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Re: lvm module can't see USB disks' volume group

From: Alexandre Oliva
Subject: Re: lvm module can't see USB disks' volume group
Date: Wed, 08 Feb 2017 19:51:45 -0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux)

On Feb  1, 2017, Alexandre Oliva <address@hidden> wrote:

> On Feb  1, 2017, Andrei Borzenkov <address@hidden> wrote:
>> 31.01.2017 22:27, Alexandre Oliva пишет:
>>> Note: I haven't yet determined that it's the USBness of the disks that's
>>> a factor; it might be disk size, though I find that unlikely.  So far,
>>> I've only converted some sub-TB disks to LVM.  The new USB disks that I
>>> set up with LVM are 5TB with 512b logical sectors; I've got another
>>> slightly older 5TB non-USB disk that I intend to convert over the next
>>> few days, and other smaller but still few-TB-sized disks, some plain
>>> SATA, some in USB3 enclosures that make the disks seem to support only
>>> 4KiB sectors, and so I may shortly know whether GRUB has issues with big
>>> PVs or USB disks. 

>> GRUB itself should not, but BIOS support for 4K disks is pretty flaky.

> I think I may have had trouble with the 4K enclosures before, but I
> haven't got to them yet.  The problems I reported in my earlier email
> involved newer enclosures that exported 512b logical sectors.

I've finally got to the 4K enclosures, and to a 5TB disk outside an
enclosure.  Symptoms were different.

5TB disk, no enclosure, 512B logical sectors: GRUB2 can see and grab
data from the PV encompassing (almost) the entire disk (all but the
first MB)

5TB disk, USB enclosure that exposes disk as 512B logical sectors: GRUB2
can see the (GPT) partition table, but it won't expose the PV contents,
nor can it access files in a separate 256MB btrfs partition created for
testing at the very end of the disk.  It can, however, read the GRUB
core embedded in the boot partition in the beginning of the disk.

4TB disk, USB enclosure that exposes disk as 4KiB logical sectors: GRUB2
can't even see the (GPT) partition table.

I can't figure out why GRUB would be able to read the embedded core
image, but not the LVM metadata, that's all in the 2nd MiB of the disk,
the beginning of the PV.  Sure enough, reading through to the end of the
disk might run into some LBB limit or BIOS bug, but accessing the
initial sectors should be fine, and GRUB2 *can* access PVs larger than
4TB, even all the way to the end, without an enclosure.

Now, does it make any sense to test things any further, e.g., try to
create a regular filesystem outside the PV, closer to the beginning of
the 5TB disk in the 512B enclosure?

Any other tests you can think of to try to make disks in (512B)
enclosures to work just like those outside enclosures, as far as GRUB2
is concerned?


Alexandre Oliva, freedom fighter
You must be the change you wish to see in the world. -- Gandhi
Be Free! --   FSF Latin America board member
Free Software Evangelist|Red Hat Brasil GNU Toolchain Engineer

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