On Sun, May 07, 2006 at 06:08:03PM +1200, Martin Langhoff wrote:
> >> And in any case commits and trees are lightweight and compress well...
> >Commit maybe, but is this based on a hard fact?
> No hard facts here :( but I think it's reasonable to assume that the
> trees delta/compress reasonably well, as a given commit will change
> just a few entries in each tree.
A few hard facts (using Linus' linux-2.6 tree):
- original packsize: 120996 kilobytes
- unpacked: 233338 objects, 1417476 kilobytes
This is an 11.7:1 compression ratio (of course, much of this is
wasted space from the 4k block size in the filesystem)
- There were 87915 total blob objects, of which 19321 were in the
current tree. I removed all non-current blobs to produce a "shallow"
tree.
- The shallow tree unpacked: 164744 objects, 761960 kilobytes
IOW, about half of the unpacked disk usage was old blobs.
- Shallow commit/tree/tag objects packed (using 1.3.1
git-pack-objects):
Total 164744, written 164744 (delta 92322), reused 0 (delta 0)
size: 108088
The compression ratio here is only 7.0:1
- Total savings by going shallow: 10.7%
So basically, trees and commits DON'T compress as well as historical
blobs (potentially because git-pack-objects isn't currently optimized
for this -- I haven't checked). As a result, we're saving only 10% by
going shallow instead of a potential 50%.
-Peff
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