Git as a filesystem

Git as a filesystem

[Peter Stahlir]

Hi!

Is it possible/feasible to use git as a filesystem?
Like having git on top of ext3.

This way I could do a gitfs-gc and there is only one
pack file sitting on the disk which is a compressed
version of the whole system.
I am not interested in a version controlled filesystem,
only in the space saving aspects.

Thanks,

Peter

Re: Git as a filesystem

[Johannes Schindelin]

Hi,

On Fri, 21 Sep 2007, Peter Stahlir wrote:

> Is it possible/feasible to use git as a filesystem?
> Like having git on top of ext3.

I haven't looked at it closely, but there is a GitFS:

http://git.or.cz/gitwiki/InterfacesFrontendsAndTools#head-f354b40618742b976c13700fe1fea28387ad5c89

(I am pointing you to the Git Wiki, so that you can find more pointers 
should you not be happy with this one.)

Ciao,
Dscho

Re: Git as a filesystem

[Peter Stahlir]

2007/9/21, Johannes Schindelin <Johannes.Schindelin@gmx.de>:
> On Fri, 21 Sep 2007, Peter Stahlir wrote:
>
> > Is it possible/feasible to use git as a filesystem?
> > Like having git on top of ext3.
>
> I haven't looked at it closely, but there is a GitFS:
>
> http://git.or.cz/gitwiki/InterfacesFrontendsAndTools#head-f354b40618742b976c13700fe1fea28387ad5c89
>
> (I am pointing you to the Git Wiki, so that you can find more pointers
> should you not be happy with this one.)

Thank you.
This is was I was looking for. My motivation is whether it is possible
to run a system, for example Debian on a computer on top of gitfs,
and then have a huge mirror on it, for example a complete 252GB
Debian mirror as space efficient as possible.

I wonder how big a deltified Debian mirror in one pack file would be. :)

Peter

Re: Git as a filesystem

[Karl Hasselström]

On 2007-09-21 13:41:07 +0200, Peter Stahlir wrote:

> My motivation is whether it is possible to run a system, for example
> Debian on a computer on top of gitfs, and then have a huge mirror on
> it, for example a complete 252GB Debian mirror as space efficient as
> possible.
>
> I wonder how big a deltified Debian mirror in one pack file would
> be. :)

Very, very close to 252 GB, since .deb files are already compressed.

If it's just the gzip compression you want, surely there must be real
filesystems that can do that.

-- 
Karl Hasselström, kha@treskal.com
      www.treskal.com/kalle

Re: Git as a filesystem

[Peter Stahlir]

> > I wonder how big a deltified Debian mirror in one pack file would
> > be. :)
>
> Very, very close to 252 GB, since .deb files are already compressed.

Yes, but if there were deb and tar support in git (to automatically unpack
archives and store the contents), together with the best available
binary diffs I think the repository could be significantly smaller because
files common to all architectures could be deltified,

I did a quick check with 100MB of deb archives; the result was nearly 100MB
as you said.
I also did a quick check with all .so files in my /usr/lib directory; it shrunk
from 50MB to 20MB, the same is achieved with tar + bz2.

But the thing is, I think there is a lot of redundancy in
a) a Debian mirror or
b) your disk at home.

Telling git to handle -for example- deb archives and storing
everything in a pack file would take advantage of redundancy across
_all_ files.
So the /usr/share/doc of all architectures could be compressed.

Right?

Re: Git as a filesystem

[Michael Poole]

Peter Stahlir writes:

> Telling git to handle -for example- deb archives and storing
> everything in a pack file would take advantage of redundancy across
> _all_ files.
> So the /usr/share/doc of all architectures could be compressed.
>
> Right?

You're proposing to trade off lots of CPU time in fetching many files
from a pack and making the package file -- paid every time someone
requests a package -- for at most 250 GB of space (cf Amdahl's law).

How long are your users willing to wait in exchange for 250 GB of
saved space?  How much CPU are you willing to spend for it?  Compare
those to the cost of a 300 GB hard drive (roughly $65).

There's also the cost to make git support the package format, and to
maintain that code going forward.  Those costs are also large.

Michael Poole

Re: Git as a filesystem

[jlh]

Peter Stahlir wrote:
> But the thing is, I think there is a lot of redundancy in
> a) a Debian mirror or

Yes, surely.  Your idea suggests that you want any file to be
reconstructed on-the-fly whenever it's being requested.  Isn't
there the danger of killing performance, the CPU being the
bottleneck?  I imagine such a debian mirror has quite some
traffic.

> b) your disk at home.

I doubt so.  There sure is lots of redundancy within each file and
that's what compressed file systems are good for.  But what you
talk about is redundancy across (unversioned) files, and I don't
feel there is a lot of it.  Yes, I might have a few copies of the
file COPYING on my disk, and maybe some of my sources share a few
functions, but this won't save me tons of space.  All my binaries,
libraries, MP3s, videos, config files, etc don't really have any
redundancy across file boundaries.  And even if there is, finding
that redundancy is an O(whatever-but-not-n) operation that would
be rather slow.

I definitely see gitfs (or similar ideas) as potentially being
useful in some cases (maybe debian mirrors could be one), but not
for my disk at home, which I generally would prefer to be faster
than more compressed.

jlh

Re: Git as a filesystem

[Dmitry Potapov]

On Fri, Sep 21, 2007 at 03:28:20PM +0200, Peter Stahlir wrote:
> Yes, but if there were deb and tar support in git (to automatically unpack
> archives and store the contents), together with the best available
> binary diffs I think the repository could be significantly smaller because
> files common to all architectures could be deltified,

You can unpack contain of gzipped or bzipped files and deltify it, but
you cannot restore exactly the same gzip or bzip file based on its
content unless you use exactly the same version of compressor that was
used to create the original file. So, if you put any .deb file in such
a system, you will get back a different .deb file (with a different SHA1).
So, aside high CPU and memory requirements, this system cannot work in
principle unless all users have exactly the same version of a compressor.

Dmitry

Re: Git as a filesystem

[Martin Langhoff]

On 9/22/07, Dmitry Potapov <dpotapov@gmail.com> wrote:
> used to create the original file. So, if you put any .deb file in such
> a system, you will get back a different .deb file (with a different SHA1).
> So, aside high CPU and memory requirements, this system cannot work in
> principle unless all users have exactly the same version of a compressor.

Was thinking the same - compression machinery, ordering of the files,
everything. It'd be a nightmare to ensure you get back the same .deb,
without a single different bit.

Debian packaging toolchain could be reworked to use a more GIT-like
approach - off the top of my head, at least

  - signing/validating the "tree" of the package rather than the
completed package could allow the savings in distribution you mention,
decouple the signing from the compression, and simplify things like
debdiff

  - git or git-like strategies for source packages

cheers,


m

Re: Git as a filesystem

[Sam Vilain]

Martin Langhoff wrote:
> On 9/22/07, Dmitry Potapov <dpotapov@gmail.com> wrote:
>   
>> used to create the original file. So, if you put any .deb file in such
>> a system, you will get back a different .deb file (with a different SHA1).
>> So, aside high CPU and memory requirements, this system cannot work in
>> principle unless all users have exactly the same version of a compressor.
>>     
>
> Was thinking the same - compression machinery, ordering of the files,
> everything. It'd be a nightmare to ensure you get back the same .deb,
> without a single different bit.
>
> Debian packaging toolchain could be reworked to use a more GIT-like
> approach - off the top of my head, at least
>
>   - signing/validating the "tree" of the package rather than the
> completed package could allow the savings in distribution you mention,
> decouple the signing from the compression, and simplify things like
> debdiff
>
>   - git or git-like strategies for source packages
>   

Nightmare indeed.  I actually wrote a proof of concept for this idea for
gzip.

http://git.catalyst.net.nz/gw?p=git.git;a=shortlog;h=archive-blobs
(see also
http://planet.catalyst.net.nz/blog/2006/07/17/samv/xteddy_caught_consuming_rampant_amounts_of_disk_space)

I usually warn people that this undertaking is "slightly insane".

My implementation was designed to be called like "git-hash-object". 
What it did was look at the input stream, and detect quickly whether it
looked like a gzip stream.  If it was, it would decompress it and then
try to compress the first few blocks using different compression
libraries and settings to determine what settings were used.  If it
could find the right settings for the first meg or so, then it would
bank on the rest being identical as well, record which compressor and
what settings were used and write the uncompressed object, as well as
the information needed to reconstruct the gzip header, to a new type of
object called an "archive" object.  If the stream could not be
reproduced then it would save the raw stream instead.  For something
like a Debian archive, it is very likely that all compressed streams
will be reproducible, because they will almost all be compressed using
the same implementation of gzip.

For tar and .ar files, this can be slightly more deterministic of
course.  It doesn't even need to be particularly savvy of what all the
fields are - just locate the files in the .tar, write out a tree, and
then write a TOC that lists tree entries and contains any extra data (ie
headers, etc).

In hindsight, making a new object type was probably a mistake.  If I
were to re-undertake this I would not go down that path, though I'd
certainly consider using tag objects for the extra data, and throwing
them in the tree like submodules.  It would also be essential in a
"real" solution to bundle reference copies of the zlib and gzip
compressors (yes, their output streams differ with longer inputs and
even some short ones).

Sam.

Re: Git as a filesystem

[Nicolas Pitre]

On Fri, 21 Sep 2007, Peter Stahlir wrote:

> This is was I was looking for. My motivation is whether it is possible
> to run a system, for example Debian on a computer on top of gitfs,
> and then have a huge mirror on it, for example a complete 252GB
> Debian mirror as space efficient as possible.
> 
> I wonder how big a deltified Debian mirror in one pack file would be. :)

It would be just as big as the non gitified storage on disk.

The space saving with git comes from efficient delta storage of 
_versioned_ files, i.e. multiple nearly identical versions of the same 
file where the stored delta is only the small difference between the 
first full version and subsequent versions.  Unless you plan on storing 
many different Debian versions together, you won't benefit from any 
delta at all. And since Debian packages are already compressed, git 
won't be able to compress them further.

So don't waste your time.


Nicolas

Re: Git as a filesystem

[Peter Stahlir]

> > I wonder how big a deltified Debian mirror in one pack file would be. :)
>
> It would be just as big as the non gitified storage on disk.
>
> The space saving with git comes from efficient delta storage of
> _versioned_ files, i.e. multiple nearly identical versions of the same
> file where the stored delta is only the small difference between the
> first full version and subsequent versions.  Unless you plan on storing
> many different Debian versions together, you won't benefit from any
> delta at all. And since Debian packages are already compressed, git
> won't be able to compress them further.
>
> So don't waste your time.

The 252GB stem from the fact that there are more than 10 architectures.
I guess the /usr/share/doc of all architectures could be deltified (as could
be all files that are architecture-independent)

Right?

Re: Git as a filesystem

[Nicolas Pitre]

On Fri, 21 Sep 2007, Peter Stahlir wrote:

> > > I wonder how big a deltified Debian mirror in one pack file would be. :)
> >
> > It would be just as big as the non gitified storage on disk.
> >
> > The space saving with git comes from efficient delta storage of
> > _versioned_ files, i.e. multiple nearly identical versions of the same
> > file where the stored delta is only the small difference between the
> > first full version and subsequent versions.  Unless you plan on storing
> > many different Debian versions together, you won't benefit from any
> > delta at all. And since Debian packages are already compressed, git
> > won't be able to compress them further.
> >
> > So don't waste your time.
> 
> The 252GB stem from the fact that there are more than 10 architectures.
> I guess the /usr/share/doc of all architectures could be deltified (as could
> be all files that are architecture-independent)
> 
> Right?

Indeed.

But how much does this represents, once compressed, compared to the 
rest?  I doubt it is significant enough for the trouble.


Nicolas

Re: Git as a filesystem

[Christian von Kietzell]

Am Freitag, den 21.09.2007, 15:35 +0200 schrieb Peter Stahlir:
> > > I wonder how big a deltified Debian mirror in one pack file would be. :)
> >
> > It would be just as big as the non gitified storage on disk.
> >
> > The space saving with git comes from efficient delta storage of
> > _versioned_ files, i.e. multiple nearly identical versions of the same
> > file where the stored delta is only the small difference between the
> > first full version and subsequent versions.  Unless you plan on storing
> > many different Debian versions together, you won't benefit from any
> > delta at all. And since Debian packages are already compressed, git
> > won't be able to compress them further.
> >
> > So don't waste your time.
> 
> The 252GB stem from the fact that there are more than 10 architectures.
> I guess the /usr/share/doc of all architectures could be deltified (as could
> be all files that are architecture-independent)
> 
> Right?

I don't think so. Architecture-independent files are usually separated
out into separate packages (think of the -doc and -data packages) that
get architecture "all" and land in the Debian archive only once. So you
probably won't save too much there.


  Chris

Re: Git as a filesystem

[Eric Wong]

Nicolas Pitre <nico@cam.org> wrote:
> On Fri, 21 Sep 2007, Peter Stahlir wrote:
> 
> > This is was I was looking for. My motivation is whether it is possible
> > to run a system, for example Debian on a computer on top of gitfs,
> > and then have a huge mirror on it, for example a complete 252GB
> > Debian mirror as space efficient as possible.
> > 
> > I wonder how big a deltified Debian mirror in one pack file would be. :)
> 
> It would be just as big as the non gitified storage on disk.
> 
> The space saving with git comes from efficient delta storage of 
> _versioned_ files, i.e. multiple nearly identical versions of the same 
> file where the stored delta is only the small difference between the 
> first full version and subsequent versions.  Unless you plan on storing 
> many different Debian versions together, you won't benefit from any 
> delta at all. And since Debian packages are already compressed, git 
> won't be able to compress them further.
> 
> So don't waste your time.

On a similar note, has anybody experimented with using git to
store maildirs or news spools?  I'd imagine the quoted portions of
most message threads could be delta-compressed quite efficiently.

-- 
Eric Wong

Re: Git as a filesystem

[Johannes Schindelin]

Hi,

On Fri, 21 Sep 2007, Eric Wong wrote:

> Nicolas Pitre <nico@cam.org> wrote:
> > On Fri, 21 Sep 2007, Peter Stahlir wrote:
> > 
> > > This is was I was looking for. My motivation is whether it is possible
> > > to run a system, for example Debian on a computer on top of gitfs,
> > > and then have a huge mirror on it, for example a complete 252GB
> > > Debian mirror as space efficient as possible.
> > > 
> > > I wonder how big a deltified Debian mirror in one pack file would be. :)
> > 
> > It would be just as big as the non gitified storage on disk.
> > 
> > The space saving with git comes from efficient delta storage of 
> > _versioned_ files, i.e. multiple nearly identical versions of the same 
> > file where the stored delta is only the small difference between the 
> > first full version and subsequent versions.  Unless you plan on storing 
> > many different Debian versions together, you won't benefit from any 
> > delta at all. And since Debian packages are already compressed, git 
> > won't be able to compress them further.
> > 
> > So don't waste your time.
> 
> On a similar note, has anybody experimented with using git to
> store maildirs or news spools?  I'd imagine the quoted portions of
> most message threads could be delta-compressed quite efficiently.

I store all my mail in a git repository.  Works beautifully.  Except that 
the buffers on my laptop are constantly full :-(  So a simple commit takes 
some waiting.

Should be no issue on normal (desktop) machines.

Ciao,
Dscho

Re: Git as a filesystem

[Eric Wong]

Johannes Schindelin <Johannes.Schindelin@gmx.de> wrote:
> Hi,

Hi,

> On Fri, 21 Sep 2007, Eric Wong wrote:
> > 
> > On a similar note, has anybody experimented with using git to
> > store maildirs or news spools?  I'd imagine the quoted portions of
> > most message threads could be delta-compressed quite efficiently.
> 
> I store all my mail in a git repository.  Works beautifully.  Except that 
> the buffers on my laptop are constantly full :-(  So a simple commit takes 
> some waiting.
> 
> Should be no issue on normal (desktop) machines.

D'oh.  I already have maildir performance problems on my laptop.

I wonder how well only having an index and no commits (no versioning),
and manual packing with pack-objects would work.  Packing could be
optimized to order objects based on the Message-Id, References, and
In-Reply-To headers, too.

-- 
Eric Wong

Re: Git as a filesystem

[Johannes Schindelin]

Hi,

On Fri, 21 Sep 2007, Eric Wong wrote:

> Johannes Schindelin <Johannes.Schindelin@gmx.de> wrote:
> 
> > On Fri, 21 Sep 2007, Eric Wong wrote:
> > > 
> > > On a similar note, has anybody experimented with using git to store 
> > > maildirs or news spools?  I'd imagine the quoted portions of most 
> > > message threads could be delta-compressed quite efficiently.
> > 
> > I store all my mail in a git repository.  Works beautifully.  Except 
> > that the buffers on my laptop are constantly full :-( So a simple 
> > commit takes some waiting.
> > 
> > Should be no issue on normal (desktop) machines.
> 
> D'oh.  I already have maildir performance problems on my laptop.

Umm.  Regular operation is not affected, since I (add and) commit only 
when I weeded out all those spams and other unwanted mail.

> I wonder how well only having an index and no commits (no versioning), 
> and manual packing with pack-objects would work.  Packing could be 
> optimized to order objects based on the Message-Id, References, and 
> In-Reply-To headers, too.

The most efficient way would be to have a mailer backend accessing the 
database, and then not have a working directory, methinks (especially with 
these amounts of mail I am juggling ATM).

Time forbids working on this, though.

Ciao,
Dscho

Re: Git as a filesystem

[Miklos Vajna]

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On Fri, Sep 21, 2007 at 12:11:41PM +0100, Johannes Schindelin <Johannes.Schindelin@gmx.de> wrote:
> I haven't looked at it closely, but there is a GitFS:
> 
> http://git.or.cz/gitwiki/InterfacesFrontendsAndTools#head-f354b40618742b976c13700fe1fea28387ad5c89
> 
> (I am pointing you to the Git Wiki, so that you can find more pointers 
> should you not be happy with this one.)

fyi, last time i had a look at it, it did not compile with git 1.5.2.x

thanks,
- VMiklos

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