From: Sammy (mesammy@gmail.com)
Date: Thu Jul 21 2005 - 06:42:57 EDT
Hello,
and HELP! - Im going mad trying to work out what is probably really a
very simple and common Disksuite scenario:
I am building a very simple system with two disks - one mirroring the other.
I came to the idea that providing I have 4 replica's - 2 on each disk
- the system should reboot if one of the mirrors (and hence 2 of the
metadb's) are lost.
However, after thinking further, what is the difference logically?
The reference guide says:
"The majority consensus algorithm is conservative in the
sense that it will fail if a majority consensus cannot be
reached, even if one replica actually does contain the most
up-to-date data. This approach guarantees that stale data
will not be accidentally used, regardless of the failure
scenario. The majority consensus algorithm accounts for the
following: the system will stay running with exactly half or
more replicas; the system will panic when less than half the
replicas are available; the system will not reboot without
one more than half the total replicas."
So it seems that in will stay running with two but not reboot (it will
go into single user mode...)
The only reason then to have two disks on each disk would be that when
in single user, you could maybe delete one of the replica's on the
failed disk from the remaining metadb, so that you then have "1 more
than half" of the replica's left, and then reboot again -??? or is
that silly? (and is it therefore impossible to have a system with only
2 disks that wont fall into single user when it loses one of them?)
Any help much appreciated,
many thanks,
Sammy
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