Re: kernel: device-mapper: table: 254:1: adding target device sda1 caused an alignment inconsistency

[Rick Thomas <rbthomas@pobox.com>]

On Jul 28, 2018, at 1:28 PM, Henrique de Moraes Holschuh <hmh@debian.org> wrote:

> On Sat, 28 Jul 2018, David Wright wrote:
>> On Sat 28 Jul 2018 at 10:57:45 (-0300), Henrique de Moraes Holschuh wrote:
>>> On Sat, 28 Jul 2018, Rick Thomas wrote:
>>>>> rbthomas@small:~$ lsblk -t
>>>>> NAME           ALIGNMENT MIN-IO   OPT-IO PHY-SEC LOG-SEC ROTA SCHED       RQ-SIZE  RA WSAME
>>>>> sda                    0   4096 33553920    4096     512    1 mq-deadline      60 128    0B
>>>>> `-sda1                 0   4096 33553920    4096     512    1 mq-deadline      60 128    0B
>>>>>  |-small-swap        -1   4096        0    4096     512    1                 128 128   32M
>>>>>  |-small-root        -1   4096        0    4096     512    1                 128 128   32M
>>>>>  `-small-home        -1   4096        0    4096     512    1                 128 128   32M
>>>>> mmcblk2                0    512        0     512     512    0 mq-deadline     128 128    0B
>>>>> |-mmcblk2p1            0    512        0     512     512    0 mq-deadline     128 128    0B
>>>>> `-mmcblk2p2            0    512        0     512     512    0 mq-deadline     128 128    0B
>>>>> rbthomas@small:~$ 
>>>> 
>>>> Note the alignment values of “-1” for the lvm entries but not for the GPT partition or the whole disk.
>>>> Why do you suppose that is?
>>> 
>>> Keep in mind that you *offset*-align the outer container *only*, and then inside
>>> you just keep the size alignment.
>>> 
>>> So, the above ensures correct use of the partitions even if sda1 is
>>> unaligned.
>>> 
>>> If you offset-align sda1 to -1, everything inside it should have an offset of
>>> zero to keep the alignment correct.
>> 
>> I don't think I fully understand the explanation. Can you point out
>> the number(s) that's wrong, and how it should be corrected.
> 
> There is nothing wrong on the table above as far as I can tell,
> *assuming* the device does need the -1 alignment.  Since sda1 isn't
> aligned, everything inside it at the first level must be (and is)
> aligned at -1 to compensate.
> 
> Where sda1 aligned at -1, nothing inside it should be, as sda1 would
> already provide the required alignemnt to anything inside it.


Thanks for the clarification, Henrique!

Let me see if I have this right…

1) If I had heeded the warning in “man pvcreate” and set up the physical volume with
    pvcreate —dataalignmentoffset 7s /dev/sda
to accommodate the -1 offset (provided either by the drive itself or the USB enclosure it is in — we don’t know) then all the “-1”s in the above table would be “0”s and I would not be getting the error messages at boot time.  Is this correct?

2) But as it is, I did not do that, so when I used vgcreate to make the volume group called “small”, it realized that the partition was out of alignment and compensated — thus causing the alignment of “-1” for all of the logical volumes.  And, most importantly, therefor the logical volumes are, in fact, properly aligned and I can stop worrying.  Furthermore, the warnings at boot time are just noting that an alignment of “-1” has been applied by the device-mapper and all is well.  Is this correct?

3) Or are you saying that the messages are _not_ just warnings:  I should go back and re-format the disk to use the “—dataalignmentoffset 7s” option.  (I can do this if I do it soon. I don’t have much data on the disk yet.)

Thanks,
Rick