Re: [SLUG] UML Sanity Check

From: Ian C. Blenke (icblenke@nks.net)
Date: Thu May 22 2003 - 12:17:45 EDT


On Thursday 22 May 2003 03:18, Greg Schmidt wrote:
> What's this COW stuff? I found copy on write. I found uml_moo. It
> sounds like you keep a shared read-only filesystem in one place and a
> delta in another. Somehow a virtual filesystem keeps them straight?
> (Yeah, that's a question mark.)
>
> I read some HOW_TOs and if I read them a time or two more I might be
> able to pull it off. In the spirit of another thread that seems to be
> active lately, I guess what I'm missing is the WHY_TO.
>
> More importantly, what's this talk about a painless restore?

A "COW File" (Copy On Write file) is a sparse file that only grows as data is
written. You define a COW file by:

        ubd0=initial_root_fs,current_root_fs

This tells UML to present a /dev/ubd0 device that really maps to these two
files. The first file is the "backing store" file, the initial filesystem
image. The second file is the COW file, where all data writes will be
committed. The first file will NOT be modified by the running UML kernel. If
you wish to combine a backing store together with a COW file, you can use the
"uml_moo" tool, but onlt AFTER you shut down any running UML images using
those files.

A COW file is more than just a simple sparse file. It contains a header that
specifies the full path to the backing store file as well as a bitmap table
for showing which blocks in the sparse file have been written to.

Another neat thing that James and I have recently discover, you can "stack"
COW files:

        ubd0=initial_root_fs,previous_root_fs,current_root_fs

This allows you to spawn new images using previous COW files. IE, you do not
need to "merge" a backing store and COW file to make it a backing store for
another image. This also permits "rollback" to a known good state simply by
removing the current_root_fs file and rebooting the image.

COW files can speed up disk access across a number of UML images running at
the same time. From recent testing, however, we have also found that using
COW files can decrease the MTBF of running UML images.

Yes, bugs still remain in UML - we do get occasional UML crashes and hangs for
no apparent reason which we are hunting down - but we also run >250 images
now, so statistically the problems are fairly rare. My 10 personal images
running on a two VA Linux boxen cluster (web.blenke.com, mirrors.blenke.com,
mailhost.blenke.com, etc) haven't had a problem - but I don't pump the volume
of traffic through them that we do at the office.

Has anyone heard about Microsoft's new top-secret Windows 2003 based "virtual
server" platform? I've heard rumors that it may run Linux images, and I'm
trying to find out more information about it.

-- 
- Ian C. Blenke <icblenke@nks.net>

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