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

[Marc Branchaud <marcnarc@xiplink.com>]

On 2020-09-17 7:28 a.m., Ævar Arnfjörð Bjarmason wrote:
> 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

Typo: containng

		M.

> +	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'
>