Re: Mercurial vs Updated git HOWTO for kernel hackers

From: Matt Mackall (mpm_at_selenic.com)
Date: 06/29/05

  • Next message: Grzegorz Kulewski: "Re: [patch] latest inotify." 
    Date:	Tue, 28 Jun 2005 15:49:46 -0700
To: Sean <seanlkml@sympatico.ca>

    On Tue, Jun 28, 2005 at 06:23:09PM -0400, Sean wrote:
    > On Tue, June 28, 2005 6:14 pm, Matt Mackall said:
    > >> You can even have a setup where objects
    > >> are archived onto write-once media like DVD and still participate in a
    > >> live repository, where new objects are written to hard disk, but older
    > >> object are (automatically) sourced from the DVD.
    > >
    > > Have fun with that. It's an excellent way to destroy your DVD drive.
    >
    > Oh come on, stop the FUD. You pack all the objects up into a single pack
    > file (see new feature in Git) and you burn it _once_ to dvd or cdrom.

    And even as one big file, it will _still_ be layed out on disk in
    pessimal order.

    > > Git's completely structureless filename hashing pretty much guarantees
    > > that disk layout will degrade to worst-case random access behavior
    > > over time. Just walking through the 2000 commit blobs in the current
    > > tree can take minutes cold cache on a fast hard disk. Walking the 1700
    > > tree blobs in a given version takes quite a while too.
    > >
    > > Put that on a DVD and that could easily turn into hours of continuous
    > > seeking for a simple operation like checking out tip of tree.
    > >
    > > And as far as I know, ISO9660 and UDF don't really handle huge
    > > directories well. So if you try and put the whole kernel history (200k
    > > files, some huge number of directory blobs, and 30k-60k commit blobs)
    > > on a DVD, you'll be really hurting.
    > >
    > > Meanwhile the whole history (>30k changesets) with Mercurial fits on a
    > > regular CD, with reasonable directory sizes and I/O patterns.
    > >
    > > Not that it's really worth the trouble. It takes longer for me to burn
    > > an ISO image to disc than to download a complete kernel repo from
    > > kernel.org.
    > >
    >
    > Git is still developing, there will be new ways to seek and cache things
    > etc eventually that remove any performance issue.

    Again, have fun with that. Mercurial already went down this path a
    month ago, discovered it couldn't reasonably be fixed without
    abandoning the hashes as file name scheme, and changed repo layout.

    Git's going to have a much harder time as it's pretty solidly tied to
    lookup by contents hash. If you throw that out, you might as well use
    Mercurial. 

    -- 
Mathematics is the supreme nostalgia of our time.
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