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

On 5/12/2025 1:41 AM, Anton Ertl wrote:

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.

Caveat - my knowledge on this subject is old and may be obsolete. But it was correct.
Close.  They complete the current sector (so that a subsequent read doesn't cause a messy error due to partially written sector), then do an emergency retract of the heads.  This is done so when the disk stops spinning and the heads land, they do so in the "landing zone" which is an area that is a little rougher than the rest of the disk.  This prevents the heads from becoming stuck to the disk surface, a problem known as "stiction" without this, when power is restored and the disk starts spinning again, the heads would stick to the disk surface and be ripped off the arm, essentially destroying the drive.  The rougher landing zone area of the disk prevents stiction.
If you want the write operation to complete after a power outage, you need a battery backup.

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.

Actually, both a cache and skewing are done.  There are two "levels" of skew.  A small skew is used at the end of each track except the last on on the cylinder to allow for a small "microseek" to allow the drive to compensate for the small differences in track location in each surface. A larger skew is used after the last track on each cylinder to allow the heads to seek to the next cylinder without, as you say, incurring a full rotation delay.
--
  - Stephen Fuld
(e-mail address disguised to prevent spam)