fdisk reports partitions are misaligned even when they don't appear to be

Question

I have a new 4 TB Western Digital SN850X NVMe SSD drive that I am trying to partition as a boot drive for Ubuntu 20.04. No matter what I do, fdisk reports that the partitions are misaligned, even though they appear to be aligned to both the logical (512 byte) and physical (8388608 byte) sector sizes:

$ sudo fdisk -l /dev/sda
Disk /dev/sda: 3.65 TiB, 4000787030016 bytes, 7814037168 sectors
Disk model:                 
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 8388608 bytes
I/O size (minimum/optimal): 8388608 bytes / 8388608 bytes
Alignment offset: 6832128 bytes
Disklabel type: gpt

Device      Start        End    Sectors  Size Type
/dev/sda1   16384     671743     655360  320M EFI System
/dev/sda2  671744 7814035455 7813363712  3.7T Linux filesystem

Partition 1 does not start on physical sector boundary.
Partition 2 does not start on physical sector boundary.

Note that partition 1 starts at sector 16384, exactly 8 MiB (16384 * 512) into the disk, which matches the physical sector size, but fdisk claims that it's misaligned by the strange value of 6832128 bytes.

parted align-check reports the same:

(parted) align-check
alignment type(min/opt)  [optimal]/minimal? minimal                       
Partition number? 1                                                       
1 not aligned: 16384s % 16384s != 13344s
(parted) align-check                                                      
alignment type(min/opt)  [optimal]/minimal? minimal                       
Partition number? 2                                                       
2 not aligned: 671744s % 16384s != 13344s

Here parted appears to be saying that the partitions are misaligned by 13344 sectors, which is 6832128 bytes, the same value reported by fdisk. But clearly "16384s % 16384s" is aligned.

Where is this bizarre misalignment value of 6832128 bytes / 13344 sectors coming from? Any suggestions / thoughts much appreciated!

Answer 1

Note the text:

Alignment offset: 6832128 bytes

in the fdisk -l output.

This SuSE support article about partition alignment has this nugget of information on alignment offset:

There's one special case: When internal 4k block sizes were introduced, some HDD manufacturers actually addressed the classical DOS partition table misalignment by shifting the logical sector counting by one, so a start at sector 63 would translate to sector 64 (i.e. internal block 8). Some HDDs even were configurable with a switch to do this shifting by one. The SATA spec did even provide a mechanism for the drives to report such an offset, so the OS can take the appropriate steps to optimize performance. To our knowledge not many such drives exist; and only a subset of them reports the offset correctly.

If the drive reports any alignment offset, the Linux kernel in SLE11-SP1 (or later) will report this via the attribute /sys/block/$DEV/alignment_offset (in sectors).

So your disk seems to be a very special case: not only it seems to use 8 MiB physical blocks, the first block also has a non-uniform size of 6832128 bytes for some reason... assuming the reported information is correct.

Since Western Digital SN850X is a NVMe drive (as you correctly stated), the fact that you are accessing it as /dev/sda implies there is a converter device of some kind in between the PCIe bus and the drive: a directly-connected non-enterprise NVMe device would be presented as /dev/nvmeNn1 and its partitions as /dev/nvmeNn1pP instead of /dev/sd* (where N = the NVMe device number and P = partition number).

(In NVMe device names, the n1 is a namespace number: most non-enterprise NVMe devices support only a single namespace, but the device naming scheme is designed to support more than one. Think of namespaces as meta-partitions with potential for some enterprise storage features like snapshotting.)