On 3/2/2014 11:02 PM, Jan Kundrát wrote:

Hi,
I'm having troubles using WD40EZRX [1] in a USB3 -> SATA dock. The
trouble I'm hitting is that the data I get back when reading the first
partition differ depending on whether I access the drive via the
enclosure, or directly as a SATA disk.

Here is how it looks directly attached:

plejtvak ~ # sg_readcap -l /dev/sdc
Read Capacity results:
  Protection: prot_en=0, p_type=0, p_i_exponent=0
  Logical block provisioning: lbpme=0, lbprz=0
  Last logical block address=7814037167 (0x1d1c0beaf), Number of logical
blocks=7814037168
  Logical block length=512 bytes
  Logical blocks per physical block exponent=3
  Lowest aligned logical block address=0
Hence:
  Device size: 4000787030016 bytes, 3815447.8 MiB, 4000.79 GB

And this is how it looks through the USB convertor:

svist ~ # sg_readcap -l /dev/sdb
Read Capacity results:
  Protection: prot_en=0, p_type=0, p_i_exponent=0
  Logical block provisioning: lbpme=0, lbprz=0
  Last logical block address=976754645 (0x3a3817d5), Number of logical
blocks=976754646
  Logical block length=4096 bytes
  Logical blocks per physical block exponent=0
  Lowest aligned logical block address=0
Hence:
  Device size: 4000787030016 bytes, 3815447.8 MiB, 4000.79 GB

My partition table is probably borked -- IIRC I created it via `cfdisk`
over the USB connection, but I am not sure about that anymore. This is
what gdisk reports when directly attached:

Command (? for help): p
Disk /dev/sdc: 7814037168 sectors, 3.6 TiB
Logical sector size: 512 bytes
Disk identifier (GUID): AD1C809E-45FC-4CDD-A9EC-2855D4AF1E6E
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 7814037134
Partitions will be aligned on 8-sector boundaries
Total free space is 4294972395 sectors (2.0 TiB)

Number  Start (sector)    End (sector)  Size       Code  Name
  1              63      3519064768   1.6 TiB     0700  Linux/Windows data

...and through the USB dock:

Command (? for help): p
Disk /dev/sdb: 976754646 sectors, 3.6 TiB
Logical sector size: 4096 bytes
Disk identifier (GUID): DFE2C96D-8A4C-4516-86B2-4F6DA0EB8609
Partition table holds up to 128 entries
First usable sector is 6, last usable sector is 976754640
Partitions will be aligned on 8-sector boundaries
Total free space is 57 sectors (228.0 KiB)

Number  Start (sector)    End (sector)  Size       Code  Name
  1              63      3519064768   13.1 TiB    8300  Linux filesystem

I have a filesystem on the first partition. Accessing it works fine via
the USB reduction, but mount refuses to proceed when accessing the SATA
disk directly. I assume that this is because the partition starts at
byte (63*4096) from the beginning of the disk, which matches the
detected partition start when using the USB thingy. However, when
accessing the disk directly over SATA, the partition is presumably
expected to begin at (63*512), which is a very different location, and
stuff therefore breaks.

I have already read the discussion from January [3] and believe that
this is a different situation -- I haven't disassembled anything, this
is a standalone USB->SATA convertor and a standalone SATA disk.

I would appreciate a couple of pointers here:

- Did I screw up somehow? If so, how? 

Yes.

- Can I salvage the data by rewriting the partition table in some magic
way which would work with both "physical 4k, logic 4k" and "physical
512, logic 4k" modes at the same time?

Presumably you have some md raid level with fault tolerance, since you
posted this to the linux-raid list.  Why not simply kick the drive from
the array, install it "direct" inside the chassis, repartition it the
same as the other members of the array that are direct connected to the
SATA controller, re-add the partition to the array, and rebuild.

- Is the USB enclosure which I'm using just a crappy piece of hardware
that I should stay away from?

All USB drive enclosures are crap.  Why you prepped this drive in it
with intention of moving it into the PC chassis leaves me scratching my
head.  Is this some habit you developed years ago?  Prepping a new drive
in a USB before permanently installing it in the chassis?  If so, I'd
recommend using a hot swap SATA chassis that installs in a 5.25" drive
bay instead.  They're $15-30 USD, cheaper, and more reliable, than a USB
dock, by far.

Never use USB connected disk drives for RAID.  This is but one of many
problems you'll run into.

- What is the cleanest way of accessing the drive in the safest possible
manner? I would like to put my backups in there, and I wasn't really
happy when I plugged the drive directly in order to test the contents
and found out that it won't even mount.

See above.

- I guess I could just give up on partitioning altogether and use
filesystem on the whole device without much trouble. However, I'm afraid
that this might confuse the kernel's partition layout detection, and it
also seems a bit ugly.

The problem isn't use of partitions, but use a USB.

Help is much appreciated.

Cheers.

-- 
Stan

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