"disabling bitmap writing, as some objects are not being packed"?

I'm a bit confused by the message "disabling bitmap writing, as some
objects are not being packed".  I see it the my gc.log file on my git
server.

1. Its presence in the gc.log file prevents future automatic garbage
collection.  This seems bad.  I understand the desire to avoid making
things worse if a past gc has run into issues.  But this warning is
non-fatal; the only consequence is that many operations get slower.  But
a lack of gc when there are too many packs causes that consequence too
(often a much worse slowdown than would be caused by the missing
bitmap).

So I wonder if it would be better for auto gc to grep gc.log for fatal
errors (as opposed to warnings) and only skip running if any are found.
Alternately, we could simply put warnings into gc.log.warning and
reserve gc.log for fatal errors. I'm not sure which would be simpler.  

2. I don't understand what would cause that message.  That is, what bad
thing am I doing that I should stop doing?  I've briefly skimmed the
code and commit message, but the answer isn't leaping out at me.

On Fri, Dec 16, 2016 at 04:05:31PM -0500, David Turner wrote:

1. Its presence in the gc.log file prevents future automatic garbage
collection.  This seems bad.  I understand the desire to avoid making
things worse if a past gc has run into issues.  But this warning is
non-fatal; the only consequence is that many operations get slower.  But
a lack of gc when there are too many packs causes that consequence too
(often a much worse slowdown than would be caused by the missing
bitmap).

So I wonder if it would be better for auto gc to grep gc.log for fatal
errors (as opposed to warnings) and only skip running if any are found.
Alternately, we could simply put warnings into gc.log.warning and
reserve gc.log for fatal errors. I'm not sure which would be simpler.  

Without thinking too hard on it, that seems like the appropriate
solution to me, too.

2. I don't understand what would cause that message.  That is, what bad
thing am I doing that I should stop doing?  I've briefly skimmed the
code and commit message, but the answer isn't leaping out at me.

Do you have alternates and are using --local? Do you have .keep packs
and have set repack.packKeptObjects to false?

There are other ways (e.g., an incremental repack), but I think those
are the likely ones to get via "git gc".

-Peff