Disks in mainframe days had no onboard cache, and the O/S had to do all
that. Today that has been built into much cheaper disks. While there is
no mechanism to determine what is cached by the onboard disk memory and
what is not, there are situations where you need disk cacheing to be
disabled for the full disk, and that can be done in Linux. Its not
something commonly done anymore with the use of COW filesystems and
actually-working redundant UPS systems, but there are complex
mission-critical db apps and complex multi-machine apps that need to
make guarantees that something is really on disk after a 2-phase commit
completes. The default disk performance suffers a huge hit when doing
this though, as the rotational latency becomes the critical speed limit
instead of the bus write speed. In general, these apps would use their
own dedicated disks and the rest of the system is not then limited by
the same handicapping of the disk.
In the Disks app, select the disk, go to the 3 dots on the top right,
select write cache and select disable. I'm unsure, but you might need
to reboot as well. You can also do this in the CLI, but its been years
since I have had to do that.
On 2024-11-20 22:45, Tim via users wrote:
Stephen Morris wrote:
40 years ago mainframe storage controllers provided functionality to
control what files were loaded into the storage cache and how much of
the file was loaded, I just thought hard disks had advanced enough to
now provide similar functionality.
Tim:
I'm wondering how a storage device is going to know which files to
treat differently from other files? That's going to be OS dependent.
Stephen Morris:
The storage device doesn't, you tell it what to do.
Then why did you think that disc drives had advanced to the point where
they could control what's cached or not? (Your message on 19 Nov.)
Storage control is an OS-controlled thing through device controller on
your motherboard or daughterboard. Or, much more likely, completely in
software with the hardware just being a simple interface.
The storage device - the disc drive itself - is a pretty dumb device.
To optimise use of a storage device (to that degree) you'd need to make
OS- and file-system-specific decisions. That's really outside the
scope of a simple storage device that's used on many (very) different
kinds of systems. Otherwise, you'd need custom disk drives for each
system. Generic hardware is just one of the reasons PCs were cheaper
than mainframe equipment.
And yes, your (outside the disc drive) controller could possibly send
commands to the drive about caching priority, and maybe the drive could
do different things, but the decisions would be external. And I
suspect new bottlenecks would arise.
--
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