Efficient handling of sparse files

[Matthew Wilcox <matthew@wil.cx>]

This problem came up with the systemimager program which uses rsync to
install files from a master server to many clients.  Red Hat has a system
user with uid 2^32-1 which causes lastlog to grow to 1.2GB in size.
rsync does understand the concept of sparse files (with the -S flag), but
it has to read every block to discover that it is indeed empty.  This sucks.

I was wondering if we could introduce a new system call (or ioctl?) that,
given an fd would find the next block with data in it.  We could use the
->bmap method ... except that has dire warnings about adding new callers
and viro may soon be in testicle-gouging range.

One system interface hack would be to introduce lseek(fd, 0, SEEK_DATA)
... but without permission to reuse ->bmap for this purpose, it's
pointless to discuss user interfaces.

Suggestions?

-- 
"Next the statesmen will invent cheap lies, putting the blame upon 
the nation that is attacked, and every man will be glad of those
conscience-soothing falsities, and will diligently study them, and refuse
to examine any refutations of them; and thus he will by and by convince 
himself that the war is just, and will thank God for the better sleep 
he enjoys after this process of grotesque self-deception." -- Mark Twain