On Wed, Dec 11, 2024 at 09:51:01AM +0200, Anssi Saari wrote:
Michael Stone <mst...@debian.org> writes:
On Thu, Dec 05, 2024 at 10:55:48AM -0500, e...@gmx.us wrote:
How do I tell how many lanes a given drive uses (preferably before purchase)?
It would be buried in the technical docs. I've only seen 4x drives
(but I'm sure there may be some cheaper drives with fewer).
While we're on the topic PCIe lanes and SSDs, I've been looking into
some way of usiing old NVME SSDs when they get replaced by bigger
ones. I don't really want to have a stack of little m.2 USB boxes.
There are some PCIe adapter boards that take two or more SSDs but what
isn't clear to me is if those cards can work in the typically free x1
PCIe slot, if the cards are x4 or x8 and drives are x4?
As a general matter PCIe devices can/will downgrade, so e.g., if you
plug a x16 video card into a x1 slot it will just work, but at x1 speed.
But... The first gotcha is that many "dual m.2" boards have one sata and
one nvme slot, and are effectively single slot adapters if you're
dealing with nvme drives. To have more than one nvme means one of two
things: 1) a pcie switch chip 2) port bifurcation. Switch chips are
expensive, and would probably make this exercise cost more than an old
nvme drive is worth. Port bifurcation requires a certain number of
physical lanes, typical would be a x8 card with x4 going to each of two
m.2 slots: in that case, the second slot *will not* work if the adapter
is plugged into a x1 slot. Check the documentation carefully to
understand what each adapter does, and plan accordingly. To set
expectations, a single nvme/pcie adapter costs a few bucks, a nvme+sata
adapter a couple of bucks more, a dual nvme with a switch will cost over
a hundred, and a dual bifurcated card maybe fifty to a hundred. There
are cards that are way overpriced, but rarely are they underpriced--if
you see a really good deal on a dual m.2, it's probably nvme+sata. Be
warned: I've seen a lot of incompatibilities between various adapters &
motherboards in this space. (Beyond the obvious issues with whether a
motherboard supports bifurcation, the cards which have pcie switches are
using functionality that's in the spec but not used all that much and
not necessarily tested on a particular motherboard's implementation--
especially consumer motherboards which aren't expected to use anything
other than a video card.)
Yes. Also not many drives can sustain a multi-gigabyte write rate
anyway...
I have to say I was quite disappointed when I cloned a 1TB SSD to a 2TB
one, average speed wasn't much higher than writing to an HD. I don't
remember what the target drive was though. Since I don't intend to make
a habit of this, no big deal, but I wonder what kind of write speed one
could expect in a sustained write of 1TB?
Depends absolutely on the drive. Assuming something fairly recent, a
gigabyte or two per second is easy to obtain with a simple cp. (If
you're copying large files; small files will run much slower.) A
transfer to a hard drive will max out around 100-200MByte/s, and a sata
SSD around 500-600MByte/s--which shouldn't be hard to exceed with an
NVMe unless it's older/cheaper or being throttled by running in the
wrong slot.
