Re: [Xen-users] How to best share disk for speed?

["Grant McWilliams" <grantmasterflash@xxxxxxxxx>]

On Mon, Jan 12, 2009 at 11:16 AM, Dylan Martin <dmartin@xxxxxxxxxxxx> wrote:

I have a number of DomUs running the same OS & distro (CentOS 5.2) so
they're using the same files for most operations.  Because I'm just
sharing access to the same RAID drive for disk, I assume disk I/O is
my biggest performance bottleneck / area of contention. It seems like
I should be able to set up a shared disk in a way that nearly all
reads are done from a single shared memory cache, and this should make
things zippy indeed.  Here's a few schemes I've thought of:

- tmpfs (ram disk) on dom0 and share read-only with domUs
- ramdisk backed GFS shared with domUs
- tap:ram backed GFS (if tap:ram actually exists)
- Maybe I'm a dork, and Xen does this already?
- Configure DomUs to cache a lot
- Maybe DomUs cache so much already that this is a moot point?

Just to try and clarify again, I'm not trying to save disk space, I'm
trying to save disk seeks.  My 2nd priority after I/O performance is
to avoid wasting RAM on redundant caches.

And while I'm at it:  Does a request for data in a ram-disk
masquerading as a hard drive arrive any slower than cached data from a
previous request?  IE, if I could disable caching on the domU and
serve from a cache on Dom0, would that work?

Okay, enough rambling, basically, I'm trying to get the best disk I/O,
ram usage out of my system.  Any advice?

Thanks
-Dylan

You might find that running out of tmpfs is slower because Linux caches ram as apposed to caching HD which doesn't really do anything but use more ram. I'm working on a completely different type of project but also using tmpfs. I write large files into it and then duplicate them to many drives. Not related to VMs but for my purpose it works great.

Until recently I didn't know tap:ram existed so I can't comment. It would be nice to hear your updates though as I have another project where I'll have a qcow backing file that 30-50 VMs will be accessing and each would have their own writable snapshot. That means a lot of reads out of one file and all the writes go somewhere else. It may be advantageous to load that one file into ram somehow.

Grant McWilliams

Some people, when confronted with a problem, think "I know, I'll use Windows."
Now they have two problems.
 

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