Re: [Xen-users] RAID-1 strategy for a Xen/CentOS server?

[Tom Brown <xensource.com@xxxxxxxxxxxxxxxxxxx>]

> FWIW, "this box" has 4GB RAM atm, expandale to 8GB.
IMHO that means nothing... if it's handling 4 mailboxes, it's overkill, if 
it's handling 40,000 you're probably dead before you even started... but 
then you could come up with particular examples where the roles of that 4 
and 40,000 could be reversed... e.g. support@xxxxxxxxxxxxx, 
abuse@xxxxxxxxxxx, etc versus 40,000 never used freebie accounts that are 
included with some other product...
> "If you are using RAID-1 to help to ensure that your system stays up in the
> event of a hard disk partition failure, you should consider raiding your
> swap partition(s).
No argument. Generally my thinking with respect to raid 1 is to reduce the 
chance of data loss, but as per the introductory clause, there can 
be different goals.
> My broad-brushstroke goals -- for the whole system,as well as for individual
> DomUs -- include:
>
> (1) Maximize failover capability
??? By definition, "failover" requires redundant hardware, sort of the 
opposite of cutting up a big machine into smaller pieces (xen). If you 
want failover capability, see linux-ha.org ... and you need to be able to 
get at the data from the redundant hardware... it's a rather different 
picture than a normal server setup.
Stuffing DRBD under the vbd's so that a redundant server can mirror the 
block devices is fine start :) ... but that certainly isn't what I thought 
you meant by hardware vs software RAID... and you're entering the world of 
network attached storage....
> (2) Minimize potential fordata loss due to HW failure
> (3) Maximize the performance of the system
>
> (I've given up on running at less than 10W total power utilization ... ;-) )
>
>
> You're correct -- in answering my misstated question.  I *meant* to ask:
> What's the recommended FS for the Dom0 Xen host?
it is basically irrelevent, as anything that will boot xen is pretty much 
sufficient. dom0 isn't supposed to do much work (beyond having the kernel 
relay I/O requests to the real hardware). If you're using file images 
instead of block devices for your domU vbd's then it may be relevent... 
but ext3 should be fine. I can't really imagine a scenario where the more 
esoteric filesystems would be needed... but I'm no expert, and when you 
get into journalling filesystems (in the domUs) there are some complex 
interactions (e.g. preserving the order of writes, which is one reason why 
using real block devices instead of files may be better).
(Then again, using disk files for filesystem images probably conflicts 
with maximizing failover capabilities... this may depend on how many domUs 
you have, and whether you need/want to be able to fail them over 
independently.)
-Tom

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