Re: [PATCH 1/1] read-cache: update index format default to v4

From: Patrick Steinhardt <ps@pks.im>
Date: Sep 25, 2018, 07:06 UTC
Message-ID: <20180925070618.GB1596@xps>
In-Reply-To: <20180924213223.GG27036@localhost>

On Mon, Sep 24, 2018 at 11:32:23PM +0200, SZEDER Gábor wrote:

Show 10 quoted lines

> On Mon, Sep 24, 2018 at 02:15:30PM -0700, Derrick Stolee via GitGitGadget wrote:
> > From: Derrick Stolee <dstolee@microsoft.com>
> > 
> > The index v4 format has been available since 2012 with 9d22778
> > "reach-cache.c: write prefix-compressed names in the index". Since
> > the format has been stable for so long, almost all versions of Git
> > in use today understand version 4, removing one barrier to upgrade
> > -- that someone may want to downgrade and needs a working repo.
> 
> What about alternative implementations, like JGit, libgit2, etc.?

Speaking of libgit2, we are able to read and write index v4 since commit c1b370e93 (Merge pull request #3837 from novalis/dturner/indexv4, 2016-08-17), released with v0.25 in December 2016. Due to insufficient tests, our read support was initially broken, which got fixed with commit 3bc95cfe3 (Merge pull request #4236 from pks-t/pks/index-v4-fixes, 2017-06-07) and released with v0.26 in June 2017.

Right now I'm not aware of additional bugs in our index v4 support, so we'd be fine with changing the default.

Patrick

Show 36 quoted lines

> > Despite being stable for a long time, this index version was never
> > adopted as the default. This prefix-compressed version of the format
> > can get significant space savings on repos with large working
> > directories (which naturally tend to have deep nesting). This version
> > is set as the default for some external tools, such as VFS for Git.
> > Because of this external use, the format has had a lot of "testing in
> > production" and also is subject to continuous integration in these
> > environments.
> > 
> > Previously, to test version 4 indexes, we needed to run the test
> > suite with GIT_TEST_INDEX_VERSION=4 (or TEST_GIT_INDEX_VERSION=4).
> > 
> > One potential, but short-term, downside is that we lose coverage of
> > the version 3 indexes. The trade-off is that we may want to cover
> > that version using GIT_TEST_INDEX_VERSION=3.
> > 
> > Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
> > ---
> >  read-cache.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/read-cache.c b/read-cache.c
> > index 372588260e..af6c8f2a67 100644
> > --- a/read-cache.c
> > +++ b/read-cache.c
> > @@ -1484,7 +1484,7 @@ struct cache_entry *refresh_cache_entry(struct cache_entry *ce,
> >   * Index File I/O
> >   *****************************************************************/
> >  
> > -#define INDEX_FORMAT_DEFAULT 3
> > +#define INDEX_FORMAT_DEFAULT 4
> >  
> >  static unsigned int get_index_format_default(void)
> >  {
> > -- 
> > gitgitgadget