On Fri, Jan 10, 2014 at 03:17:34PM -0800, Josh Berkus wrote:
> The purpose of sync rep is to know determinatively whether or not you
> have lost data when disaster strikes. If knowing for certain isn't
> important to you, then use async.
>
> BTW, people are using RAID1 as an analogy to 2-node sync replication.
> That's a very bad analogy, because in RAID1 you have a *single*
> controller which is capable of determining if the disks are in a failed
> state or not, and this is all happening on a single node where things
> like network outages aren't a consideration. It's really not the same
> situation at all.
>
> Also, frankly, I absolutely can't count the number of times I've had to
> rescue a customer or family member who had RAID1 but wan't monitoring
> syslog, and so one of their disks had been down for months without them
> knowning it. Heck, I've done this myself.
>
> So ... the Filesystem geeks have already been through this. Filesystem
> clustering started out with systems like DRBD, which includes an
> auto-degrade option. However, DBRD with auto-degrade is widely
> considered untrustworthy and is a significant portion of why DBRD isn't
> trusted today.
>
> >From here, clustered filesystems went in two directions: RHCS added
> layers of monitoring and management to make auto-degrade a safer option
> than it is with DRBD (and still not the default option). Scalable
> clustered filesystems added N(M) quorum commit in order to support more
> than 2 nodes. Either of these courses are reasonable for us to pursue.
>
> What's a bad idea is adding an auto-degrade option without any tools to
> manage and monitor it, which is what this patch does by my reading. If
> I'm wrong, then someone can point it out to me.
Yes, my big take-away from the discussion is that informing the admin in
a durable way is a requirement for this degraded mode. You are right
that many ignore RAID degradation warnings, but with the warnings
heeded, degraded functionality can be useful.
-- Bruce Momjian <bruce@momjian.us> http://momjian.us EnterpriseDB
http://enterprisedb.com
+ Everyone has their own god. +