Re: LUKS2 on disk format

On 5/12/20 7:48 AM, Maksim Fomin wrote:
> ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
> On Monday, May 11, 2020 7:58 PM, Milan Broz <[email protected]> wrote:
> 
>> On 11/05/2020 21:07, Maksim Fomin wrote:
>>
>>> After reading LUKS2 on-disk format specification, I have one main
>>> question - does LUKS2 data resides entirely on header, which occupies
>>> first 16 mib (at most) of block device?
>>
>> Yes, all LUKS metadata are stored in th eLUKS heaer.
>> But the LUKS header size is configurable (it is not fixed to 16MB,
>> 16MB is just the default size)
>> (And most of the area is reserved for keyslots, used in online reencryption.)
> 
> Can default size (16MiB) be extended after setting up LUKS partition? What is recommended size? What are reasons to increase default size?

There were basically two reasons for the increase in default LUKS2 
metadata size IIRC:

1) As Milan pointed out online reencryption performs much better when 
there's enough consecutive free space in LUKS2 keyslots area (the binary 
area for keyslots material).

While testing we figured out with 16 MiB LUKS2 metadata we hit good 
balance of getting good enough performance for reencryption and not 
consuming too much free space on a drive so that it's not a big issue 
for general use case and for users not interested in reencryption.

To get smaller metadata size you can format the device with:
cryptsetup luksFormat --offset 8192 and data offset (and LUKS2 metadata 
size) will be at 4MiB exactly (similar to current LUKS1 default).

2) LUKS2 supports up to 32 keyslots. With 4 MiB  keyslots area there 
were not enough space to fill all keyslots.

In general I don't think 16MB is big issue nowadays and for special use 
cases you can create smaller LUKS2 metadata. Even smaller than LUKS1. 
Milan gave example here: 
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=932437#10

----

So I'd say 16 MiB is indeed recommended default LUKS2 metadata size in 
most cases.

-----

About extending existing LUKS2 metadata. Technically it's possible, but 
currently you have to use reencryption for it since we have to reduce 
data device size first before extending LUKS2 metadata.

cryptsetup reencrypt --reduce-device-size 4M /dev/sdx

(but do NOT forget to shrink residing filesystem/data first!)

By reducing data device by 4 MiBs we can extend LUKS2 metadata by same 
value. In the process the data are shifted backwards towards tail of the 
device.

In future we may add option to change LUKS2 metadata in-place w/o data 
shifting (reencryption) but that would be feature that requires 
cooperation with i.e. LVM2 or other volume management tools. If LVM2 
could extends device by adding extents in head of LV, it would be 
possible to simplify also online encryption etc.

Kind regards
O.

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