[Info-vax] Do any disks still lie about writing data to permanent storage ?

[Jan-Erik Söderholm]

Den 2021-07-06 kl. 20:11, skrev Simon Clubley:
> On 2021-07-06, Jan-Erik Söderholm <jan-erik.soderholm at telia.com> wrote:
>> Den 2021-07-06 kl. 15:04, skrev Simon Clubley:
>>> Talking about disks has reminded me of something else.
>>>
>>> In the not too distant past, some disks lied about having written data
>>> to permanent storage instead of merely to some internal cache that
>>> would be lost on a power failure.
>>>
>>
>> They did not lie, of course. They said that the data was in the cache,
>> which was perfectly true.
>>
> 
> We are not talking about SAN systems, we were talking about disk drives
> such as Sata/SCSI/IDE disks. It's clear you have no experience working
> with disk drives at the hardware level or you would have recognised the
> issue I am raising.

BS! I have worked with PDP-11/VAX/Alpha system from the 80s and forward
incl MV-II, MicroVAX and up from that. And now including large IBM SAN
systems. Using all sorts of different physical disk drives.

> 
> Those disks have APIs built into them that tells the operating system
> when the data in question has been committed to permanent storage by
> the disk itself.
> 
> Unfortunately, in the past some disks have lied about this and have
> told the operating system that the data has been committed to permanent
> storage when in fact it is still in a cache on the disk.

That was not a "lie", it did so by design. And in many cases (or at least
some) you could configure the drive to not do so, if you wanted.

> 
> That means if one of these disks suffers a power failure before the data
> _is_ written to permanent storage, then that data is lost. This is after
> the operating system has been told the data is safe and has potentially
> done other things based on that. :-(

*IF* this has been a real issue, well, it had been known as a real issue.
And if you concidered this to be a real issue, you selected your disk
hardware accordingly.

> 
>>> Do any disks still lie about having committed data to permanent storage
>>> or have we moved past that ?
>>
>> I would expect any modern SAN system to have extensive caching
>> to reach good performance. Adn that the I/O finish when the data
>> is in the cache.
>>
>> And why would the cache be lost at power failure?
>>
>> I think that your way of putting your question is "questionable".
>>
> 
> Next time Jan-Erik, before saying my question is "questionable", you
> may want to ask yourself if in fact I am asking about something that
> you are unaware of as is clearly the case here.

BS! What I'm saying is that your are asking basied questions by
calling something a "lie", that might just as well be well known
design decisions. And yes, this was well know from 80s and forward.
Anyone working with disk drives knew this and made their own decissions.

I certenly know what you are asking about.

It is still not a "lie", which is what I'm pointing out. It was just
the way that (some) disk drives worked (and maybe works).

But you asked about today. I would not concider this an issue today,
since most storage is not based on single physical disks, at least
not in storage used from VMS systems of today.

Lying is something a human does, not some bit of hardware.