RE: [Xen-users] RE: Best way to store domU's. NFS? NBD?

[Jeff Sturm <jeff.sturm@xxxxxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-users-
> bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Wiebe Cazemier
> Sent: Thursday, March 25, 2010 5:31 AM
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-users] RE: Best way to store domU's. NFS? NBD?
> 
> On Wednesday 24 March 2010 19:34, Jeff Sturm wrote:
> > Decide whether you want files or block devices as the backing store
of
> > your domU's.  There are pros and cons to each.  In my installations
I
> > work solely with block devices, so I won't discuss NFS further.
> 
> Is storing disk images on NFS even an option? I can imagine that this
gives
> problems with syncs not actually being written when the driver reports
it, and
> similar problems. I have found out that when you do use NFS, the
tap:aio:
> back-end should be used, as opposed to the file: back-end.

I confess I don't really know the answer to your question.  I suspect
NFS provides a "good enough" consistency model for many users.

I've been burned by early implementations of NFS (v2) which colors my
opinion somewhat.  Recent implementations of NFSv4 likely fix some of
the problems, but I'm really not an NFS expert, as there are enough
alternatives these days for network storage that I don't have to be one.

Use tap:sync: if you are paranoid.  I've had consistency problems with
GFS and tap:aio: over block devices.

> Let me put it this way, what are common and reliable storage
solutions, using
> only Linux hosts? My problem here is that there is documentation for a
whole
> bunch of methods, but still I don't really know which of those methods
is
> reliable and commonly used, and which are just legacy's from the past,
and so
> forth.

My opinion is but a single data point.  In my last reply I'd given you
some guidelines and general info that you could use to do your own
evaluation.

But for my money...

- High-availability block storage on Linux only--DRBD + CLVM.  Make sure
you invest in a good disk subsystem for each storage host.  No need for
15K RPM drives everywhere, but the more spindles, the better.

- SAN storage. EqualLogic for iSCSI--a bit pricey but solid, reliable,
simple to configure and great performance.  Coraid for AoE--simple,
easy, very inexpensive, also reliable.

I wouldn't bother with the standard protocols like iSCSI unless I'm also
buying hardware from a commercial storage vendor.  If you're committed
to Linux only, take a long look at DRBD.  It's actively developed and
works great.  (There are "soft" appliances for iSCSI like Openfiler.
I've had lukewarm results with those.  Commercial products are generally
much more polished.)

> And, if you say iSCSI is ubiquitous, why are there 0 hits when I
search for
> iscsi on wiki.xensource.org?

Do you mean wiki.xensource.com?  The search
http://www.google.com/#hl=en&q=site%3Awiki.xensource.com+iscsi gives me
quite a few hits.

-Jeff



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