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

[gah4]

On Tuesday, July 6, 2021 at 6:04:49 AM UTC-7, Simon Clubley wrote:
> 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. 

> Do any disks still lie about having committed data to permanent storage 
> or have we moved past that ? 

There was a big discussion of this some years ago related to NFS servers.

With synchronous writes (traditional default and still recommended) 
the server is not supposed to acknowledge the write until (as you note)
it is on permanent storage. 

For most computers, and most uses, this isn't really a problem.
If the computer powers off, along with the drive, the indication that it
was or wasn't written is likely lost at the same time. 

In the case of an NFS server, though, the server could be powered down
after acknowledging the write, but before it was out of the drives cache.
The client might be somewhere else, and stay powered on.

Many systems, though, now use journaling file systems, which helps with
some of the problems. In theory, the drive has a small amount of time
before the filter capacitors discharge and the disk stops, to still get
data onto disk. 

As noted above, in many cases it is the drive acknowledging the
transfer of data across the bus, and releasing the bus for the
next operation, and not that the data is on permanent storage.

NFS has read-ahead and write-behind, so that it can avoid the
large latency that would otherwise exist.