Re: [PATCH v5 2/5] core.fsyncmethod: add writeout-only mode

"Neeraj Singh via GitGitGadget" [off-list ref] writes:

From: Neeraj Singh <redacted>

This commit introduces the `core.fsyncMethod` configuration
knob, which can currently be set to `fsync` or `writeout-only`.

The new writeout-only mode attempts to tell the operating system to
flush its in-memory page cache to the storage hardware without issuing a
CACHE_FLUSH command to the storage controller.

Writeout-only fsync is significantly faster than a vanilla fsync on
common hardware, since data is written to a disk-side cache rather than
all the way to a durable medium. Later changes in this patch series will
take advantage of this primitive to implement batching of hardware
flushes.

When git_fsync is called with FSYNC_WRITEOUT_ONLY, it may fail and the
caller is expected to do an ordinary fsync as needed.

On Apple platforms, the fsync system call does not issue a CACHE_FLUSH
directive to the storage controller. This change updates fsync to do
fcntl(F_FULLFSYNC) to make fsync actually durable. We maintain parity
with existing behavior on Apple platforms by setting the default value
of the new core.fsyncMethod option.

Signed-off-by: Neeraj Singh <redacted>
---

OK.  This seems to be quite reasonable in that the pieces of code
that use fsync_or_die() do not have to change at all, and all of
them will keep behaving the same way.  In other words, the "how" of
fsync is very much well isolated.

Nice.