[dm-crypt] crypetsetup and GPT partitions

David Christensen dpchrist at holgerdanske.com
Fri Feb 10 19:23:28 CET 2017


On 02/10/17 08:22, Sven Eschenberg wrote:
> Am 10.02.2017 um 17:05 schrieb David Christensen:
>> On 02/10/17 01:07, Michael Kjörling wrote:
>>> On 10 Feb 2017 00:15 -0800, from dpchrist at holgerdanske.com (David
>>> Christensen):
>>>> The available space of the LUKS mapped device is going to be smaller
>>>> than the partition size.  On one of my 3 TB drives, it's about ~44
>>>> GB smaller (~1.6%).  The LUKS meta-data is going to be in there,
>>>> including the header.
>>>
>>> That doesn't make sense. The LUKS header is a shade over 1 MiB,
>>> depending on the specific options (the FAQ has details). The size of
>>> the header isn't related to the size of the container. Something else
>>> is going on in your case.
>>
>> Here's the data:
>>
>> 2017-02-10 07:38:40 root at cd2533 ~
>> # parted /dev/sda u s p
>> Model: ATA ST3000DM001-1ER1 (scsi)
>> Disk /dev/sda: 5860533168s
>> Sector size (logical/physical): 512B/4096B
>> Partition Table: gpt
>>
>> Number  Start  End          Size         File system  Name     Flags
>>  1      2048s  5860532223s  5860530176s               primary
>>
>> 2017-02-10 07:40:57 root at cd2533 ~
>> # df | egrep 'File|mnt'
>> Filesystem                1K-blocks      Used  Available Use% Mounted on
>> /dev/mapper/i3000d_crypt 2884281560 848596104 1889172304  31% /mnt/i3000d
>>
>>
>> Here's the math:
>>
>>    5860530176 s / (2 s/kB) - 2884281560 kB
>>  = 2930265088 kB           - 2884281560 kB
>>  = 45983528 kB
>> ~= 44905.8 MB
> That's the size of the filesystem afterall.

D'oh!


> Lookt at blockdev --report to see the blockdev sizes (i.e. physical
> disk, partition, crypt device).


RTFM blockdev(8):

2017-02-10 10:01:08 root at cd2533 ~
# blockdev --report /dev/sda /dev/sda1 /dev/mapper/i3000d_crypt
RO    RA   SSZ   BSZ   StartSec            Size   Device
rw   256   512  4096          0   3000592982016   /dev/sda
rw   256   512  4096       2048   3000591450112   /dev/sda1
rw   256   512  4096          0   3000589352960   /dev/mapper/i3000d_crypt


So, it looks like LUKS consumes:

3000591450112 - 3000589352960 = 2097152 = 2 MB


Thanks for the correction.  :-)


> Check against dmsetup

RTFM dmsetup(8):

2017-02-10 10:14:06 root at cd2533 ~
# dmsetup info /dev/mapper/i3000d_crypt
Name:              i3000d_crypt
State:             ACTIVE
Read Ahead:        256
Tables present:    LIVE
Open count:        1
Event number:      1
Major, minor:      254, 2
Number of targets: 1
UUID: CRYPT-LUKS1-<redacted>-i3000d_crypt_unformatted


> --table

RTFM dmsetup(8), the --table option requires a <table> value.  WAG:

2017-02-10 10:14:17 root at cd2533 ~
# dmsetup info --table LIVE /dev/mapper/i3000d_crypt
Name:              i3000d_crypt
State:             ACTIVE
Read Ahead:        256
Tables present:    LIVE
Open count:        1
Event number:      1
Major, minor:      254, 2
Number of targets: 1
UUID: CRYPT-LUKS1-<redacted>-i3000d_crypt_unformatted


I'm not sure what I am checking...


David



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