On Sat, Apr 8, 2023 at 10:37 AM Charles Forsyth
<charles.fors...@gmail.com> wrote:
> It was the different characteristics of hard drives, even decent SATA,
> compared to SSD and nvme that I had in mind.
Since details have been requested about this. I wouldn't presume to
speak from Charles, but some of those differences _may_ include:
1. Optimizing for the rotational latency of spinning media, and its effects vis:
a. the layout of storage structures on the disk,
b. placement of _data_ on the device.
2. Effects with respect to things that aren't considerations for rotating disks
a. Wear-leveling may be the canonical example here
3. Effects at the controller level.
a. Caching, and the effect that has on how operations are ordered to
ensure consistency
b. Queuing for related objects written asynchronously and
assumptions about latency
In short, when you change storage technologies, assumptions that were
made with, say, a filesystem was initially written may be invalidated.
Consider the BSD FFS for example: UFS was written in an era of VAXen
and slow, 3600 RPM spinning disks like RA81s attached to relatively
unintelligent controllers; it made a number of fundamental design
decisions based on that, trying to optimize placement of data and
metadata near each other (to minimize head travel--this is the whole
cylinder group thing), implementation that explicitly accounted for
platter rotation with respect to scheduling operations for the
underlying storage device, putting multiple copies of the superblock
in multiple locations in the disk to maximize the chances of recovery
in the event of the (all-too-common) head crashes of the era, etc.
They also did very careful ordering of operations for soft-updates in
UFS2 to ensure filesystem consistency when updating metadata in the
face of a system crash (or power failure, or whatever). It turns out
that many of those optimizations become pessimizations (or at least
irrelevant) when you're all of a sudden writing to a solid-state
device, nevermind battery-backed DRAM on a much more advanced
controller.
- Dan C.
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