Subj : Re: Move bookworm system from SSD to NVME
To   : All
From : The Natural Philosopher
Date : Sat Aug 03 2024 12:59:17

On 03/08/2024 12:52, Björn Lundin wrote:
> On 2024-08-03 12:07, The Natural Philosopher wrote:
>
>> 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
>
> And that is my point. sync will do no harm either,
> but it might save you, especially when you are dd()ing an image smaller
> that the RAM of the computer.
> Like I have 32 Gb RAM, and dd an image of 4 Gb (like a headless) onto a
> SD-card to run an old pi. The interface is slow, yet dd reports done
> within a minute. sync takes a long time to flush it over.
>
> Just waiting a few secs will end in disappointment
>
Well I will be at some stage dd-ing about 60GByte across to a 2TB disk,
but given the hassle, sync and a cup of coffee wait before unplugging
it, is likely.

RK claims the wait afterwards isn't necessary.  Completing 'sync' is
enough. He generally doesn't comment unless he knows.

I simply do not know how a command to something purporting to resemble a
hard drive, that isnt can guarantee all data in it is flushed.


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