Re: RAID performance

From: Chris Murphy <lists@colorremedies.com>
To: Adam Goryachev <mailinglists@websitemanagers.com.au>
Cc: Phil Turmel <philip@turmel.org>,
	Dave Cundiff <syshackmin@gmail.com>,
	"stan@hardwarefreak.com Hoeppner" <stan@hardwarefreak.com>,
	"linux-raid@vger.kernel.org list" <linux-raid@vger.kernel.org>
Subject: Re: RAID performance
Date: Fri, 8 Feb 2013 00:35:00 -0700	[thread overview]
Message-ID: <DFB376BB-32B9-48DE-A7E9-A1B8F97982EF@colorremedies.com> (raw)
In-Reply-To: <51149A66.6000308@websitemanagers.com.au>

On Feb 7, 2013, at 11:25 PM, Adam Goryachev <mailinglists@websitemanagers.com.au> wrote:
> 
> 
> On the remote machine....
> NFS mount
> loop to present the NFS file as a block device
> Xen which passes through the block device to domU (Windows)
> disk partition
> partition is formatted NTFS

Assuming the domU gets it's own IP, Windows will mount NFS directly. You don't need to format it. On the storage server, storage is ext4 or XFS and can be on LVM if you wish.

> I'm not sure, but it was my understanding that using block devices was
> the most efficient way to do this….

Depends on the usage. Files being copied and/or moved on the same storage array sounds like a file sharing context to me, not a block device requirement. And user report of write failures over iSCSI bothers me also. NFS is going to be much more fault tolerant, and all of your domUs can share one pile of storage. But as you have it configured, you've effectively over provisioned if each domU gets its own LV, all the more reason I don't think you need to do more over provisioning. And for now I think NFS vs iSCSI can wait another day, and that your problem lies elsewhere on the network.

Do you have internet network traffic going through this same switch? Or do you have the storage network isolated such that *only* iSCSI traffic is happening on a given wire?

> ie, if a user logs into terminal server 1, and copies a
> large file from the desktop to another folder on the same c:, then this
> terminal server will get busy, possibly using a full 1Gbps through the
> VM, physical machine, switch, to the storage server. However, the
> storage server has another 3Gbps to serve all the other systems.

I think you need to replicate the condition that causes the problem, on the storage server itself first, to isolate this from being a network problem. And I'd do rather intensive read tests first and then do predominately write tests to see if there's a distinct difference (above what's expected for the RAID 5 write hit). And then you can repeat these from your domUs.

I'm flummoxed off hand if an NTFS formatted iSCSI block device behaves exactly as an NTFS formatted LV; ergo, is it possible (and OK) to unmount the volumes on the domUs, and then mount the LV as NTFS on the storage server so that your storage server can run local tests, simultaneously to those LVs. Obviously you should not mount the LVs on the storage server while they are mounted over iSCSI or you'll totally corrupt the file system (and it will let you do this, a hazard of iSCSI).

Chris Murphy--
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