  1 file changed, 26 insertions(+), 2 deletions(-)
> Hm.  I would expect a preallocated image to read 0.  But if you just
> pass this through to the protocol layer, it won’t read 0.

Yes, it will be zeros at the physical layer, but unintelligble
garbage from POV of the virtual disk.

I don't think this is really a problem though - this is what you
get already if you create a LUKS volume on top of a block device

AFAIK, we've not documented that preallocation guarantees future
reads will return zeros. Preallocation simply ensures that all
required space is allocated upfront. We do mention that it might
be achieved by writing zeros to the underlying storage but never
said you'll get zeros back.

IOW I think its at most a docs problem to more clearly explain
that preallocation != guaranteed zeros for reads.

> (In fact, I don’t even quite see the point of having LUKS as an own
> format still.  It was useful when qcow2 didn’t have LUKS support, but
> now it does, so...  I suppose everyone using the LUKS format should
> actually be using qcow2 with LUKS?)

Certainly not. LUKS on raw is going to be very common, not least because
that's directly compatible with what Linux kernel supports. If you don't
want the features of qcow2 like snapshots, it just adds overhead and mgmt
complexity for no gain, especially if dealing with block device backed
storage (iSCSI, RBD).

OpenStack will use cryptsetup when initializing its block storage with
LUKS, then tell QEMU to run with the raw + LUKS driver.

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