On Tue, 22 May 2018 21:16:44 +0100, Paul wrote:

Jimmy Wilkinson Knife wrote:

At what age does the data become unreadable if the drive has not been
powered up?

Flash is generally quoted as holding charge on a floating
gate for around 10 years.

That means, if you're using SSDs for archival storage, they
should be plugged in and re-written every five years, at a guess.

For a given design, I don't know how to guess at that value,
and the 10 year number is merely a "starting point, ball park number".

If I had an SSD today, that was as old as the oldest hard drive
in the room, chances are it would throw a CRC error or two, signifying
the error correction couldn't fix the number of errors accumulated
in a sector.

In my experience, hard drives start making nasty noises and or getting bad sectors well before SSDs show any signs of problems. Back in the days of not much RAM, pagefiles wore the poor things out really quickly.

It's a mirrored array, and the drives have different SMART data, even
though they're identical and were installed together, so one should fail
well before the other and prevent a problem.

Well, I don't want to propose something stupid to you,
and cause the mirror to break as a side effect. You have
to be careful that any soft-raid methods don't "track"
what you do to them, and then the next time you boot
into the "working" configuration, the status is
degraded and it costs you another rebuild. If you move
one of those drives somewhere so you can read the SMART,
you might upset the array status.

I'd mess about if it was a machine with not much on it. I've got 4 PCs doing nothing but science research. An OS and one program to reinstall, no data. But the SSDs in question are my main PC with everything on it. When I built it 5 years ago to replace an older one, it took me weeks to get everything installed and setup the way I wanted it.

If you don't have SMART visibility, and you insist
on running a RAID 1 mirror, I would recommend
to you that you mix drives from different manufacturers.
Pair an Intel branded 512GB drive with a Samsung branded
512GB drive. That should de-correlate things enough, that
there won't be any unfortunate accidents. I personally
would not pair two identical Intel drives in a RAID 1
mirror, if you paid me :-) I'm be a "lucky enough guy"
to have Windows Defender and Search Indexer keep writing
to C: just after the first drive fails, until the second
drive fails and I'm toasted. That's what would happen
to me if I tried that.

I tend to use identical drives in mirrors as I thought otherwise you just degrade it to the speed of the slowest one. I also only use Crucial SSDs for reliability.

Anyway, the SMART I get from Speedfan indicates one of my SSDs will fail a long time before the other. I doubt they're identical enough to fail within a short time period. If I was that worried, maybe I'd use RAID 6.

With mirrored drives *you still need backups*.

Yip, I have what used to be USB3 external drives I swapped around for full backups. But I've now put one into each of the science computers so I can backup over 10Gbit network (into another building, so there's fire protection too). The drives are fast enough, USB3 is fast enough, but the ****ty interface in the external caddies was causing ridiculous bottlenecks and backups took days.

If the 5V rail on your PSU overvolts, and burns both
SSDs at the same time, "you got nuttin".

I use quality PSUs, I've never known even a ****ty one to overvolt (undervolt, yes). Even the cheap ones tend to commit suicide without killing anything else. I've had a couple of cheap ones go bang in the past when I "overloaded" them, without frying anything else. "Overloaded" means daring to draw 500W from an 800W supply. If it's not a big brand name, don't expect more than 50% of the rated watts before an explosion. The decent Corsair ones are wonderful, they don't even power up their own fan until you draw a lot of current - virtually no heat from them at all.

We do backups
to protect against lightning and PSU failures and ransomware.
The mirror idea, isn't "the Space Shuttle". It's not sufficient
redundancy for disaster planning. It's *not* a substitute
for backups.

I know, I never said it was. But it's damn convenient. No reinstalling, no downtime, just shove in another drive with the PC still turned on. I've never had to restore much from backups when using a mirror or any other redundant array. Just files I accidentally deleted. And decent RAID controllers let you specify one or more "spare drives", so as soon as one in any of your mirrors fails, even if you're not at the machine, it will immediately copy the data onto that drive so you still have redundancy. I did once have a motherboard fail, but I just bought another board with a similar RAID controller on it and it understood the array.

--
A worried father confronted his daughter one night.
"I don't like that new boyfriend, he's rough and common and bloody stupid with it."
"Oh no, Daddy," the daughter replied, "Fred's ever so clever, we've only been going out nine weeks and he's cured me of that illness I used to get once a month."