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Re: [Linux-cluster] Home-brew SAN/iSCSI



Madison Kelly wrote:
Andrew A. Neuschwander wrote:
Madison Kelly wrote:
Hi all,

Until now, I've been building 2-node clusters using DRBD+LVM for the shared storage. I've been teaching myself clustering, so I don't have a world of capital to sink into hardware at the moment. I would like to start getting some experience with 3+ nodes using a central SAN disk.

So I've been pricing out the minimal hardware for a four-node cluster and have something to start with. My current hiccup though is the SAN side. I've searched around, but have not been able to get a clear answer.

Is it possible to build a host machine (CentOS/Debian) to have a simple MD device and make it available to the cluster nodes as an iSCSI/SAN device? Being a learning exercise, I am not too worried about speed or redundancy (beyond testing failure types and recovery).

Thanks for any insight, advice, pointers!

Madi


If you want to use a Linux host as a iscsi 'server' (a target in iscsi terminiology), you can use IET, the iSCSI Enterprise Target: http://iscsitarget.sourceforge.net/. I've used it and it works well, but it is a little CPU hungry. Obviously, you don't get the benefits of a hardware SAN, but you don't get the cost either.

-Andrew

Thanks, Andrew! I'll go look at that now.

I was planning on building my SAN server on an core2duo-based system with 2GB of RAM. I figured that the server will do nothing but host/handle the SAN/iSCSI stuff, so the CPU consumption should be fine. Is there a way to quantify the "CPU/Memory hungry"-ness of running a SAN box? Ie: what does a given read/write/etc call "cost"?

As an aside, beyond hot-swap/bandwidth/quality, what generally is the "advantage" of dedicated SAN/iSCSI hardware vs. white box roll-your-own?

Thanks again!

Madi


I think what makes being an iSCSI target CPU hungry is that it is handling a block layer protocol in user space. So while what it does is fairly simple (i.e. no filesystem), it has to do a lot of it. Storage performance is usually discussed in IOPS (I/Os Per Second), but when rolling my own, I just throw enough spindles/raid/cpu/memory at it saturate a GigE link and call it a day.

I've not used a hardware iSCSI SAN, just FC. The biggest benefits, in my mind, of something like an EMC Clariion are the fully redundant hardware path and the fast fabric.

Hmm, I may be getting off-topic here. Sorry about that.

-Andrew


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