On Wed, Nov 07, 2018 at 08:08:41PM -0500, Fulko Hew wrote:
> old brain remembers ... sync; sync; sync;
Yep, that was the mantra.
> Supposedly,
> #1 for the data blocks,
> #2 to ensure the updated inodes got written,
> #3 to ensure that superblock updates got written (if necessary).
Well, that wasn't so. I was doing Unix internals at Bell Labs in 1980.
Sync flushed all pending disk I/O operations for the entire kernel
...that's all processes and *all users* on a multi-user machine.
The idea was apparently that multiple "syncs" would assure flushing
operations that would get submitted; even then it was a bit of apocrypha.
However, the early days *were* interesting; BTL was pushing Unix hard by
1980, especially for the 5ESS project, and more people than had ever been
using a Unix system concurrently were on the development systems.
One week started out badly for us; for some reason everyone was
experiencing *incredibly* poor performance. We figured out that we were
all on the same server, but the poor guys in the data center couldn't see
anything wrong; it was just, suddenly, and inexplicably ***slow***.
I was chatting with a new hire who'd started that morning, and he proudly
showed me a vi macro he'd written to "guarantee that he never lost data".
Really? Let's see...hmm...every command issued was intercepted...AND
CALLED SYNC. On a machine hosting a boatload of other users. He was
calling sync for every single editor command he typed.
Cluebat applied, macro removed, life returned to normal.
(And no, I don't know when they stopped letting a single user do that to
the whole system, and lost access to Unix source when I left the Labs Way
Back When, but they *must* have, right? And no, I've not cared enough to
go see how Linux does it.)
Cheers,
--
Dave Ihnat
dih...@dminet.com
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