[DRBD-user] Ideas for splitting data between A and C protocol devices - safe or not?
Casey Allen Shobe
cshobe at bepress.com
Wed Aug 13 06:50:48 CEST 2008
I've been tossing around a couple ideas in my head with a mind to
improve overall performance. I'd just like some feedback on the
sanity of these ideas, especially if any seem inherently risky.
Reading through some random DRBD documents, I read something
familiar: "Applications like databases and journaling ﬁle systems
maintain the consistency of their data by keeping track of
modiﬁcations of their data sets in special places (logs, journals).
In case of a system crash, the information of such a journal is
sufﬁcient to bring the data set in consistent state again."
This makes me wonder, well just how much can I get away with if the
log/journal isn't damaged?
The idea that occurs to me is that since I'm already using two DRBD
partitions for our PostgreSQL database, one for the data and another
separate one for the write-ahead log, maybe it's tolerable to make the
data partition use Protocol A (async replication) rather than B or
C?. And/or if I reconfigure the underlying JFS filesystem on the data
partition to store it's journal on a synchronously-replicated device,
making the main data replicated using protocol A, is this safe?
I ask because I see almost no difference between protocols B and C (I
believe B is plenty safe for our needs, but no point in using it if
it's no faster, eh?), but protocol A is significantly speedier.
Protocol A "sounds" unsafe, but maybe since the filesystem and/or
database can store their journals on Protocol C devices, it really is
By "safe", I mean to say that "as long as one node always remains up,
data is never lost". I am not concerned about simultaneous failure of
both nodes, and next year I hope to add a third node to boost
redundancy further (such that when a secondary is intentionally
offline, we're not in single point of failure mode).
Thanks in advance!
Casey Allen Shobe
Database Architect, The Berkeley Electronic Press
cshobe at bepress.com (email/jabber/aim/msn)
http://www.bepress.com | +1 (510) 665-1200 x163
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