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Fwd: Local clones aka forks disk size optimization

Subject: Fwd: Local clones aka forks disk size optimization

## tl;dr

13 messages between Nov 14, 2012 and Nov 18, 2012.

replies: 12people: 7as markdown or json

Javier Domingo· Nov 14, 2012, 23:42 UTC · lore
Hi,

I have come up with this while doing some local forks for work. Currently, when you clone a repo using a path (not file:/// protocol) you get all the common objects linked.

But as you work, each one will continue growing on its way, although they may have common objects.

Is there any way to avoid this? I mean, can something be done in git, that it checks for (when pulling) the same objects in the other forks?

Thought this doesn't make much sense in clients, when you have to maintain 20 forks of very big projects in server side, it eats precious disk space.

I don't know how if this should have [RFC] in the subject or what. But here is my idea.

As hardlinking is already done by git, if it checked for how many links there are for its files, it would be able to find other dirs where to search. The easier way is checking for the most ancient pack.

Hope you like this idea,
Javier Domingo
Andrew Ardill· Nov 15, 2012, 00:18 UTC · re: Javier Domingo · lore

Re: Local clones aka forks disk size optimization

On 15 November 2012 10:42, Javier Domingo <javierdo1@gmail.com> wrote:
Show 11 quoted lines
> Hi,
>
> I have come up with this while doing some local forks for work.
> Currently, when you clone a repo using a path (not file:/// protocol)
> you get all the common objects linked.
>
> But as you work, each one will continue growing on its way, although
> they may have common objects.
>
> Is there any way to avoid this? I mean, can something be done in git,
> that it checks for (when pulling) the same objects in the other forks?
Have you seen alternates? From [1]:
Show 15 quoted lines
> How to share objects between existing repositories?
> ---------------------------------------------------------------------------
>
> Do
>
> echo "/source/git/project/.git/objects/" > .git/objects/info/alternates
>
> and then follow it up with
>
> git repack -a -d -l
>
> where the '-l' means that it will only put local objects in the pack-file
> (strictly speaking, it will put any loose objects from the alternate tree
> too, so you'll have a fully packed archive, but it won't duplicate objects
> that are already packed in the alternate tree).
[1] https://git.wiki.kernel.org/index.php/GitFaq#How_to_share_objects_between_existing_repositories.3F
Regards,
Andrew Ardill
Javier Domingo· Nov 15, 2012, 00:40 UTC · re: Andrew Ardill · lore

Re: Local clones aka forks disk size optimization

Hi Andrew,

The problem about that, is that if I want to delete the first repo, I will loose objects... Or does that repack also hard-link the objects in other repos? I don't want to accidentally loose data, so it would be nice that althought avoided to repack things, it would also hardlink them. Javier Domingo

2012/11/15 Andrew Ardill <andrew.ardill@gmail.com>:
Show 37 quoted lines
> On 15 November 2012 10:42, Javier Domingo <javierdo1@gmail.com> wrote:
>> Hi,
>>
>> I have come up with this while doing some local forks for work.
>> Currently, when you clone a repo using a path (not file:/// protocol)
>> you get all the common objects linked.
>>
>> But as you work, each one will continue growing on its way, although
>> they may have common objects.
>>
>> Is there any way to avoid this? I mean, can something be done in git,
>> that it checks for (when pulling) the same objects in the other forks?
>
> Have you seen alternates? From [1]:
>
>> How to share objects between existing repositories?
>> ---------------------------------------------------------------------------
>>
>> Do
>>
>> echo "/source/git/project/.git/objects/" > .git/objects/info/alternates
>>
>> and then follow it up with
>>
>> git repack -a -d -l
>>
>> where the '-l' means that it will only put local objects in the pack-file
>> (strictly speaking, it will put any loose objects from the alternate tree
>> too, so you'll have a fully packed archive, but it won't duplicate objects
>> that are already packed in the alternate tree).
>
> [1] https://git.wiki.kernel.org/index.php/GitFaq#How_to_share_objects_between_existing_repositories.3F
>
>
> Regards,
>
> Andrew Ardill
Andrew Ardill· Nov 15, 2012, 00:53 UTC · re: Javier Domingo · lore

Re: Local clones aka forks disk size optimization

On 15 November 2012 11:40, Javier Domingo <javierdo1@gmail.com> wrote:
Show 7 quoted lines
> Hi Andrew,
>
> The problem about that, is that if I want to delete the first repo, I
> will loose objects... Or does that repack also hard-link the objects
> in other repos? I don't want to accidentally loose data, so it would
> be nice that althought avoided to repack things, it would also
> hardlink them.
Hi Javier, check out the section below the one I linked earlier:
Show 14 quoted lines
> How to stop sharing objects between repositories?
>
> To copy the shared objects into the local repository, repack without the -l flag
>
> git repack -a
>
> Then remove the pointer to the alternate object store
>
> rm .git/objects/info/alternates
>
> (If the repository is edited between the two steps, it could become corrupted
> when the alternates file is removed. If you're unsure, you can use git fsck to
> check for corruption. If things go wrong, you can always recover by replacing
> the alternates file and starting over).
Regards,
Andrew Ardill
Javier Domingo· Nov 15, 2012, 01:15 UTC · re: Andrew Ardill · lore

Re: Local clones aka forks disk size optimization

Hi Andrew,

Doing this would require I got tracked which one comes from which. So it would imply some logic (and db) over it. With the hardlinking way, it wouldn't require anything. The idea is that you don't have to do anything else in the server.

I understand that it would be imposible to do it for windows users (but using cygwin), but for *nix ones yes... Javier Domingo

2012/11/15 Andrew Ardill <andrew.ardill@gmail.com>:
Show 29 quoted lines
> On 15 November 2012 11:40, Javier Domingo <javierdo1@gmail.com> wrote:
>> Hi Andrew,
>>
>> The problem about that, is that if I want to delete the first repo, I
>> will loose objects... Or does that repack also hard-link the objects
>> in other repos? I don't want to accidentally loose data, so it would
>> be nice that althought avoided to repack things, it would also
>> hardlink them.
>
> Hi Javier, check out the section below the one I linked earlier:
>
>> How to stop sharing objects between repositories?
>>
>> To copy the shared objects into the local repository, repack without the -l flag
>>
>> git repack -a
>>
>> Then remove the pointer to the alternate object store
>>
>> rm .git/objects/info/alternates
>>
>> (If the repository is edited between the two steps, it could become corrupted
>> when the alternates file is removed. If you're unsure, you can use git fsck to
>> check for corruption. If things go wrong, you can always recover by replacing
>> the alternates file and starting over).
>
> Regards,
>
> Andrew Ardill
Andrew Ardill· Nov 15, 2012, 01:34 UTC · re: Javier Domingo · lore

Re: Local clones aka forks disk size optimization

On 15 November 2012 12:15, Javier Domingo <javierdo1@gmail.com> wrote:
Show 10 quoted lines
> Hi Andrew,
>
> Doing this would require I got tracked which one comes from which. So
> it would imply some logic (and db) over it. With the hardlinking way,
> it wouldn't require anything. The idea is that you don't have to do
> anything else in the server.
>
> I understand that it would be imposible to do it for windows users
> (but using cygwin), but for *nix ones yes...
> Javier Domingo
Paraphrasing from git-clone(1):

When cloning a repository, if the source repository is specified with /path/to/repo syntax, the default is to clone the repository by making a copy of HEAD and everything under objects and refs directories. The files under .git/objects/ directory are hardlinked to save space when possible. To force copying instead of hardlinking (which may be desirable if you are trying to make a back-up of your repository) --no-hardlinks can be used.

So hardlinks should be used where possible, and if they are not try upgrading Git.

I think that covers all the use cases you have?
Regards,
Andrew Ardill
Sitaram Chamarty· Nov 15, 2012, 03:44 UTC · re: Andrew Ardill · lore

Re: Local clones aka forks disk size optimization

On Thu, Nov 15, 2012 at 7:04 AM, Andrew Ardill <andrew.ardill@gmail.com> wrote:
Show 26 quoted lines
> On 15 November 2012 12:15, Javier Domingo <javierdo1@gmail.com> wrote:
>> Hi Andrew,
>>
>> Doing this would require I got tracked which one comes from which. So
>> it would imply some logic (and db) over it. With the hardlinking way,
>> it wouldn't require anything. The idea is that you don't have to do
>> anything else in the server.
>>
>> I understand that it would be imposible to do it for windows users
>> (but using cygwin), but for *nix ones yes...
>> Javier Domingo
>
> Paraphrasing from git-clone(1):
>
> When cloning a repository, if the source repository is specified with
> /path/to/repo syntax, the default is to clone the repository by making
> a copy of HEAD and everything under objects and refs directories. The
> files under .git/objects/ directory are hardlinked to save space when
> possible. To force copying instead of hardlinking (which may be
> desirable if you are trying to make a back-up of your repository)
> --no-hardlinks can be used.
>
> So hardlinks should be used where possible, and if they are not try
> upgrading Git.
>
> I think that covers all the use cases you have?
I am not sure it does.  My understanding is this:

'git clone -l' saves space on the initial clone, but subsequent pushes end up with the same objects duplicated across all the "forks" (assuming most of the forks keep up with some canonical repo).

The alternates mechanism can give you ongoing savings (as long as you push to the "main" repo first), but it is dangerous, in the words of the git-clone manpage. You have to be confident no one will delete a ref from the "main" repo and then do a gc or let it auto-gc.

He's looking for something that addresses both these issues.

As an additional idea, I suspect this is what the namespaces feature was created for, but I am not sure, and have never played with it till now.

Maybe someone who knows namespaces very well will chip in...
Michael J Gruber· Nov 16, 2012, 11:25 UTC · re: Sitaram Chamarty · lore

Re: Local clones aka forks disk size optimization

Sitaram Chamarty venit, vidit, dixit 15.11.2012 04:44:
Show 47 quoted lines
> On Thu, Nov 15, 2012 at 7:04 AM, Andrew Ardill <andrew.ardill@gmail.com> wrote:
>> On 15 November 2012 12:15, Javier Domingo <javierdo1@gmail.com> wrote:
>>> Hi Andrew,
>>>
>>> Doing this would require I got tracked which one comes from which. So
>>> it would imply some logic (and db) over it. With the hardlinking way,
>>> it wouldn't require anything. The idea is that you don't have to do
>>> anything else in the server.
>>>
>>> I understand that it would be imposible to do it for windows users
>>> (but using cygwin), but for *nix ones yes...
>>> Javier Domingo
>>
>> Paraphrasing from git-clone(1):
>>
>> When cloning a repository, if the source repository is specified with
>> /path/to/repo syntax, the default is to clone the repository by making
>> a copy of HEAD and everything under objects and refs directories. The
>> files under .git/objects/ directory are hardlinked to save space when
>> possible. To force copying instead of hardlinking (which may be
>> desirable if you are trying to make a back-up of your repository)
>> --no-hardlinks can be used.
>>
>> So hardlinks should be used where possible, and if they are not try
>> upgrading Git.
>>
>> I think that covers all the use cases you have?
> 
> I am not sure it does.  My understanding is this:
> 
> 'git clone -l' saves space on the initial clone, but subsequent pushes
> end up with the same objects duplicated across all the "forks"
> (assuming most of the forks keep up with some canonical repo).
> 
> The alternates mechanism can give you ongoing savings (as long as you
> push to the "main" repo first), but it is dangerous, in the words of
> the git-clone manpage.  You have to be confident no one will delete a
> ref from the "main" repo and then do a gc or let it auto-gc.
> 
> He's looking for something that addresses both these issues.
> 
> As an additional idea, I suspect this is what the namespaces feature
> was created for, but I am not sure, and have never played with it till
> now.
> 
> Maybe someone who knows namespaces very well will chip in...
> 
I dunno about namespaces, but a safe route with alternates seems to be:

Provide one "main" clone which is bare, pulls automatically, and is there to stay (no pruning), so that all others can use that as a reliable alternates source.

Michael
Enrico Weigelt· Nov 16, 2012, 18:04 UTC · re: Michael J Gruber · lore

Re: Local clones aka forks disk size optimization

> Provide one "main" clone which is bare, pulls automatically, and is
> there to stay (no pruning), so that all others can use that as a
> reliable alternates source.

The problem here, IMHO, is the assumption, that the main repo will never be cleaned up. But what to do if you dont wanna let it grow forever ?

hmm, distributed GC is a tricky problem.
maybe it could be easier having two kind of alternates:
a) classical: gc+friends will drop local objects that are 
   already there
b) fallback: normal operations fetch objects if not accessible
   from anywhere else, but gc+friends do not skip objects from there.

And extend prune machinery to put some backup of the dropped objects to some separate store.

This way we could use some kind of rotating archive:
* GC'ed objects will be stored in the backup repo for some while
* there are multiple active (rotating) backups kept for some time,
  each cycle, only the oldest one is dropped (and maybe objects
  in a newer backup are removed from the older ones)
* downstream repos must be synced often enough, so removed objects
  are fetched back from the backups early enough
You could see this as some heap:
* the currently active objects (directly referenced) are always
  on the top
* once they're not referenced, they sink a lever deeper
* when the're referenced again, they immediately jump up to the top
* at some point in time unreferenced objects sink too deep that
  they're dropped completely
cu
-- 
Mit freundlichen Grüßen / Kind regards 

Enrico Weigelt 
VNC - Virtual Network Consult GmbH 
Head Of Development 

Pariser Platz 4a, D-10117 Berlin
Tel.: +49 (30) 3464615-20
Fax: +49 (30) 3464615-59

enrico.weigelt@vnc.biz; www.vnc.de
Sitaram Chamarty· Nov 18, 2012, 10:42 UTC · re: Enrico Weigelt · lore

Re: Local clones aka forks disk size optimization

On Fri, Nov 16, 2012 at 11:34 PM, Enrico Weigelt <enrico.weigelt@vnc.biz> wrote:
Show 8 quoted lines
>
>> Provide one "main" clone which is bare, pulls automatically, and is
>> there to stay (no pruning), so that all others can use that as a
>> reliable alternates source.
>
> The problem here, IMHO, is the assumption, that the main repo will
> never be cleaned up. But what to do if you dont wanna let it grow
> forever ?

That's not the only problem. I believe you only get the savings when the main repo gets the commits first. Which is probably ok most of the time but it's worth mentioning.

>
> hmm, distributed GC is a tricky problem.

Except for one little issue (see other thread, subject line "cloning a namespace downloads all the objects"), namespaces appear to do everything we want in terms of the typical use cases for alternates, and/or 'git clone -l', at least on the server side.

Enrico Weigelt· Nov 18, 2012, 17:02 UTC · re: Sitaram Chamarty · lore

Re: Local clones aka forks disk size optimization

Hi,
> That's not the only problem.  I believe you only get the savings when
> the main repo gets the commits first.  Which is probably ok most of
> the time but it's worth mentioning.

Well, the saving will just be deferred to the point where the commit finally went to the main repo and downstreams are gc'ed.

Show 7 quoted lines
> > hmm, distributed GC is a tricky problem.
> 
> Except for one little issue (see other thread, subject line "cloning
> a
> namespace downloads all the objects"), namespaces appear to do
> everything we want in terms of the typical use cases for alternates,
> and/or 'git clone -l', at least on the server side.

hmm, not sure about the actual internals, but that namespace filtering should work in a way that local clone should never see (or consider) remote refs that are outside of the requested namespace. Perhaps that should be handled entirely on server side, so all called commands treat these refs as nonexisting.

By the way: what happens if one tries to clone from an broken repo (which has several refs pointing to nonexisting objects ?

cu
-- 
Mit freundlichen Grüßen / Kind regards 

Enrico Weigelt 
VNC - Virtual Network Consult GmbH 
Head Of Development 

Pariser Platz 4a, D-10117 Berlin
Tel.: +49 (30) 3464615-20
Fax: +49 (30) 3464615-59

enrico.weigelt@vnc.biz; www.vnc.de
Pyeron, Jason J CTR (US)· Nov 16, 2012, 14:55 UTC · re: Javier Domingo · lore

RE: Local clones aka forks disk size optimization

Show 12 quoted lines
> -----Original Message-----
> From: Javier Domingo
> Sent: Wednesday, November 14, 2012 8:15 PM
> 
> Hi Andrew,
> 
> Doing this would require I got tracked which one comes from which. So
> it would imply some logic (and db) over it. With the hardlinking way,
> it wouldn't require anything. The idea is that you don't have to do
> anything else in the server.
> 
> I understand that it would be imposible to do it for windows users
Not true, it is a file system issue not an os issue. FAT does not support hard links, but ext2,3,4 and NTFS do.
> (but using cygwin), but for *nix ones yes...
> Javier Domingo
Jörg Rosenkranz· Nov 18, 2012, 17:18 UTC · lore

Re: Local clones aka forks disk size optimization

2012/11/15 Javier Domingo <javierdo1@gmail.com>
>
> Is there any way to avoid this? I mean, can something be done in git,
> that it checks for (when pulling) the same objects in the other forks?

I've been using git-new-workdir (https://github.com/git/git/blob/master/contrib/workdir/git-new-workdir) for a similar problem. Maybe that's what you're searching?

Joerg.

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