[OpenAFS] releasing ro volumes with small deltas

Olin Shivers" <olin.afs.7ia@shivers.mail0.org Olin Shivers" <olin.afs.7ia@shivers.mail0.org
Mon, 30 Aug 2010 15:49:07 -0400 (EDT)

I have a question about releasing readonly volumes.

Suppose I have a read/write volume VRW on server 1. It is large -- VS bytes,
where VS is some number that you might find intimidatingly large. A couple
of terabytes, say.

I clone a readonly version of it, volume VRO. It takes up (almost) no extra
space over and above the storage needed for the orginal VRW volume.

I replicate VRO onto server 2. This induces a lot of net traffic -- I have to
copy over VS bytes of stuff.

Now I have VS bytes stored on server 1, and another VS bytes on server 2.

I then change *one* *tiny* file on read/write volume VRW. What I mean is:
every file on VRW is identical to its original state, except for a single
small file (say, a 1KB file). Now I re-release the readonly volume VRO,
to both partitions, on servers 1 & 2.

My question is: how much data gets shipped over the net to update volume VRO
on server 2? Does server 1 ship the entire volume (VS bytes), or does it
ship just the changed file (about 1K bytes)?

I am interested in this, because I'm considering setting up a readonly volume
for my music collection. It'd be nice to have the whole thing replicated at
home -- then I would be insulated from network outages. If I buy a new CD and
rip it into my music volume, I then have to re-release the readonly volume
after adding the new album's worth of music. I don't want the *whole
collection* (about 900GB) of bits to get jammed through my pitiful cable modem
when I do this. I'd like half a gigabyte of bits (the new music, that is) to
get shipped over the net from the main server to my home-net's replica server.

My suspicion is that this ain't on, but I thought I'd ask here.