Re: [Xen-users] File System Doubt

[Scott Serr <serrs@xxxxxxxxxxxx>]

Tomasz Chmielewski wrote:
> Artur Baruchi schrieb:
> > Hi,
> >
> > Is there any difference in using a file as disk or a big partition to 
> > Xen?
> > Im running some benchmarks and Xen shows a great performance while
> > creating and removing files, I mean, a performance much better than a
> > real machine...
> > So im figure out that the fact that im using a simple file as disk to
> > Xen is the reason of this great performance... Can this guess be true?
> I guess it can be true, if the tests were short.
>
> dom0 treats this "file image" as a regular file, so, as you write to 
> it from domU (create/remove files), dom0's kernel may decide not to 
> actually "sync" the file yet.
> That is, your changes are purely in RAM at this point, and this is why 
> it's so fast. Now, try to press reset button in dom0, and after 
> reboot, see how many files you really have - or, if you can still open 
> the image ;)
> "Creating and removing files" is not that great benchmark.
>
> Try it with a bigger amount of files (millions etc.), I expect the 
> performance won't be better for an "image file" partition anymore.
For this same reason, Win98 was much faster virtualized in VMware on 
biggish hardware (1GB RAM).  Win98 reboots were VERY FAST since about 
1/2 of the Win98 disk image was in my hosts disk cache.
Here is a question...  whats the preferred fastest backend to store a 
disk image on?  Or...
maybe the fastest Xen virtual drive is actually a raw partition or RAID 
or logical volume management?
I currently have Xen images in files (file backed?)...  Then sit on a 
software RAID-0 on 400 out of 500GB of 2 SATA drives.  I have a reiserfs 
filesystem on /dev/md0 and that is where my image files are.  I expected 
that this would be quite fast, but my drives seem to sketch around 
ALOT.  I can't imagine that LVM would buy you any more performance.  
Would ext3?  I think the shotgun approach would be to make a /dev/md1 
that was smaller (narrower slices) to fix the sketching around.  Then 
take the file image mapping out of the picture by giving the VM a raw 
/dev/md1 partition.  I know LVM is very popular for use with Xen, is it 
because of convenience (resize and management etc) or are there 
performance advantages too that I don't comprehend?
-Scott


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