[RFC PATCH 2/2] core.fsyncObjectFiles: make the docs less flippant

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

As amusing as Linus's original prose[1] is here it doesn't really explain
in any detail to the uninitiated why you would or wouldn't enable
this, and the counter-intuitive reason for why git wouldn't fsync your
precious data.

So elaborate (a lot) on why this may or may not be needed. This is my
best-effort attempt to summarize the various points raised in the last
ML[2] discussion about this.

1.  aafe9fbaf4 ("Add config option to enable 'fsync()' of object
    files", 2008-06-18)
2. https://lore.kernel.org/git/20180117184828.31816-1-hch@lst.de/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Documentation/config/core.txt | 42 ++++++++++++++++++++++++++++++-----
 1 file changed, 36 insertions(+), 6 deletions(-)

diff --git a/Documentation/config/core.txt b/Documentation/config/core.txt
index 74619a9c03..5b47670c16 100644
--- a/Documentation/config/core.txt
+++ b/Documentation/config/core.txt
@@ -548,12 +548,42 @@ core.whitespace::
   errors. The default tab width is 8. Allowed values are 1 to 63.
 
 core.fsyncObjectFiles::
-	This boolean will enable 'fsync()' when writing object files.
-+
-This is a total waste of time and effort on a filesystem that orders
-data writes properly, but can be useful for filesystems that do not use
-journalling (traditional UNIX filesystems) or that only journal metadata
-and not file contents (OS X's HFS+, or Linux ext3 with "data=writeback").
+	This boolean will enable 'fsync()' when writing loose object
+	files. Both the file itself and its containng directory will
+	be fsynced.
++
+When git writes data any required object writes will precede the
+corresponding reference update(s). For example, a
+linkgit:git-receive-pack[1] accepting a push might write a pack or
+loose objects (depending on settings such as `transfer.unpackLimit`).
++
+Therefore on a journaled file system which ensures that data is
+flushed to disk in chronological order an fsync shouldn't be
+needed. The loose objects might be lost with a crash, but so will the
+ref update that would have referenced them. Git's own state in such a
+crash will remain consistent.
++
+This option exists because that assumption doesn't hold on filesystems
+where the data ordering is not preserved, such as on ext3 and ext4
+with "data=writeback". On such a filesystem the `rename()` that drops
+the new reference in place might be preserved, but the contents or
+directory entry for the loose object(s) might not have been synced to
+disk.
++
+Enabling this option might slow git down by a lot in some
+cases. E.g. in the case of a naïve bulk import tool which might create
+a million loose objects before a final ref update and `gc`. In other
+more common cases such as on a server being pushed to with default
+`transfer.unpackLimit` settings the difference might not be noticable.
++
+However, that's highly filesystem-dependent, on some filesystems
+simply calling fsync() might force an unrelated bulk background write
+to be serialized to disk. Such edge cases are the reason this option
+is off by default. That default setting might change in future
+versions.
++
+In older versions of git only the descriptor for the file itself was
+fsynced, not its directory entry.
 
 core.preloadIndex::
 	Enable parallel index preload for operations like 'git diff'
-- 
2.28.0.297.g1956fa8f8d