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 <mlevi...@redhat.com> > > --- > > 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 today. 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. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
