[DRBD-user] DRBD + NFS

Dan Brown danb at zu.com
Tue Oct 10 19:47:17 CEST 2006

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

So I've got a couple of webservers which mirror each other without the use
of a fileserver backend.  Up until now they've been using csync2 to mirror
the data between the servers and it has been working fairly well (and the
built-in file backup has come in handy once or twice as well).  

Unfortunately csync2 has a good deal of limitations as well, such as the
more files you have, the slower it is, and you can't sync more than one
server to other servers (in this case only one) at the same time.  I'm
running it in a master/master mode.  Overall it was designed to be more of a
tool to update configs, or update relatively non-interactive systems.

So recently I've been looking more and more at DRBD and NFS (before I spend
a couple of grand on a well built fileserver).  So what I am looking at is
something like this:

server A (DRBD + active NFS)
  + mounted by itself and server B

server B (DRBD + disabled NFS)
  + mounts server A NFS share

So while server A is the active NFS server and server B is passive and only
synching, both servers would mount via NFS server A.  When server A goes
down for whatever reason (reboot, crash, etc) server B will take over and
continue where server A left off.   One thing I keep seeing in various
documents however is a warning not to have the server mount its own NFS
shares.  There is never reason (even a vague reason) given why not to
however.  I can see the obvious reasons (eg. infinitely nested filesystems
via symlinks/mounts, crossmounts, etc), but my directory structures should
not need anything like this at all.   Other than overall system complexity,
I don't see any reasons to not be able to self mount NFS with a whole lot of

The problems I see include Apache log files (I'd assume each server would
have to write to separate logs), and what happens when the servers
disconnect from each other (eg. NIC failure) but are both still active and
both decide they are masters.  How are synchronization issues then resolved?

Dan Brown
danb at zu.com

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