[Xen-users] Re: Differences in performance between file and LVM based images.

[Ligesh <myself@xxxxxxxxxx>]

On Wed, Aug 23, 2006 at 05:52:13PM +0200, Petersson, Mats wrote:
> 
> Probably yes. How much? Don't know. 
> 
> But I was more referring to the fact that different applications do
> different things to disks in the first place, so the application
> behaviour may depend on "seek time" or "write time" or "read time" in
> different proportions [1], so just using "hdparm" or something like that
> wouldn't really be a useful measure of how some particular application
> will perform on any given setup. 
> 

 Now that I have given it some thought, it seems to me that there's going to be 
some performance issues with files, and it might even be severe. For each seek, 
the control has to go through the ext3 driver, which is the only guy who knows 
how the file is structured. So if you are doing a seek on loop device, the OS 
needs the help of the ext3 driver to translate this into a position inside the 
file.

 The steps involved in doing a an operation on a loop device would be:

 1) Linux will have to first locate the file on the main filesystem. (Or does 
linux use the file's harddisk postion as the identifier for the loop 
device?...).

 2) Then it has to find out how the file is structured in the harddisk.

 3) To read/write anything, it will again need the entire ext3 logic for file 
structure.

 Anyway, some benchmarks would be great. And I think this should be explicitly 
mentioned in the documentation. Primary purpose of virtualization is to squeeze 
the maximum out of hardware, and so we cannot really afford performance 
penalties arising from wrong implementation decisions.

 I will see if I can do some benchmarks.
 

 Thanks.

--
:: Ligesh :: http://ligesh.com 



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