[wplug] NFS Availability Issues -- Linux HA for web (even NFS shared web data) v. traditional NFS home dirs

[Bryan J. Smith]

Brandon Poyner <bpoyner at gmail.com> wrote:
> If you're willing to consider increasing the complexity, I'll throw
> out the suggestion of DRBD + Linux HA.  I use them for high
> availability on the cheap and it works well.
>  ...  
> DRBD has never let me down, but Linux HA has once in 4 years.  The
> latest version of DRBD can actually permit both servers to write to
> the block device at the same time and some people use clustering
> filesystems on it (almost like cheap SAN disk storage, only without
> some of the best features of a SAN).

First off, I don't have any issue with DRBD per-se.  Yes, it's more
ideal to have true, multi-targettable storage, but I will leave that
out for the purposes of this discussion and DRBD.

Secondly, I've seen DRBD+HA work for _web_ applications where data is
shared over NFS mounts.  That's because _web_ applications "don't
leave files open."  They are stateless from the standpoint of file
locks and usage.

Now keeping those two things in mind ...

There are just some aspects where NFS really doesn't work with the
DRBD + Linux HA approach.  Beyond the fact that NFS wasn't designed
for it at all, the majority of people I've seen using this
configuration are using it, again, for web.  The web servers are not
only tuned into the Linux HA part, but they are _not_ "keeping files
open."

Traditional NFS clients, like those mounting their home directories,
"keep files open."  ;)

E.g., A common "workaround "is a "reboot" to severe the session and
then reconnect is not my idea of HA.  ;)

So I don't consider Linux HA "viable" for NFS failover of home
directories.  For web servers where you want to "share" data via NFS,
yes, it "does the job."  But not for workstations with home
directories that are "always mounted" and "always open" and, in some
cases, the only way to "reset state" is to reboot.  ;)


-- 
Bryan J. Smith   Professional, Technical Annoyance
b.j.smith at ieee.org    http://thebs413.blogspot.com
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