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

From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH] LUKS: support preallocation in qemu-img
Date: Wed, 10 Jul 2019 23:52:24 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0

On 10.07.19 23:24, 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?
>> 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.
> (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?)

Kevin just pointed out to me that our LUKS format is compatible to the
actual layout cryptsetup uses.  OK, that is an important use case.

Hm.  Unfortunately, that doesn’t really necessitate preallocation.

Well, whatever.  If it’s simple enough, that shouldn’t stop us from
implementing preallocation anyway.

Now I found that qapi/block-core.json defines PreallocMode’s falloc and
full values as follows:

> # @falloc: like @full preallocation but allocate disk space by
> #          posix_fallocate() rather than writing zeros.
> # @full: preallocate all data by writing zeros to device to ensure disk
> #        space is really available. @full preallocation also sets up
> #        metadata correctly.

So it isn’t just me who expects these to pre-initialize the image to 0.
 Hm, although...  I suppose @falloc technically does not specify whether
the data reads as zeroes.  I kind of find it to be implied, but, well...


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