Re: Is Parallel Programming Hard, And, If So, What Can You Do About It?

Terje Mathisen <terje.mathisen@tmsw.no> writes:

Lawrence D'Oliveiro wrote:

One of my pet peeves is disk drives with memory caches in them. Why?
=20

>
For reads it allows the disk to always read full sets of sectors, the=20
following blocks are likely to be needed soon anyway.

Yes.

For writes, as long as the drive has enough energy (maybe in the form of =
>
spinning inertia, or a hefty cap?) the always be able to save the buffer =
>
cache to spinning rust, it can allow operations to complete immediately, =
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or as soon as the data has been transferred into the disk cache.

I used to think so, but someone (who appeared knowledgable) corrected
that view and told me that all that HDDs ever do on power loss is to
complete the current sector.

Since all disks are using linear sector (or 4K block?) addressing these=20
days, instead of head/cylinder/sector, a little bit of cache can help=20
hide the tiny time glitches when the disk has to reposition.

An alternative is to skew the start of the first sector on the next
track to take the repositioning time into account, and from what I
have read, that is what is done.  It allows to complete a sequence of
sectors in one go, and then move on to the next sequence of sectors
elsewhere, without having to wait for another disk rotation to be able
to write the sector that was missed in an unskewed drive.

On SSDs DRAM cache is also used for storing the logical-to-physical
sector mapping of the flash translation layer; accessing it on flash
is apparently too slow.  Some DRAM-less SSDs keep that cache in host
DRAM (but AFAIK in an area that is then not accessed by the host).

In all of these caches the disk drive cache is not addressable by the
CPU and it's coherence with the CPU cache is therefore not an issue.
It's coherence with the permanent state of the drive is an issue,
though.  SCSI and SATA drives (and, I guess, NVME drives, too, but I
have read little about that) support command-queuing interfaces that
allow the file systems to manage this coherence.  File systems are not
as good at that as I would like.

- anton
--
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
  Mitch Alsup, <c17fcd89-f024-40e7-a594-88a85ac10d20o@googlegroups.com>