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From: The Natural Philosopher <tnp@invalid.invalid>
Newsgroups: comp.sys.raspberry-pi
Subject: Re: Move bookworm system from SSD to NVME
Date: Sat, 3 Aug 2024 11:07:20 +0100
Organization: A little, after lunch
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On 02/08/2024 23:08, Björn Lundin wrote:
> On 2024-08-02 16:12, The Natural Philosopher wrote:
>> On 02/08/2024 15:03, Björn Lundin wrote:
>>> On 2024-08-02 14:51, The Natural Philosopher wrote:
>>>
>>>> Never mind sync.
>>>>
>>>> It's important to wait anyway on an SSD/nvm until it has finished 
>>>> its internal business.
>>>>
>>>> For any copy process.
>>>
>>>
>>> dd'ing 500 Gb to an ssd disk, I've seen sync taking 30 s or more
>>> and sync is of course started AFTER dd is done
>>>
>>> So - how do you know it is done its internal business?
>>> Not all drives have blinking LEDs
>>>
>>> sync makes it easy to know
>>>
>>>
>> Even sync may not be enough.
>> SSDS/NVM have their own internal  caching.
>>
> 
> Then the question remains - how do you know when its done?
> 
> from man sync you get
> "sync - Synchronize cached writes to persistent storage"
> 
> In other words, doing sync at least clear the cache,
> which must be better than nothing? or?
> 
> Or are you saying that ripping the power to a newly sync()ed disk may 
> mess it up?
> 
Yes.

> as in
> 
> dd ....
> sync
> rip out usb cord for a 2.5" ssd sata in a usb case or something similar ?
> 
> 
I am on the edge of my comfort patch here.
If I were building an SSD I would have a diode and a large capacitor 
inside it to make sure all its caches were dumped to NVRAM before the 
voltage collapsed completely.

But on a big unit this could take a bit of time.

What happens between a SATA/USB plug and the actual NVRAM is a bit of a 
mystery.

We know its nothing like a 1:1 correlation between 'sector' and physical 
RAM location.
We knows that physical RAM locations are regularly shuffled for 'wear 
levelling'
When is all this done?
What happens if, during it, there is power failure?

I honestly do not know, hence the warning to leave the SSD for a few 
seconds before yanking any power cords

It can do no harm


-- 
"The most difficult subjects can be explained to the most slow witted 
man if he has not formed any idea of them already; but the simplest 
thing cannot be made clear to the most intelligent man if he is firmly 
persuaded that he knows already, without a shadow of doubt, what is laid 
before him."

    - Leo Tolstoy