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

> Ævar Arnfjörð Bjarmason  <avarab@gmail.com> writes:
> 
>> 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(-)
> 
> When I saw the subject in my mailbox, I expected to see that you
> would resurrect Christoph's updated text in [*1*], but you wrote a
> whole lot more ;-) And they are quite informative to help readers to
> understand what the option does.  I am not sure if the understanding
> directly help readers to decide if it is appropriate for their own
> repositories, though X-<.

> 
> 
> Thanks.
> 
> [Reference]
> 
> *1* https://public-inbox.org/git/20180117193510.GA30657@lst.de/
> 
>>
>> 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'
>