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Re: [Linux-cluster] SSI, Virtual Servers, ShareRoot, Etc
- From: gordan bobich net
- To: linux clustering <linux-cluster redhat com>
- Subject: Re: [Linux-cluster] SSI, Virtual Servers, ShareRoot, Etc
- Date: Fri, 25 Jan 2008 10:20:38 +0000 (GMT)
On Thu, 24 Jan 2008, isplist logicore net wrote:
Indeed. That sounds rather like you were using a SAN just for the sake of
using a SAN, and taking all the disadvantages without any of the
I'm not sure what you mean. I use an FC SAN because it allows me to separate
the storage from the machine.
But is that really cost effective? SANs aren't exactly cheap. Even if you
build one yourself, it takes a lot of disks before you actually break even
on disk costs.
What I was hoping to do with the partitions was to give each blade it's own
boot and scratch space, then allow each blade to have/use shared storage for
the rest. I was hoping to boot the machines from FC or perhaps PXE.
The point I was making was that SAN isn't cost effective unless you are
reaping other benefits (such as simplified administration) in addition to
saving on storage space (by the time you put the rest of the SAN box
together, the chances are that your net price per GB will increase). SAN
is also typically slower than local disks (don't believe the marketting
someone mentioned OpenSharedroot which sounded more interesting than carving
up a chassis into a bunch of complicated partitions. Just never got back to it
but want to again now. I badly want to eliminate the drives from each blade in
place of PXE or FC boot with something such as a sharedroot.
Sure, PXE+OSR is exactly the sort of administration simplification that I
was talking about. It also saves you extra space on top. The fact that
most configuration files (with only a few exceptions) are shared saves
you from having to implement really naff things like coming up with
complex frameworks to push the configuration to all machines and keep them
Same applies to keeping installed packages the same across the cluster
(you install on any node, and all nodes have it), and most importantly,
the data itself. All of it is always going to be consistent. It
effectively reduces the administration and maintainance complexity from
O(n) to O(1).
The only thing to watch out for is that swapping onto SAN is likely to be
relatively slow, as is scratch space. The only time I'd use completely
diskless nodes is when I can get away without swap/scratch partitions and
just use shared space for everything.
A shared root system would be better because then I don't have
to have fixed partitions, just files.
Then, each node would have it's storage over other storage chassis on
Not sure what you mean there, the last two sentences didn't quite parse.
Can you please elaborate?
These two statements you mean?
What I meant was first, that when I was trying this out, I was not aware of
the sharedroot project. However, I could take one of my 12-drive FC chassis
and partition it into say 32 partitions. My hope was to be able to boot 32
machines from that storage chassis. So, for the cost of running 12 drives in a
RAID5 array, I would eliminate 32 drives of all sorts of sizes for 12.
But does that really end up being cheaper, when you factor in the cost of
the chasis itself?
Since I was not able to boot from FC for what ever reason, I found that I
could install a small cheap flash IDE card into each blade for their /boot
partition, then it's main / partition would be the storage device. This worked
but of course I ran into other problems.
The problem had to do with not just zoning complications but that the storage
device was static in that if say I needed to do something with a certain
partition, I was unable to make any changes unless I changed all of the
partitions. Not a good idea.
This is where PXE booting the OSR initrd is useful. Update the initrd, do
a rolling reboot, and it's all updated. :-)
maybe before going right into SSI which seems to be quite an effort to
get running, you might want to think about "cloning" system images by
using any kind of volume manager and copy on write volumes. lvm2 for
instance does support this.
That sounds like misusing LVM for what DRBD is designed for...
But my thinking is not about ease of creating servers, it is about wasting the
resources of servers which are sitting there idle for the most part.
Actually, unless the administrator's time is worthless, you'll likely find
that cost of the extra man-hours required to admin the system is a greater
saving than what you'll achieve with cost saving on the hardware.
machines yet when they aren't doing anything, really, just wasted resources.
My curiosity was about creating a cluster of machines, which could use all of
the processing power of the others when needed. Instead of having a bunch of
machines sitting around mostly idle, when something came up, any one of them
could use what it needed for resources, better utilizing the resources.
The point is that all the resources you have are spinning all the time
anyway, and if you are using SSI/OSR, the chances are that you have a
homogenous cluster and all the machines are evenly load balanced.
That, of course, is making the assumption that I would be using applications
which put to use such resources as an SSI cluster.
One of the neat things you CAN achieve using SSI and clustering is that
you don't need all the nodes up all the time. Say you have 31 nodes. You
need 16 of those to maintain quorum. You want another 2 in there just to
cover any unexpected failures, so that's 18. The other 13 can be powered
off to conserve power, with their customer facing IPs HA failed over to
the 18 working nodes, with weighting adjusted accordingly. If the load
goes up you can bring the additional nodes online to cope with the load,
fail it's IP back to it and adjust the load balancing weights.
It gives you a reasonably nice way of only using the hardware you need for
normal load, while giving you a transparent way to bring up extra capacity
when your load spikes up (e.g. after a big marketing campaign). The
hardware that is powered down isn't using power and it isn't depleting
it's MTBF, so it prolongs the operational live of your cluster, too.
That would make things more complicated to set up AND more complicated to
maintain and keep in sync afterwards. SSI isn't hard to set up. Follow the
OSR howto and you'll have it up and running in no time.
From what I understand of SSI clusters, the applications have to be able to
put that sort of processing to proper use. While I'm using the term SSI, I am
really just asking everyone here for thoughts :).
SSI in a very simple setup is just shared-root. It doesn't necessarily
include Mosix-like things where all the CPUs from all the nodes appear as
local CPUs. That adds an additional layer of complexity that is a lot less
useful for most applications. SR SSI, OTOH, gives you immediate and
obvious benefits such as reduced space usage and more importantly, reduced
All I really mean is kind of where computing is going anyhow. Build something
very powerful (SSI for lack of better word), allow it to be sliced as many
times as required (VM), allow all of it's power to be used by any one or more
requests or share it amongst request automatically.
You'll find that even with Mosix type node unification, it's more
efficient to use the virtualizer's own VM migration solution. Mosix is
useful if you want to be lazy, but it isn't the magic bullet and doesn't
work well in all scenarios. Most of the time application-level clustering
is better in terms of performance, and it is usually a lot better at
handling error conditions (e.g. node failure) gracefully.
On the resources side, make it easy to add more power by easily being able to
add servers, memory, storage, what ever, into the mix. Isn't this where
computing is heading?
That's exactly what I described above. SR makes that really easy. You can
add additional nodes in the time it takes you to add it to the dhcp.conf
In my case, I just want to stop wasting all the power I'm wasting, have
something flexible, easy to grow and manage.
See above. :-)
the top of my head. It's useful to keep things like /var/cache shared. But
that's all fairly minor stuff. The howto will get you up and running.
I'll take a peek, thanks much.
The major upshot of SSI is that you only need to manage one file system,
which means you can both use smaller disks in the nodes, and save yourself
the hassle of keeping all the packages/libraries/configs in sync.
The very first thing I'd like to achieve is being able to get rid of the
drives in all of my machines in place of a FC HBA or using PXE. Then using
central storage for each servers needs from there.
You might as well do it all in one go. If you have a single machine set
up, you can get the OSR set up pretty quickly. Certainly less time than
it'll take you to create volumes for all the nodes and copy their FS to
their SAN volume.
On SSI, again, this is where it is unclear to me and perhaps I am using the
wrong term. I understand SSI as meaning a single system image but one which
you can only take advantage of with special applications. In other words, a
LAMP system would not take advantage of it.
In that case you are misunderstanding. Any homogenous cluster can take
advantage of it. In fact, you can do it even where all nodes don't have
the exact same job, but then you have to unshare the configs, which makes
it more complicated. You could still work around that by having / shared
via GFS and then having a separate /etc volume per cluster, but then you
still have the same package set on all the nodes (which may not be a
To summarize - most clusters can take advantage of SSI.
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