Re: [Xen-users] Optimizing I/O

["Fajar A. Nugraha" <fajar@xxxxxxxxx>]

On Fri, Jan 23, 2009 at 11:05 PM, lists@xxxxxxxxxxxx <lists@xxxxxxxxxxxx> wrote:
>> What I'm doing is have all webservers on DomU's off an iSCSI array and
>> have the databases in separate DomU's running off the servers' local
>> disks.
>> This keeps the performance of the databases predictable, latency low
>> and takes a big chunk of IO out of the iSCSI.
>
> This is an area where I am having conflicting thoughts.
> In testing, I've found that the only real way of serving LAMP services is 
> distributed stand alone servers rather than on shared environments such as 
> virtual servers. The speed differences are instantly obvious when connecting 
> to a VM based server compared to stand alone.

Again, you caught me by surprise here. The newbie vs experienced thing :)
Seems like you're a seasoned VM users. In general, I'd say any I/O
optimization you previously used on other virtualization platform or
standalone boxes should also work on Xen. This includes noatime, which
makes huge difference when you're mostly serving static content (since
you didn't give the article link that recommends against it, all I can
say for now is "noatime is good").

Regarding VM vs stand-alone, I'm not sure about your setup (were you
still using vmware?), but in my experience when serving the same load,
a VM server (with local storage) is comparable to stand alone machine
with the same resource and load.

Now if we're talking about :
- using shared storage, or
- converting several stand alone boxes into one using VM without adding resource
Then yes, your point about "instantly obvious" is valid.

>
> The cost is a bit higher but who wants to suffer slowness on LAMP services?
>
> It seems to me that the best machines to virtualize are only machines which 
> aren't required in such a demanding role as web/mysql servers are. Maybe I'm 
> missing something.

Best machines to virtualize are those which aren't using resource
close to 100%. By virtualizing them you increase utilization and
reducing idle resource, thus saving money. I think that was the basic
principle.

Regards,

Fajar

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