New to MogileFS have a few questions

Mark Smith smitty at
Tue Jun 10 20:52:14 UTC 2008

> 1. How come i have to do a "mogadm fsck reset" before a "mogadm fsck
> start" in order to repair a file i corrupted/deleted? Is there any
> downside to this?

The fsck process starts at the beginning and runs to the end.  The
reset forces it to start back over at the beginning.  (This is so that
you can analyze the results of the fsck after it finishes going

99% sure about this one anyway, not TOO familiar with the fsck code nowadays.

> 2. If a disk fails, when i replace it, is MogileFS smart enough to
> finish reaping it before putting that node back into production?

Not automatically, no.  It doesn't know if a disk is down or dead.
When a disk dies, you do two things:

1) bring up a new disk as a NEW device ID (very important! do not
reuse device IDs!)
2) mark the old device as "dead"

This will tell MogileFS to start removing files from the old one, and
the replicator will then pick up on this and start replicating them
out according to the policy.

> 3. The content length checking works for files that are corrupted with
> wrong length. Are there any patches that apply MD5 capability to the
> database/fsck?

You can choose the fsck level, and the higher levels will do MD5 (or
SHA1, I forget) on the files on all devices and ensure that they
match.  Not too sure what happens if they don't.  :-)

> 4. Which version should i use for production on a Fedora 8 server? The
> SVN, the stable or the RPMs?

Depends on what functionality you need.  I always suggest the SVN
version, MogileFS has been fairly stable for a long time, it doesn't
get tons of new functionality or things that will break it.  But if
you feel safer, use the RPMs.

Hachi or Alan can correct me if I'm wrong, but I believe that Six
Apart is using the SVN version (or some revision that is non-packaged)
on their production stacks.

> 5. I've had "weird things" (sorry didn't document them) happen when
> I've used host-names in the configuration files. Should i stick to IPs
> instead?

I've always used IPs.  If nothing else, it reduces dependence on DNS
which is historically flaky in my experience.  (I don't care how good
of a stack you have, DNS always seems to fart.)

> 6. If i roll MogileFS out to 3 servers each with 3 hard drives, and
> set the class to 3 copies, will MogileFS put all 3 copies on one
> server, or one copy on each server?

Yep.  Well, it depends on the replication policy, but the typical
minimum replica count ones are all based on hosts.

Thanks for your interest in MogileFS.  Feel free to ask any questions
you might have, or run your setup/configuration by the list.  We're
generally pretty helpful and don't often bite (and if we do, we have
our shots).  :-)

Mark Smith / xb95
smitty at

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