Commit 76cc3800 authored by Mike Hibler's avatar Mike Hibler

Add some notes about how we might use frisbee in the future.

We have talked about these in the past, and I just happened to be thinking
about it this morning.
parent f063e020
NEW FEATURES:
1. Accommodate new usage.
There are a couple of other places where we could make good use of
frisbee technology.
One is in the Emulab infrastructure for using frisbee!
Our current technique for running frisbee involves running a memory
filesystem based FreeBSD system. So each node downloads not only a
FreeBSD kernel but a multi-megabyte MFS image as well. We could
modify the FreeBSD bootloader (from which our initial pxeboot program
is derived) to support a frisbee client. In this way, multiple nodes
could download their kernels and MFSs more efficiently and we could
scale better. The big problem here is that the bootload uses the PXE
API for its network support and it isn't clear that the UDP-level
services provided support multicast. Lower-level (UNDI) services
do, so that might be a possibility. Another possibility is to use
stub "real" drivers say from NetBSD.
Another use is for distributing arbitrary files. In the Emulab
context this would be useful for implementing the features allowing
installation of tar and rpm files. We could multicast distribute
the files to all nodes in an experiment at once. A couple of things
are needed for this. One is a discovery protocol in frisbee itself.
A frisbee client should be able to contact a known address and ask
for an addr/port to use to download a specific file. While we can
get this info "out of band" (like we do now), it would be generally
useful to include this in frisbee itself. The other needed feature
is the ability for the frisbee server to generate images on the fly.
I would propose that it not bother with compression when creating the
image, it would only do the chunking. Three reasons for this: 1) it is
simpler and more efficient for the server, 2) the types of files I
envision us really wanting to distribute this way are already
compressed (tgz and rpm files), and 3) it makes out-of-order delivery
of blocks much easier. To elaborate on the final point, one of the
nice things about frisbee is that clients can request blocks in any
order at any time. For static images, the frisbee server can
handle this easily, every chunk is at a fixed offset in the image
file. If we were to dynamically generate compressed images in the
server, and the first request it got was for the final chunk of the
image, the server would have to compress/chunk the entire source
file before being able to present that chunk. By not compressing,
we know in advance, for any chunk, how much and which data will go
into that chunk. Note that we could just say that exactly 1MB of
data will go into every chunk and still compress that, but there
is no point since we pad out every chunk to 1MB.
Now one could imagine a super, caching frisbee server that creates
compressed images on the fly and caches them for later use. Perhaps
it would become more of a chunk server where it caches every chunk
it has ever fed up along with a hash of the chunk. Here when a client
requests a specific file, we send back not only the addr/port to use
for the request, we also send back a list of chunk hashes for every
chunk in the file. This is very like the HCP work at Stanford.
ENHANCEMENTS:
1. Better throttling of block re-requests at the client.
Currently we have a variable, redodelay, in the client which is a
......@@ -53,7 +112,7 @@
4. Multi-thread the frisbee server.
We can make our network output intervals more consistant if we
We can make our network output intervals more consistent if we
separate the disk reader from the network writer. This would have a
performance benefit for the imageunzip program which currently
combines the reader and decompresser having only a separate writer
......@@ -69,7 +128,7 @@
Maybe should be a multiple of 32 to ensure bitmap is a multiple of 4
in size. Large chunk issues: 1) more potential wasted space per chunk,
though mostly only in the last chunk, 2) It takes longer to accumulate
chunks at the client, potentially idling the decompesser and writer,
chunks at the client, potentially idling the decompresser and writer,
3) takes more space to accumulate chunks, allowing for fewer in progress
chunks. So maybe 1448B/blk * 768 blks/chunk == 1.06MB/chunk. PREQUEST
BlockMaps come down from 128 bytes to 96.
......
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