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

[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.

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 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. :-(

>> 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.

This was a well-known problem in the past for some cases for disk device
driver writers. I am just curious if it is still a problem these days
because that is something I don't know.

Simon.

-- 
Simon Clubley, clubley at remove_me.eisner.decus.org-Earth.UFP
Walking destinations on a map are further away than they appear.