Re: [Xen-users] Optimizing I/O

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

On Mon, Jan 26, 2009 at 6:41 AM, lists@xxxxxxxxxxxx <lists@xxxxxxxxxxxx> wrote:
> Basically, it talked about a guy doing tests and finding that with noatime, 
> the data could become inconsistent and other things. So, in your experience, 
> this has not been a problem?
>

I believe some software would use atime to determine whether or not
data should be compressed or not. There's also some distros that uses
atime or mtime to determine which files on /tmp can be deleted. So far
there hasn't been any data corruption or inconsistency that I know of
that results from using noatime.

> The thing about optimizing I/O is that the guests are still really just data 
> on Ethernet ports, no matter how you cut it.

On your setup, yes.

> Whether I ran many instances of a web server or multiple separate guests, 
> does it not always add up to more resources being used from the host? Just 
> seems that you'd want to have stand alone servers working full bore on 
> serving up web pages in a distributed manner rather than just taxing a host 
> with guests and all the extras needed for similar redundancy. Guess I'll have 
> to run some tests once I get everything together and see what happens.

That's only logical. If your primary goal is redundancy and your
servers are quite busy, then using virtualization doesn't help much.


>> 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.
>
> Well, shared storage on the host for the guests but the web servers would 
> serve from network storage.

Usually that whould be where the bottle neck is. Using shared storage
(SAN/NAS) requires that :
- the SAN/NAS box is capable of providing I/O needed for all clients
- storage network has enough bandwidth for I/O needs

Usually this translates to using lots and lots of 72GB disks and
dedicated switches/ports for SAN/NAS.


Regards,

Fajar

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