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From: Derrick Stolee <stolee@gmail.com>
To: Junio C Hamano <gitster@pobox.com>, Jeff King <peff@peff.net>
Cc: Taylor Blau <me@ttaylorr.com>,
	git@vger.kernel.org, dstolee@microsoft.com
Subject: Re: [PATCH] builtin/repack.c: invalidate MIDX only when necessary
Date: Tue, 25 Aug 2020 12:18:42 -0400	[thread overview]
Message-ID: <c78b3108-b760-d252-9428-6f03549fea11@gmail.com> (raw)
In-Reply-To: <xmqqtuwq1zux.fsf@gitster.c.googlers.com>

On 8/25/2020 11:58 AM, Junio C Hamano wrote:
> Jeff King <peff@peff.net> writes:
> 
>> Does "git repack" ever remove just one pack? Obviously "git repack -ad"
>> or "git repack -Ad" is going to pack everything and delete the old
>> packs. So I think we'd want to remove a midx there.
> 
> AFAIK, the pack-redundant subcommand is used by nobody in-tree, so
> nobody is doing "there are three packfiles, but all the objects in
> one of them are contained in the other two, so let's remove that
> redundant one".
> 
>> And "git repack -d" I think of as deleting only loose objects that we
>> just packed. But I guess it could also remove a pack that has now been
>> made redundant? That seems like a rare case in practice, but I suppose
>> is possible.
> 
> Meaning it can become reality?  Yes.  Or it already can happen?  I
> doubt it.
> 
>> E.g., imagine we have a midx that points to packs A and B, and
>> git-repack deletes B. By your logic above, we need to remove the midx
>> because now it points to objects in B which aren't accessible. But by
>> deleting it, could we be deleting the only thing that mentions the
>> objects in A?
>>
>> I _think_ the answer is "no", because we never went all-in on midx and
>> allowed deleting the matching .idx files for contained packs.
> 
> Yeah, it has been my assumption that that part of the design would
> never change.

Yes, we should intend to keep the .idx files around forever even when
using the multi-pack-index. The duplicated data overhead is not typically
a real problem.

The one caveat I would consider is if we wanted to let a multi-pack-index
cover thin packs, but that would be a substantial change to the object
database that I do not plan to tackle any time soon.

>> I'm also a little curious how bad it is to have a midx whose pack has
>> gone away. I guess we'd answer queries for "yes, we have this object"
>> even if we don't, which is bad. Though in practice we'd only delete
>> those packs if we have their objects elsewhere.
> 
> Hmph, object O used to be in pack A and pack B, midx points at the
> copy in pack B but we deleted it because the pack is redundant.
> Now, midx says O exists, but midx cannot be used to retrieve O.  We
> need to ask A.idx about O to locate it.
> 
> That sounds brittle.  I am not sure our error fallback is all that
> patient.

The best I can see is that prepare_midx_pack() will return 1 if the
pack no longer exists, and this would cause a die("error preparing
packfile from multi-pack-index") in nth_midxed_pack_entry(), killing
the following stack trace:

+ find_pack_entry():packfile.c
 + fill_midx_entry():midx.c
  + nth_midxed_pack_entry():midx.c

Perhaps that die() is a bit over-zealous since we could return 1
through this stack and find_pack_entry() could continue searching
for the object in the packed_git list. However, it could start
returning false _negatives_ if there were duplicates of the object
in the multi-pack-index but only the latest copy was deleted (and
the object does not appear in a pack-file outside of the multi-
pack-index).

Thanks,
-Stolee

  parent reply	other threads:[~2020-08-25 16:18 UTC|newest]

Thread overview: 78+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-25  2:01 [PATCH] builtin/repack.c: invalidate MIDX only when necessary Taylor Blau
2020-08-25  2:26 ` Jeff King
2020-08-25  2:37   ` Taylor Blau
2020-08-25 13:14     ` Derrick Stolee
2020-08-25 14:41       ` Taylor Blau
2020-08-25 15:14         ` Derrick Stolee
2020-08-25 15:42           ` Taylor Blau
2020-08-25 16:56             ` Jeff King
2020-08-25 15:58   ` Junio C Hamano
2020-08-25 16:08     ` Taylor Blau
2020-08-25 16:18     ` Derrick Stolee [this message]
2020-08-25 17:34       ` Jeff King
2020-08-25 17:22     ` Jeff King
2020-08-25 18:05       ` Junio C Hamano
2020-08-25 18:27         ` Jeff King
2020-08-25 22:45           ` [PATCH] pack-redundant: gauge the usage before proposing its removal Junio C Hamano
2020-08-25 23:09             ` Taylor Blau
2020-08-25 23:22               ` Junio C Hamano
2020-08-26  1:17             ` [PATCH v1 0/3] War on dashed-git Junio C Hamano
2020-08-26  1:17               ` [PATCH v1 1/3] transport-helper: do not run git-remote-ext etc. in dashed form Junio C Hamano
2020-08-26  1:24                 ` Eric Sunshine
2020-08-26  7:55                   ` Johannes Schindelin
2020-08-26 16:27                   ` Junio C Hamano
2020-08-26  1:17               ` [PATCH v1 2/3] cvsexportcommit: do not run git programs " Junio C Hamano
2020-08-26  1:28                 ` Eric Sunshine
2020-08-26  1:42                   ` Junio C Hamano
2020-08-26 16:08                   ` Junio C Hamano
2020-08-26 16:28                     ` Junio C Hamano
2020-08-26  8:02                 ` Johannes Schindelin
2020-08-26  1:17               ` [PATCH v1 3/3] git: catch an attempt to run "git-foo" Junio C Hamano
2020-08-26  1:19                 ` Junio C Hamano
2020-08-26  8:06                 ` Johannes Schindelin
2020-08-26 16:30                   ` Junio C Hamano
2020-08-28  2:13                 ` Johannes Schindelin
2020-08-28 22:03                   ` Junio C Hamano
2020-08-31  9:59                     ` Johannes Schindelin
2020-08-31 17:45                       ` Junio C Hamano
2020-12-20 15:25                   ` Johannes Schindelin
2020-12-21 22:24                     ` Junio C Hamano
2020-12-30  5:30                       ` Johannes Schindelin
2020-08-26  8:09               ` [PATCH v1 0/3] War on dashed-git Johannes Schindelin
2020-08-26 16:45                 ` Junio C Hamano
2020-08-26 19:46                   ` [PATCH v2 0/2] avoid running "git-subcmd" in the dashed form Junio C Hamano
2020-08-26 19:46                     ` [PATCH v2 1/2] transport-helper: do not run git-remote-ext etc. in " Junio C Hamano
2020-08-26 19:46                     ` [PATCH v2 2/2] cvsexportcommit: do not run git programs " Junio C Hamano
2020-08-26 21:37                       ` [PATCH v2 3/2] credential-cache: use child_process.args Junio C Hamano
2020-08-26 22:25                         ` [PATCH] run_command: teach API users to use embedded 'args' more Junio C Hamano
2020-08-27  4:21                           ` Jeff King
2020-08-27  4:30                             ` Junio C Hamano
2020-08-27  4:31                             ` Eric Sunshine
2020-08-27  4:44                               ` Jeff King
2020-08-27  5:03                                 ` Eric Sunshine
2020-08-27  5:25                                   ` [PATCH] worktree: fix leak in check_clean_worktree() Jeff King
2020-08-27  5:56                                     ` Eric Sunshine
2020-08-27 15:31                                       ` Junio C Hamano
2020-08-27  4:13                         ` [PATCH v2 3/2] credential-cache: use child_process.args Jeff King
2020-08-27  4:22                           ` Jeff King
2020-08-27  4:31                           ` Junio C Hamano
2020-08-27  4:14                         ` Jeff King
2020-08-27 15:34                           ` Junio C Hamano
2020-08-31 22:56                         ` Junio C Hamano
2020-09-01  4:49                           ` Jeff King
2020-09-01 16:11                             ` Junio C Hamano
2020-08-27  0:57                     ` [PATCH v2 0/2] avoid running "git-subcmd" in the dashed form Derrick Stolee
2020-08-27  1:22                       ` Junio C Hamano
2020-08-28  9:14             ` [PATCH] pack-redundant: gauge the usage before proposing its removal Jeff King
2020-08-28 22:45               ` Junio C Hamano
2020-08-25  7:55 ` [PATCH] builtin/repack.c: invalidate MIDX only when necessary Son Luong Ngoc
2020-08-25 12:45   ` Derrick Stolee
2020-08-25 14:45   ` Taylor Blau
2020-08-25 16:04     ` [PATCH v2] " Taylor Blau
2020-08-26 20:51       ` Derrick Stolee
2020-08-26 20:54         ` Junio C Hamano
2020-08-25 16:47     ` [PATCH] " Jeff King
2020-08-25 17:10       ` Derrick Stolee
2020-08-25 17:29         ` Jeff King
2020-08-25 17:34           ` Taylor Blau
2020-08-25 17:42             ` Jeff King

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