Decided that a new approach was called for, and pared down the storage chapter. I think the chapter, as we've got it now, does a good job of presenting the basics of storage with Xen. Stuff related to copy-on-write has been mostly passed to hosting, while network storage is mostly sent to migration.
Casualties of the process included QCOW images, which worked at one point, then stopped working, and dmuserspace, which I get the strong impression no one actually uses.
It's a pity, because it's actually a pretty cool technology -- but it's got too much bit rot. At times I almost had it working, but there was no guarantee that these steps would continue working, and I couldn't in good conscience advice people to download patches from a mailing list's archive.
Ah well. Here are the first few paragraphs of the section that I wrote about it, just to frustrate future web searchers coming and looking for useful information:
Casualties of the process included QCOW images, which worked at one point, then stopped working, and dmuserspace, which I get the strong impression no one actually uses.
It's a pity, because it's actually a pretty cool technology -- but it's got too much bit rot. At times I almost had it working, but there was no guarantee that these steps would continue working, and I couldn't in good conscience advice people to download patches from a mailing list's archive.
Ah well. Here are the first few paragraphs of the section that I wrote about it, just to frustrate future web searchers coming and looking for useful information:
DmUserspace
DmUserspace is an extension to the basic device-mapper concept that
allows the kernel to forward requests for blocks on pseudo-devices to
a userspace program, which responds with an appropriate destination
device and set of block addresses. This allows you, the
administrator, to have block devices that dynamically resize and move
themselves as needed.
This facility works well with Copy-on-Write, because it avoids the
need to pre-allocate backing store that one finds with LVM, or the
simpler CoW scripts on the Xen liveCD. Instead, DmUserspace works
with a userspace daemon that finds and allocates sectors to
pseudo-devices automatically as needed. Because this is a userspace
program, it can be flexible -- for example, one suggestion on the
Xen-devel mailing list was to have a device that transparently fetched
and cached from Amazon's S3.
Although this sounds great (and is pretty great,) it doesn't come with
Xen, and it's not trivial to set up. You'll need a fully set-up
compilation environment, including a kernel source tree. If you've
got your ducks in a row, read on
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