[dm-crypt] Alignment issue with 4K disk

Sven Eschenberg sven at whgl.uni-frankfurt.de
Tue Jan 5 00:02:43 CET 2016


Hi Alexis,

I stumbled over this line:

I/O size (minimum/optimal): 4096 bytes / 33553920 bytes

The latter resembles 65535 sectors (512 Byte logical ones), which 
explains why the automated alignment was done at sector 65535. This 
value seems extremely weird though.

You said the drive was external, connected how? USB maybe?

Can you take a look at hdparm -I /dev/sdc? Anything unusual there?

Regards

-Sven


Am 04.01.2016 um 23:20 schrieb Yves-Alexis Perez:
> On lun., 2016-01-04 at 22:54 +0100, Sven Eschenberg wrote:
>> Hi,
>>
>> start=4194304 -> assumind this is in 512 byte blocks, the start sector
>> is aligned to MByte boundary and thus 4k boundary.
>>
>> Anyway, if your device only has one partition which is encrypted with
>> dm-crypt, set it up to start at sector 2048 (which should be default for
>> parted, fdisk and so on). Pass align-payload 2048 during creation of the
>> LUKS header, which will give you an MByte boundary and should always be
>> okay.
>>
>> If the kernel still complains, then something else might be going wrong.
>
> Right now I have:
>
> root at scapa:~# lsblk -o +ALIGNMENT /dev/sdc
> NAME             MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT        ALIGNMENT
> sdc                8:32   0 931.5G  0 disk                            0
> └─sdc1             8:33   0 931.5G  0 part                          512
>    └─scapa-backup 253:4    0 931.4G  0 crypt /mnt/scapa-backup        -1
>
> And:
>
> scapa-backup: 0 1953336209 crypt aes-xts-plain64
> 0000000000000000000000000000000000000000000000000000000000000000 0 8:33 65536
>
> root at scapa:~# fdisk -l /dev/sdc
> Disk /dev/sdc: 931.5 GiB, 1000204886016 bytes, 1953525168 sectors
> Units: sectors of 1 * 512 = 512 bytes
> Sector size (logical/physical): 512 bytes / 4096 bytes
> I/O size (minimum/optimal): 4096 bytes / 33553920 bytes
> Disklabel type: gpt
> Disk identifier: EBA98857-89BA-4B2C-8366-4A77A527A8E0
>
> Device     Start        End    Sectors   Size Type
> /dev/sdc1  65535 1953467279 1953401745 931.5G Linux filesystem
>
> Partition 1 does not start on physical sector boundary.
>
> [I'm a bit concerned with that, but since I'm using a GPT and not MBR it might
> be related]
>
> root at scapa:~# gdisk -l /dev/sdc
> GPT fdisk (gdisk) version 1.0.1
>
> Partition table scan:
>    MBR: protective
>    BSD: not present
>    APM: not present
>    GPT: present
>
> Found valid GPT with protective MBR; using GPT.
> Disk /dev/sdc: 1953525168 sectors, 931.5 GiB
> Logical sector size: 512 bytes
> Disk identifier (GUID): EBA98857-89BA-4B2C-8366-4A77A527A8E0
> Partition table holds up to 128 entries
> First usable sector is 34, last usable sector is 1953525134
> Partitions will be aligned on 8-sector boundaries
> Total free space is 123356 sectors (60.2 MiB)
>
> Number  Start (sector)    End (sector)  Size       Code  Name
>     1           65535      1953467279   931.5 GiB   8300  scapa-backup
>
> Regards,
>
>
>
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