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At a company I work for we have such a thing called "playlists" which are small files ~100-300 bytes each. There's about a million of them. About 100,000 of them get changed every hour. These playlists need to be uploaded to 10 other remote servers on different continents every hour and it needs to happen quick in under 2 mins ideally. It's very important that files that are deleted on the master are also deleted on all the replicas. We currently use Linux for our infrastructure.

I was thinking about trying rsync with the -W option to copy whole files without comparing contents. I haven't tried it yet but maybe people who have more experience with rsync could tell me if it's a viable option?

What other options are worth considering?

Update: I have chosen the lsyncd option as the answer but only because it was the most popular. Other suggested alternatives are also valid in their own way.

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Do you have a log indicating what files have been changed or deleted? –  Oliver Jun 15 '12 at 9:45
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If only the playlists were mysql records. You could then use database replication and get mysql to work out what's needed to be sent/received. –  Matt Jun 15 '12 at 10:04
    
@oliver we do. However then you need to trust that log meaning the code generating it must be correct and then you need custom code to process that log which also needs to be correct. I'd rather avoid in house built code to do it over something that has been extensively tested by the community. –  Zilvinas Jun 15 '12 at 10:31
    
Do you want the change to only get applied every hour? Or is instant replication also acceptable? –  faker Jun 15 '12 at 10:31
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Don't underestimate the time it takes rsync to work through a million files. Just try it and you will see what you are up to. If you have that log, use it or try any other of the proposed solutions. –  Oliver Jun 15 '12 at 11:20
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7 Answers

up vote 33 down vote accepted

Since instant updates are also acceptable, you could use lsyncd.
It watches directories (inotify) and will rsync changes to slaves.
At startup it will do a full rsync, so that will take some time, but after that only changes are transmitted.
Recursive watching of directories is possible, if a slave server is down the sync will be retried until it comes back.

If this is all in a single directory (or a static list of directories) you could also use incron.
The drawback there is that it does not allow recursive watching of folders and you need to implement the sync functionality yourself.

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Again a brilliant tip :) –  Zilvinas Jun 15 '12 at 11:19
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+1 This is essentially a cache coherency problem, a monitor that pushes changes is the easiest solution. lsyncd implements that... –  Chris S Jun 15 '12 at 14:14
    
I would investigate lsyncd and inotify deeply as applies to your specific server OS. There is a limit on the number of inotify watches available. I believe the default is around 1500 or 8000 depending on your particular Linux version. Most kernels let you raise the limit, but monitoring 1 million files may be more than is practical. It didn't work for me in 2008. Also, the inotify event queue can overflow causing you to lose events, and you need to have a way to recover from that. A carefully tuned lsyncd implementation plus a daily rsync might work now in 2012 to cover your bases. –  Old Pro Jun 16 '12 at 6:07
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Actually it does an iontify on the directory not the individual files. How many directories can you watch? Check /proc/sys/fs/inotify/max_user_watches (usually 8192). –  faker Jun 16 '12 at 12:54
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With ~50k directories inotify will quite possibly not scale well. When we tried a similar approach in 2009 with 100k directories, it took the kernel way to long to subscribe to all the directories. As for @OldPro it did not work for us. –  neovatar Jun 19 '12 at 18:05
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Consider using a distributed filesystem, such as GlusterFS. Being designed with replication and parallelism in mind, GlusterFS may scale up to 10 servers much more smoothly than ad-hoc solutions involving inotify and rsync.

For this particular use-case, one could build a 10-server GlusterFS volume of 10 replicas (i.e. 1 replica/brick per server), so that each replica would be an exact mirror of every other replica in the volume. GlusterFS would automatically propagate filesystem updates to all replicas.

Clients in each location would contact their local server, so read access to files would be fast. The key question is whether write latency could be kept acceptably low. The only way to answer that is to try it.

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+1 for Glusterfs –  Tom O'Connor Jun 18 '12 at 15:54
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I doubt rsync would work for this in the normal way, because scanning a million files and comparing it to the remote system 10 times would take to long. I would try to implement a system with something like inotify that keeps a list of modified files and pushes them to the remote servers (if these changes don't get logged in another way anyway). You can then use this list to quickly identify the files required to be transferred - maybe even with rsync (or better 10 parallel instances of it).

Edit: With a little bit of work, you could even use this inotify/log watch approach to copy the files over as soon as the modification happens.

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Thanks for the inotify idea. Very interesting. –  Zilvinas Jun 15 '12 at 10:35
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Some more alternatives:

  • Insert a job into RabbitMQ or Gearman to asynchronously go off and delete (or add) the same file on all remote servers whenever you delete or add a file on the primary server.
  • Store the files in a database and use replication to keep the remote servers in sync.
  • If you have ZFS you can use ZFS replication.
  • Some SANs have file replication. I have no idea if this can be used over the Internet.
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This seems to be an ideal storybook use case for MongoDB and maybe GridFS. Since the files are relatively small, MongoDB alone should be enough, although it may be convenient to use the GridFS API.

MongoDB is a nosql database and GridFS is a file storage build on top of it. MongoDB has a lot of built in options for replication and sharding, so it should scale very well in your use case.

In your case you will probably start with a replica set which consists of the master located in your primary datacenter (maybe a second one, in case you want to failover on the same location) and your ten "slaves" distributed around the world. Then do load tests to check if the write performance is enough and check the replication times to your nodes. If you need more performace, you could turn the setup into a sharded one (mostly to distribute the write load to more servers). MongoDB has been designed with scaling up huge setups with "cheap" hardware, so you can throw in a batch of inexpensive servers to improve performance.

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I would use an S3 Backend and then just mount that on all the servers that I need - That way, everyone is in sync instantly anyway

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While the storage would be synchronized you'd have to notify the application, so you'd be back to square one, or the app would have to poll storage every time someone access on of these playlists. Performance would be horrible in either case. –  Chris S Jun 15 '12 at 14:12
    
The application doesn't need to poll the storage every time someone accesses the play lists, just enough times within the hour to ensure that the application is running without stale data. Also, if S3 is used as a backend, why would the application need to poll the files in the first place? They will always be up to date –  Mister IT Guru Jun 15 '12 at 15:52
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An option that doesn't appear to have been mentioned yet is to archive all the files into one compressed file. This should reduce the total size significantly and remove all the overhead you get from dealing with millions of individual files. By replacing the entire set of files in one big update you can also rest assured that removed files are removed on the replicas.

The downside is of course that you are transferring a many files unnecessarily. That may or may not be balanced out by the reduced size thanks to compression. Also I have no idea how long it would take to compress that many files.

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