Re: new drive is 4 sectors shorter, can it be used for swraid5 array?

[Andy Smith <andy@strugglers.net>]

Hi,

On Tue, Mar 29, 2022 at 09:55:40AM +0800, d tbsky wrote:
> In the early days different vendors made different capacity harddisks.
> But at some moment, maybe 250GB or 500GB, suddenly every vendor made
> the same capacity harddisks. It's a mystery to me. who decides the
> disk sector numbers?

There is a standard called IDEMA LBA1-03:

    http://www.idema.org/wp-content/downloads/2169.pdf

This says that a certain "marketing capacity" (i.e. when the drive
product description says "2TB" or whatever) will equal an exact
number of 512 or 4096 byte sectors.

It's been over a decade since I personally saw a SATA, SAS or NVMe
drive that did not obey this, so I've felt comfortable that I could
replace drives from one vendor with another without having to worry
about a few sectors different size here or there.

However, I did see people on this and other mailing lists such as
zfs-discuss saying they were still seeing drives that did not comply
with IDEMA LBA1-03 for capacity as recently as last year, so
apparently I did not look hard enough or have just been lucky.

If you want to play at home the formula in IDEMA LBA1-03 boils down
to:

    ($GB * 1000194048) + 10838016 bytes

using powers of ten definitions for "GB", so anything calling itself
a "2TB" drive should be exactly:

    (2000 * 1000194048) + 10838016

    = 2,000,398,934,016 bytes

Capacity presented to the OS might be lower of course if there is
hardware meddling as you mention.

Again, I've not personally seen anything recently that doesn't obey
this, but people have shown me things that don't, so it's
apparently still a thing.

Cheers,
Andy