Options for synchronising filesystems
filip at wuytack.net
Mon Sep 26 05:25:37 PDT 2005
Eric Anderson wrote:
> Brian Candler wrote:
>> I was wondering if anyone would care to share their experiences in
>> synchronising filesystems across a number of nodes in a cluster. I can
>> of a number of options, but before changing what I'm doing at the
>> moment I'd
>> like to see if anyone has good experiences with any of the others.
>> The application: a clustered webserver. The users' CGIs run in a chroot
>> environment, and these clearly need to be identical (otherwise a CGI
>> on one box would behave differently when running on a different box).
>> Ultimately I'd like to synchronise the host OS on each server too.
>> Note that this is a single-master, multiple-slave type of filesystem
>> synchronisation I'm interested in.
>> 1. Keep a master image on an admin box, and rsync it out to the frontends
>> This is what I'm doing at the moment. Install a master image in
>> /webroot/cgi, add packages there (chroot /webroot/cgi pkg_add ...), and
>> rsync it. [Actually I'm exporting it using NFS, and the frontends run
>> locally when required to update their local copies against the NFS
>> - rsyncing a couple of gigs of data is not particularly fast, even
>> when only
>> a few files have changed
>> - if a sysadmin (wrongly) changes a file on a front-end instead of on the
>> master copy in the admin box, then the change will be lost when the next
>> rsync occurs. They might think they've fixed a problem, and then (say) 24
>> hours later their change is wiped. However if this is a config file, the
>> fact that the old file has been reinstated might not be noticed until the
>> daemon is restarted or the box rebooted - maybe months later. This I
>> is the biggest fundamental problem.
>> - files can be added locally and they will remain indefinitely (unless we
>> use rsync --delete which is a bit scary). If this is done then adding
>> a new
>> machine into the cluster by rsyncing from the master will not pick up
>> extra files.
>> So, here are the alternatives I'm considering, and I'd welcome any
>> additional suggestions too.
> Here's a few ideas on this: do multiple rsyncs, one for each top level
> directory. That might speed up your total rsync process. Another
> similar method is using a content revisioning system. This is only good
> for some cases, but something like subversion might work ok here.
>> 2. Run the images directly off NFS
>> I've had this running before, even the entire O/S, and it works just
>> However the NFS server itself then becomes a critical
>> single-point-of-failure: if it has to be rebooted and is out of
>> service for
>> 2 minutes, then the whole cluster is out of service for that time.
>> I think this is only feasible if I can build a highly-available NFS
>> which really means a pair of boxes serving the same data. Since the
>> image is read-only from the point of view of the frontends, this
>> should be
>> easy enough:
>> frontends frontends
>> | | | | | |
>> NFS -----------> NFS
>> server 1 sync server 2
>> As far as I know, NFS clients don't support the idea of failing over from
>> one server to another, so I'd have to make a server pair which
>> fails over.
>> I could make one NFS server take over the other server's IP address using
>> carp or vrrp. However, I suspect that the clients might notice. I know
>> NFS is 'stateless' in the sense that a server can be rebooted, but for a
>> client to be redirected from one server to the other, I expect that these
>> filesytems would have to be *identical*, down to the level of the inode
>> numbers being the same.
>> If that's true, then rsync between the two NFS servers won't cut it. I
>> thinking of perhaps using geom_mirror plus ggated/ggatec to make a
>> block-identical read-only mirror image on NFS server 2 - this also has
>> advantage that any updates are close to instantaneous.
>> What worries me here is how NFS server 2, which has the mirrored
>> mounted read-only, will take to having the data changed under its
>> nose. Does
>> it for example keep caches of inodes in memory, and what would happen if
>> those inodes on disk were to change? I guess I can always just unmount
>> remount the filesystem on NFS server 2 after each change.
> I've tried doing something similar. I used fiber attached storage, and
> had multiple hosts mounting the same partition. It seemed as though
> when host A mounted the filesystem read-write, and then host B mounted
> it read-only, any changes made by host A were not seen by B, and even
> remounting did not always bring it up to current state. I believe it
> has to do with the buffer cache and host A's desire to keep things (like
> inode changes, block maps, etc) in cache and not write them to disk.
> FreeBSD does not currently have a multi-system cache coherency protocol
> to distribute that information to other hosts. This is something I
> think would be very useful for many people. I suppose you could just
> mount the filesystem when you know a change has happened, but you still
> may not see the change. Maybe mounting the filesystem on host A with
> the sync option would help.
>> My other concern is about susceptibility to DoS-type attacks: if one
>> frontend were to go haywire and start hammering the NFS servers really
>> it could impact on all the other machines in the cluster.
>> However, the problems of data synchronisation are solved: any change
>> made on
>> the NFS server is visible identically to all front-ends, and sysadmins
>> make changes on the front-ends because the NFS export is read-only.
> This was my first thought too, and a highly available NFS server is
> something any NFS heavy installation wants (needs). There are a few
> implementations of clustered filesystems out there, but non for FreeBSD
> (yet). What that allows is multiple machines talking to a shared
> storage with read/write access. Very handy, but since you only need
> read-only access, I think your problem is much simpler, and you can get
> away with a lot less.
>> 3. Use a network distributed filesystem - CODA? AFS?
>> If each frontend were to access the filesystem as a read-only network
>> but have a local copy to work with in the case of disconnected operation,
>> then the SPOF of an NFS server would be eliminated.
>> However, I have no experience with CODA, and although it's been in the
>> since 2002, the README's don't inspire confidence:
>> "It is mostly working, but hasn't been run long enough to be sure
>> all the
>> bugs are sorted out. ... This code is not SMP ready"
>> Also, a local cache is no good if the data you want during disconnected
>> operation is not in the cache at that time, which I think means this
>> idea is
>> not actually a very good one.
> There is also a port for coda. I've been reading about this, and it's
> an interesting filesystem, but I'm just not sure of it's usefulness yet.
>> 4. Mount filesystems read-only
>> On each front-end I could store /webroot/cgi on a filesystem mounted
>> read-only to prevent tampering (as long as the sysadmin doesn't
>> remount it
>> read-write of course). That would work reasonably well, except that being
>> mounted read-only I couldn't use rsync to update it!
>> It might also work with geom_mirror and ggated/ggatec, except for the
>> I raised before about changing blocks on a filesystem under the nose of a
>> client who is actively reading from it.
> I suppose you could mount r/w only when doing the rsync, then switch
> back to ro once complete. You should be able to do this online, without
> any issues or taking the filesystem offline.
>> 5. Using a filesystem which really is read-only
>> Better tamper-protection could be had by keeping data in a filesystem
>> structure which doesn't support any updates at all - such as cd9660 or
>> The issue here is how to roll out a new version of the data. I could push
>> out a new filesystem image into a second partition, but it would then be
>> necessary to unmount the old filesystem and remount the new on the same
>> place, and you can't really unmount a filesystem which is in use. So this
>> would require a reboot.
>> I was thinking that some symlink trickery might help:
>> /webroot/cgi -> /webroot/cgi1
>> /webroot/cgi1 # filesystem A mounted here
>> /webroot/cgi2 # filesystem B mounted here
>> It should be possible to unmount /webroot/cgi2, dd in a new image,
>> it, and change the symlink to point to /webroot/cgi2. After a little
>> hopefully all the applications will stop using files in /webroot/cgi1, so
>> this one can be unmounted and a new one put in its place on the next
>> However this is not guaranteed, especially if there are long-lived
>> using binary images in this partition. You'd still have to stop and
>> all those processes.
>> If reboots were acceptable, then the filesystem image could also be
>> in ramdisk pulled in via pxeboot. This makes sense especially for
>> where the data is pre-compressed. However I would still prefer to avoid
>> frequent reboots if at all possible. Also, whilst a ramdisk might be
>> OK for
>> the root filesystem, a typical CGI environment (with perl, php, ruby,
>> python, and loads of libraries) would probably be too large anyway.
>> 6. Journaling filesystem replication
>> If the data were stored on a journaling filesystem on the master box, and
>> the journal logs were distributed out to the slaves, then they would all
>> have identical filesystem copies and only a minimal amount of data would
>> need to be pushed out to each machine on each change. (This would be
>> like NetApps and their snap-mirroring system). However I'm not aware
>> of any
>> journaling filesystem for FreeBSD, let alone whether it would support
>> filesystem replication in this way.
> There is a project underway for UFSJ (UFS journaling). Maybe once it
> is complete, and bugs are ironed out, one could implement a journal
> distribution piece to send the journal updates to multiple hosts and
> achieve what you are thinking, however, that only distributes the
> meta-data, and not the actual data.
Have a look at dragonfly BSD for this. They are working on a journaling
filesystem that will do just that.
> Good luck finding your ultimate solution!
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