-*- indented-text -*-
-Notes towards a new version of rsync
-Martin Pool <mbp@samba.org>
+Notes towards a new version of rsync
+Martin Pool <mbp@samba.org>, September 2001.
Good things about the current implementation:
- Fairly reliable.
- - The choice of runnning over a plain TCP socket or tunneling over
+ - The choice of running over a plain TCP socket or tunneling over
ssh.
- rsync operations are idempotent: you can always run the same
- You can easily push or pull simply by switching the order of
files.
+ - The "modules" system has some neat features compared to
+ e.g. Apache's per-directory configuration. In particular, because
+ you can set a userid and chroot directory, there is strong
+ protection between different modules. I haven't seen any calls
+ for a more flexible system.
+
Bad things about the current implementation:
hard to modify/extend
- Both the program and the protocol assume a single non-interactive
- one-way transfer
+ one-way transfer
- A list of all files are held in memory for the entire transfer,
which cripples scalability to large file trees
- Error messages can be cryptic.
+ - Default behaviour is not intuitive: in too many cases rsync will
+ happily do nothing. Perhaps -a should be the default?
+
+ - People get confused by trailing slashes, though it's hard to think
+ of another reasonable way to make this necessary distinction
+ between a directory and its contents.
+
Protocol philosophy:
Questionable features:
- These are neat, but not necessarily clean or worth preserving.
+ These are neat, but not necessarily clean or worth preserving.
- The remote rsync can be wrapped by some other program, such as in
tridge's rsync-mail scripts. The general feature of sending and
These don't really require architectural changes; they're just
something to keep in mind.
-
+
- Synchronize ACLs and extended attributes
- Anonymous servers should be efficient
Alternatively, as long as transfers are idempotent, we can just
restart the whole thing. [NFSv4]
- - Scripting support.
+ - Scripting support.
- Propagate atimes and do not modify them. This is very ugly on
Unix. It might be better to try to add O_NOATIME to kernels, and
call that.
- - VFS. Useful?
-
- Unicode. Probably just use UTF-8 for everything.
+ - Open authentication system. Can we use PAM? Is SASL an adequate
+ mapping of PAM to the network, or useful in some other way?
+
+ - Resume interrupted transfers without the --partial flag. We need
+ to leave the temporary file behind, and then know to use it. This
+ leaves a risk of large temporary files accumulating, which is not
+ good. Perhaps it should be off by default.
+
+ - tcpwrappers support. Should be trivial; can already be done
+ through tcpd or inetd.
+
+ - Socks support built in. It's not clear this is any better than
+ just linking against the socks library, though.
+
+ - When run over SSH, invoke with predictable command-line arguments,
+ so that people can restrict what commands sshd will run. (Is this
+ really required?)
+
+ - Comparison mode: give a list of which files are new, gone, or
+ different. Set return code depending on whether anything has
+ changed.
+
+ - Internationalized messages (gettext?)
+
+ - Optionally use real regexps rather than globs?
+
+ - Show overall progress. Pretty hard to do, especially if we insist
+ on not scanning the directory tree up front.
+
+
+Regression testing:
+
+ - Support automatic testing.
+
+ - Have hard internal timeouts against hangs.
+
+ - Be deterministic.
+
+ - Measure performance.
+
Hard links:
become known.
+Command-line options:
+
+ We have rather a lot at the moment. We might get more if the tool
+ becomes more flexible. Do we need a .rc or configuration file?
+ That wouldn't really fit with its pattern of use: cp and tar don't
+ have them, though ssh does.
+
+
Scripting issues:
- Perhaps support multiple scripting languages: candidates include
it's not running in the users own account. So we can either
disallow it, or use some kind of sandbox system.
+ - Python is a good language, but the syntax is not so good for
+ giving small fragments on the command line.
+
+ - Tcl is broken Lisp.
+
+ - Lots of sysadmins know Perl, though Perl can give some bizarre or
+ confusing errors. The built in stat operators and regexps might
+ be useful.
+
+ - Sadly probably not enough people know Scheme.
+
+ - sh is hard to embed.
+
Scripting hooks:
- What basis file to use
- Logging
-
+
- Whether to allow transfers (for public servers)
- Authentication
- Locking
+ - Cache
+
+ - Generating backup path/name.
+
+ - Post-processing of backups, e.g. to do compression.
+
+ - After transfer, before replacement: so that we can spit out a diff
+ of what was changed, or kick off some kind of reconciliation
+ process.
+
+
+VFS:
+
+ Rather than talking straight to the filesystem, rsyncd talks through
+ an internal API. Samba has one. Is it useful?
+
+ - Could be a tidy way to implement cached signatures.
+
+ - Keep files compressed on disk?
+
Interactive interface:
- The standalone process needs to produce output in a form easily
digestible by a calling program, like the --emacs feature some
- have.
+ have. Same goes for output: rpm outputs a series of hash symbols,
+ which are easier for a GUI to handle than "\r30% complete"
+ strings.
- Yow! emacs support. (You could probably build that already, of
- course.)
+ course.) I'd like to be able to write a simple script on a remote
+ machine that rsyncs it to my workstation, edits it there, then
+ pushes it back up.
Pie-in-the-sky features:
These might have a severe impact on the protocol, and are not
clearly in our core requirements. It looks like in many of them
- having scripting hooks will allow us
+ having scripting hooks will allow us
- Transport over UDP multicast. The hard part is handling multiple
destinations which have different basis files. We can look at
with replication in place, though on some systems we will also
have to do I/O on block boundaries.
+ - Peer to peer features. Flavour of the year. Can we think about
+ ways for clients to smoothly and voluntarily become servers for
+ content they receive?
+
+ - Imagine a situation where the destination has a much faster link
+ to the cloud than the source. In this case, Mojo Nation downloads
+ interleaved blocks from several slower servers. The general
+ situation might be a way for a master rsync process to farm out
+ tasks to several subjobs. In this particular case they'd need
+ different sockets. This might be related to multicast.
+
+
+Unlikely features:
+
+ - Allow remote source and destination. If this can be cleanly
+ designed into the protocol, perhaps with the remote machine acting
+ as a kind of echo, then it's good. It's uncommon enough that we
+ don't want to shape the whole protocol around it, though.
+
+ In fact, in a triangle of machines there are two possibilities:
+ all traffic passes from remote1 to remote2 through local, or local
+ just sets up the transfer and then remote1 talks to remote2. FTP
+ supports the second but it's not clearly good. There are some
+ security problems with being able to instruct one machine to open
+ a connection to another.
+
In favour of evolving the protocol:
- If we start from scratch, it can be documented as we go, and we
can avoid design decisions that make the protocol complex or
- implementation-bound.
+ implementation-bound.
Error handling:
- We can do nonblocking network IO, but not so for disk.
- It makes sense to on the destination be generating signatures and
- applying patches at the same time.
+ applying patches at the same time.
- Can structure this with nonblocking, threads, separate processes,
etc.
http://www.ietf.org/proceedings/00jul/00july-133.htm#P24510_1276764
- Sync with PDA
-
+
- Network backup systems
- CVS filemover
would be useful.
-Moved files:
+Moved files: <http://rsync.samba.org/cgi-bin/rsync.fom?file=44>
- There's no trivial way to detect renamed files, especially if they
move between directories.
Filesystem migration:
+ NFSv4 probably wants to migrate file locks, but that's not really
+ our problem.
+
+
+Atomic updates:
+
The NFSv4 working group wants atomic migration. Most of the
- responsibility for this lies on the NFS server or OS.
+ responsibility for this lies on the NFS server or OS.
If migrating a whole tree, then we could do a nearly-atomic rename
at the end. This ties in to having separate basis and destination
files.
- NFSv4 probably wants to migrate file locks, but that's not really
- our problem.
+ There's no way in Unix to replace a whole set of files atomically.
+ However, if we get them all onto the destination machine and then do
+ the updates quickly it would greatly reduce the window.
Scalability:
-
+
We should aim to work well on machines in use in a year or two.
That probably means transfers of many millions of files in one
batch, and gigabytes or terabytes of data.
On the whole CPU usage is not normally a limiting factor, if only
because running over SSH burns a lot of cycles on encryption.
+ Perhaps have resource throttling without relying on rlimit.
+
Streaming:
pipelined. This is a problem with FTP, and NFS (at least up to
v3). NFSv4 can pipeline operations, but building on that is
probably a bit complicated.
+
+
+Related work:
+
+ - mirror.pl http://freshmeat.net/project/mirror/
+
+ - ProFTPd
+
+ - Apache
+
+ - http://freshmeat.net/search/?site=Freshmeat&q=mirror§ion=projects
+
+ - BitTorrent -- p2p mirroring
+ http://bitconjurer.org/BitTorrent/
Matt McCutchen's Web Site / rsync / rsync.git / blobdiff