Am 17.09.20 um 17:43 schrieb Junio C Hamano: > Ævar Arnfjörð Bjarmason <avarab@xxxxxxxxx> 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@xxxxxx/ >> >> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@xxxxxxxxx> >> --- >> 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-<. Not only that; the new text also uses the term "fsync" in a manner that I could be persuaded that it is actually an English word. Which, so far, I doubt that it is ;) A little bit less 1337 wording would help the users better. > > > Thanks. > > [Reference] > > *1* https://public-inbox.org/git/20180117193510.GA30657@xxxxxx/ > >> >> 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' >