On Jan 16, 2009, at 9:02 AM, Ryan How wrote:
I've also wondered the same thing with restore times. Maybe we
should give it a test?. I know backing up my virtual machine images
(around 30GB single file) takes a LONG time to make the diff. a
couple of hours. But it isn't running on a super machine. That
essentially is the same as a database - big file with many small
changes. But I would assume that applying a diff would be much much
quicker than generating one because we aren't comparing a whole
file, we are just adding bits and removing bits in certain sections.
Without testing, I would draw the opposite conclusion. When generating
a diff, we are merely asking the filesystem to read the entire file.
When applying a diff, you need to read the entire file, and change it,
while writing it back to a new file. Most OS's do not store files in
any way such that "adding bits and removing bits in certain sections"
is an in-place operation. If you edit a file in OpenOffice, it reads
the whole file into memory, you make your one change, and then it
writes the whole file back out to disk as a temp file, delete the old
one, and rename. (Or just rename over existing one for atomicity, but
that doesn't work on every OS.)
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