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Re: [Xen-users] File System Doubt

To: Tomasz Chmielewski <mangoo@xxxxxxxx>
Subject: Re: [Xen-users] File System Doubt
From: Scott Serr <serrs@xxxxxxxxxxxx>
Date: Sat, 22 Sep 2007 11:41:18 -0600
Cc: Artur Baruchi <mail.baruchi@xxxxxxxxx>, Xen-users@xxxxxxxxxxxxxxxxxxx
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Tomasz Chmielewski wrote:
Artur Baruchi schrieb:

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?


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