patchEnable core.fsyncObjectFiles by default

> Of course, it might well be the case that the actual default
> might be worth turning around. Most git users probably don't
> care about that kind of "apply two hundred patches from Andrew
> Morton" kind of workload, although "rebase a big patch-series"
> does end up doing basically the same thing, and might be more
> common.

> Linus Torvalds started a discussion[1] if we want to play rather safe
> than use defaults which make sense only for the most power users of Git:
>
>> So git is "safe" in the sense that you won't really lose any data,
>> but you may well be inconvenienced.  The "fsync each object" config
>> option is there in case you don't want that inconvenience, but it
>> should be noted that it can make for a hell of a performance impact.
>
>> Of course, it might well be the case that the actual default
>> might be worth turning around. Most git users probably don't
>> care about that kind of "apply two hundred patches from Andrew
>> Morton" kind of workload, although "rebase a big patch-series"
>> does end up doing basically the same thing, and might be more
>> common.
>
> This patch enables fsync_object_files by default.

>
> [1] https://plus.google.com/u/1/+JonathanCorbet/posts/JBxiKPe3VXa
>
> Signed-off-by: Stefan Beller <sbeller@google.com>
> ---
>  Documentation/config.txt | 8 ++++----
>  environment.c            | 2 +-
>  2 files changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/config.txt b/Documentation/config.txt
> index 43bb53c..dce2640 100644
> --- a/Documentation/config.txt
> +++ b/Documentation/config.txt
> @@ -693,10 +693,10 @@ core.whitespace::
>  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 ensures objects are written to disk instead of relying on the
> +operating systems cache and eventual write. Disabling this option will
> +yield performance with a trade off in safety for repository corruption
> +during power loss.
>  
>  core.preloadIndex::
>  	Enable parallel index preload for operations like 'git diff'
> diff --git a/environment.c b/environment.c
> index 61c685b..b406f5e 100644
> --- a/environment.c
> +++ b/environment.c
> @@ -35,7 +35,7 @@ const char *git_attributes_file;
>  int zlib_compression_level = Z_BEST_SPEED;
>  int core_compression_level;
>  int core_compression_seen;
> -int fsync_object_files;
> +int fsync_object_files = 1;
>  size_t packed_git_window_size = DEFAULT_PACKED_GIT_WINDOW_SIZE;
>  size_t packed_git_limit = DEFAULT_PACKED_GIT_LIMIT;
>  size_t delta_base_cache_limit = 96 * 1024 * 1024;

> The main issue is that non-expert users might not realize that they
> really need to run "git fsck" after a crash; otherwise, what might
> happen is that although git is only appending, that you might have
> some zero-length (or partially written) git object or pack files, and
> while you might not notice at that moment, it might come and bite you
> later.

> 
> Regarding loose object files, given that we write to a temporary,
> optionally fsync, close and then move to the final name, would we
> still see partially written file if we omit the fsync, or would the
> corruption be limited to either empty or missing?

> The reason I am wondering is because the codepath to create an
> object (i.e. "update-index --add", "hash-object -w", or "add") first
> checks if a packed or a loose object file _exists_ and if so
> bypasses writing the same thing anew, but the existence check for a
> loose object is to merely making sure that access(F_OK) (and
> optionally utime()) succeeds.  If the potential breakage is limited
> to truncation to empty, then we could replace it with stat(2) and
> st.st_size check, as no loose object file can be empty.

> Linus Torvalds started a discussion[1] if we want to play rather safe
> than use defaults which make sense only for the most power users of Git:
>
>> So git is "safe" in the sense that you won't really lose any data,
>> but you may well be inconvenienced.  The "fsync each object" config
>> option is there in case you don't want that inconvenience, but it
>> should be noted that it can make for a hell of a performance impact.
>
>> Of course, it might well be the case that the actual default
>> might be worth turning around. Most git users probably don't
>> care about that kind of "apply two hundred patches from Andrew
>> Morton" kind of workload, although "rebase a big patch-series"
>> does end up doing basically the same thing, and might be more
>> common.
>
> This patch enables fsync_object_files by default.

> Linus Torvalds started a discussion[1] if we want to play rather safe
> than use defaults which make sense only for the most power users of Git:
> 
> > So git is "safe" in the sense that you won't really lose any data,
> > but you may well be inconvenienced.  The "fsync each object" config
> > option is there in case you don't want that inconvenience, but it
> > should be noted that it can make for a hell of a performance impact.
> 
> > Of course, it might well be the case that the actual default
> > might be worth turning around. Most git users probably don't
> > care about that kind of "apply two hundred patches from Andrew
> > Morton" kind of workload, although "rebase a big patch-series"
> > does end up doing basically the same thing, and might be more
> > common.
> 
> This patch enables fsync_object_files by default.

> I had always assumed this was fine on ext4 with data=ordered (i.e.,
> either the rename and its pointed-to content will go through, or not; so
> you either get your update or the old state, but not a garbage or empty
> file). But it sounds from what Ted wrote in:
>
>   http://article.gmane.org/gmane.linux.file-systems/97255
>
> that this may not be the case. If it's not, I think we should consider
> fsyncing ref writes.