Re: Mercurial vs Updated git HOWTO for kernel hackers

On Tue, Jun 28, 2005 at 06:23:09PM -0400, Sean wrote:

On Tue, June 28, 2005 6:14 pm, Matt Mackall said:

quoted

quoted

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.

quoted

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.