How to 4k align hard drive with pre-existing partitions - Linux, Windows partitions

Question

I have a 1TB WD hard drive with Windows 10 and Devuan Linux Beowulf. I had partitioned with gparted IIRC prior to installation of both OS's. How to veerify if it is 4k aligned? If it's not so, how to adjust partitions for 4k alignment?

:~# fdisk -l -u /dev/sdb
Disk /dev/sdb: 931.53 GiB, 1000204886016 bytes, 1953525168 sectors
Disk model: WDC WD10EZEX-00B
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: ~~
Device          Start        End   Sectors   Size Type
/dev/sdb1   233455616  799057919 565602304 269.7G Linux filesystem
/dev/sdb2   799057920 1132386303 333328384   159G Linux filesystem
/dev/sdb3  1132386304 1214937087  82550784  39.4G Linux filesystem
/dev/sdb4  1214937088 1302333439  87396352  41.7G Linux filesystem
/dev/sdb5  1302333440 1631836159 329502720 157.1G Linux filesystem
/dev/sdb6  1631836160 1734492159 102656000    49G Microsoft basic data
/dev/sdb7  1734492160 1925926911 191434752  91.3G Microsoft basic data
/dev/sdb8  1925926912 1925959679     32768    16M Microsoft reserved
:~# sfdisk -d /dev/sdb
label: gpt
label-id: ~~
device: /dev/sdb
unit: sectors
first-lba: 34
last-lba: 1953525134
/dev/sdb1 : start=   233455616, size=   565602304, type=0FC63DAF-8483-4772-8E79-3D69D8477DE4, uuid=5DD63D2A-3ABF-4BB3-AE6F-A1815BC17064, name="Linux filesystem"
/dev/sdb2 : start=   799057920, size=   333328384, type=0FC63DAF-8483-4772-8E79-3D69D8477DE4, uuid=2DED301A-37F7-4A96-BCF4-01E4DE66E5E2, name="Linux filesystem"
/dev/sdb3 : start=  1132386304, size=    82550784, type=0FC63DAF-8483-4772-8E79-3D69D8477DE4, uuid=C136ED50-3AD2-4A8A-B31E-562479C46047, name="Devuan home"
/dev/sdb4 : start=  1214937088, size=    87396352, type=0FC63DAF-8483-4772-8E79-3D69D8477DE4, uuid=00F33DAC-24A9-4715-A561-CE5DC8100552, name="Devuan"
/dev/sdb5 : start=  1302333440, size=   329502720, type=0FC63DAF-8483-4772-8E79-3D69D8477DE4, uuid=0026F9E4-12E8-4C56-8E06-F18842201377, name="Linux filesystem"
/dev/sdb6 : start=  1631836160, size=   102656000, type=EBD0A0A2-B9E5-4433-87C0-68B6B72699C7, uuid=EBD4DD74-CBC0-41BC-91C0-E5C3DE1FDE07, name="Basic data partition"
/dev/sdb7 : start=  1734492160, size=   191434752, type=EBD0A0A2-B9E5-4433-87C0-68B6B72699C7, uuid=D5BDB9D1-986A-4956-B65E-8662F2471F18
/dev/sdb8 : start=  1925926912, size=       32768, type=E3C9E316-0B5C-4DB8-817D-F92DF00215AE, uuid=DF6D1F70-A8DB-4463-AE92-D355AC31981F, name="Microsoft reserved partition"

I intent to install FreeBSD 12.1 on to this drive, and it is a little tricky to partition for FreeBSD through bsdinstall (cui/tui). FreeBSD tool recommends 4k aligned boundaries. (gpart -a 4k ...)

Kamil Maciorowski · Answer 1 · 2020-08-06T06:08:10.000

3

How to verify if it is 4k aligned?

Let's take /dev/sdb1 as an example. It starts at the logical sector 233455616. Your logical sectors are of the size 512 bytes. Sectors are numbered from 0, so there are exactly 233455616 logical sectors before the partition, each taking 512 bytes.

To verify the 4k-alignment you need to check if this offset can be expressed as an integer number of sectors of the size 4096 bytes. These sectors are 8 times bigger than 512-byte sectors, so it's enough to check if 233455616 is divisible by 8. It is.

Do this for every offset, i.e. every starting sector (233455616, 799057920, ...). My calculations indicate all the partitions are aligned.

FreeBSD tool recommends 4k aligned boundaries.

Your disk uses physical sectors of the size 4096 bytes. This means the 4k-alignment is advised because of the properties of the disk itself, not because of some tool.

^{Side note: there is about 111 GiB of unpartitioned space before /dev/sdb1.}

edited Aug 06 '20 at 06:08

answered Aug 06 '20 at 05:58

Kamil Maciorowski

74,068

Yes. Divided by 8 for all starting sectors gives whole numbers. But, the fdisk output showing logical sectors of 512bytes.. Can we re-align to 4k on this drive? A way to get sector size aligned to 4k. – Number79 Aug 06 '20 at 06:56
@Number79 You mean: make the logical sector size be 4096? Why? This is how it works. Some USB enclosures may interfere and in some cases you can choose (but I suspect the choice affects the enclosure, not the disk itself). Note your current partition table is valid only for the current logical sector size. If you manage to change the size, the partition table will need to be recomputed. – Kamil Maciorowski Aug 06 '20 at 07:07

score 2 · Answer 2 · answered Aug 06 '20 at 05:49

The logical sector size is 512 bytes, so in order to be 4k-aligned (to match your physical sectors), the partition's starting sector must be an exact multiple of 4096⁄512 = 8.

So just go through all numbers in the "Start" column and divide each by 8. If you get a whole number, that partition is aligned; if you get decimals, it isn't. For example, 233455616 divided by 8 is 29181952.0, so the first partition is correctly aligned.

Many Linux-based partitioning tools default to 1MiB alignment, which is also good – it automatically means the partition will be aligned to 4k (and 8k, and 16k…). So if you're using Linux fdisk or GParted, just stick to the defaults.

How to 4k align hard drive with pre-existing partitions - Linux, Windows partitions

2 Answers2