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

["Dylan Martin" <dmartin@xxxxxxxxxxxx>]

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

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