Re: [Moosefs-users] Fwd: Fwd: Backing up MFS metadata

[Michał B. <mic...@ge...>]

 

From: Stas Oskin [mailto:sta...@gm...] 
Sent: Monday, August 09, 2010 5:18 PM
To: moosefs-users
Subject: [Moosefs-users] Fwd: Fwd: Backing up MFS metadata

 

Hi.

After reading the "Metadata files ins and outs" article, and this email
thread, I'm still left with several questions that I would like to clarify:

1) Does mfsmaster dumps to checkpoint files an up-to-date representation of
the memory, at periods of 1-2 minutes maximum?
Meaning, backing up the /var/mfs directory and restoring it back, will
provide me with a 1-2 minutes maximum of data loss?

[MB] The "mfsmaster.mfs.back" file by default is saved every hour but
"changelog.0.mfs" is saved on the fly so the delay is just in milliseconds
needed for the writing process. So the data loss of 1-2 minutes would be
really the most negative scenario. 



2) Does metalogger pull an up-to-date version of master checkpoint files as
well? If I move the master to metalogger and restore the checkpoint files,
it will provide me with a 1-2 minutes maximum of data loss as well?

[MB] This delay should also be close to 0 (on condition that metalogger has
been working all the time). The file metadata_ml.mfs.back  is dumped every
24hours but changelogs are saved on the fly. Each change that master saves
to a file is also sent to metalogger. In case of the master server failure
you would lose data stuck in buffer of outgoing network interface, times
also in milliseconds.



3) Any benefit of lowering the default 24 hours download frequency? Or the
streamed updates from master to metalogger will be enough?

[MB] When the metalogger restarts you would have incomplete change logs
(because when it is down it doesn't save the data and after restart master
server doesn't send to it the not yet saved changes - but this behavior will
be changed soon). So if you set the frequency to eg. 6 hours you decrease
probability of "holes" in changelogs in case metalogger is down.



4) In case I do want to backup to external storage (NAS for example), which
would be the better source machine - master (sounds more logical) or
metalogger?

[MB] Yes, master.



5) If I have 3 metaloggers for example, how much load it will incur on the
master server?

[MB] For the master the load is really negligible.

 

If you have any other doubts please let us know.

 

 

Regards

Michal




Thanks in advance for answers, as it would clarify the backup topic for me
completely!

Regards.