virt-sysprep --enable customize --root-password password:123456 -a /dev/sdX
Tag Archives: virt-tools
Yesterday my colleague gave me a RHEL 5 VM disk image which failed to boot after converting it using the latest virt-v2v. Because it booted before conversion but not afterwards, the fingers naturally pointed at something that we were doing during the conversion process. Which is not unusual as v2v conversion is highly complex.
The thing is that we don’t reinstall grub during conversion, but we do edit a few grub configuration files. Could editing grub configuration cause this error?
I wanted to understand what the grub-legacy “GRUB _” prompt means. There are lots and lots and lots of people reporting this bug (eg), but as is often the case I could find no coherent explanation anywhere of what grub-legacy means when it gets into this state. Lots of the blind leading the blind, and random suggestions about how people had rescued such machines (probably coincidentally), but no hard data anywhere. So I had to go back to first principles and debug qemu to find out what’s happening just before the message is printed.
Tip: To breakpoint qemu when the Master Boot Record (first sector) is loaded, do:
target remote tcp::1234 set architecture i8086 b *0x7c00 cont
After an evening of debugging, I found that it’s the first sector (known in grub-legacy as “stage 1″) which prints the
GRUB<space> message. (The same happens to be true of grub2). The stage 1 boot sector has, written into it at a fixed offset, the location of the
/boot/grub/stage2 file, ie. the literal disk start sector and length of this file. It sends BIOS
int $0x13 commands to load those sectors into memory at address 0x8000, and jumps there to start the stage 2 of grub. The boot sector is 512 bytes, so there’s no luxury to do anything except print 5 characters. It’s after the stage2 file has been loaded when all the nice graphical stuff happens.
Unfortunately in the image after conversion, the
stage2 data loaded into memory was all zeroes, and that’s why the boot fails and you see
GRUB<space><cursor> and then the VM crashes.
The mystery was how conversion could be changing the location of the
/boot/grub/stage2 file so that it could no longer be loaded at the fixed offset encoded in the boot sector.
This morning it dawned on me what was really happening …
The new virt-v2v tries very hard to avoid copying any unused data from the guest, just to save time. No point wasting time copying deleted files and empty space. This makes virt-v2v very fast, but it has an unusual side-effect: If a file is deleted on the source, the contents of the file are not copied over to the target, and turn into zeroes.
It turns out if you take the source disk image and simply zero all of the empty space in
/boot, then the source doesn’t boot either, even though virt-v2v is not involved. Yikes … this could be a bug in RHEL 5. Grub is generating a bootloader that references a deleted file.
This is where we are right now with this bug. It appears that a valid sequence of steps can make a RHEL 5 bootloader that references a deleted file, but still works as long as you never overwrite the sectors used by that file.
I have written a simple test script that you can download to find out if your RHEL ≤ 6 virtual machines could be affected by this problem. I’m interested if anyone else sees this. I ran the test over a selection of RHEL 3 – 5 guests, and could not find any which had the problem, but my collection is not very extensive, and there are likely to be common modes in how they were created.
The next steps will likely be to test a lot more RHEL 5 installs to see if this bug is really common or a strange one-off. I will also probably add a workaround to virt-v2v so it doesn’t trim the boot partition — the reason is that we cannot go back and fix old RHEL 5 installs, we have to work with them if they are broken. If it turns out to be a real bug in RHEL 5 then we will need to issue a fix for that.
virt-v2v is a small program for converting guests from VMware or Xen, to run on KVM, RHEV-M or OpenStack. For RHEL 7.1, I am rewriting and enhancing virt-v2v, so it’s much faster and easier to use.
To install, follow the instructions here for setting up the yum repository, and then you can do:
yum install virt-v2v
To use it, start with the manual here that has lots of examples and the full reference documentation.
I was asked about this today and realized it’s not very well documented in the virt-builder manual page. So here goes.
Firstly you have to (as root) drop in the file
[local] uri=file:///home/YOUR_USERNAME/builder/index proxy=off
As non-root, create the
~/builder directory (you can of course put the repository somewhere else). Drop in the disk image — in the example below it’s called
os-image-1.xz, and next to it the following
[os-1] name=OS-1 arch=x86_64 file=os-image-1.xz checksum[sha512]=place the sha512sum of the compressed file here format=raw size=place the uncompressed virtual size of the disk image here compressed_size=place the compressed size of the disk image here expand=/dev/sda3 notes=My wonderful cloud OS, version 1
You only need the
expand field if you want image resizing to work (ie. the virt-builder
--size option), and other fields are documented in the manual. To get the uncompressed virtual size of a disk image, use
Other cloud image formats like qcow2 or uncompressed raw are possible, but you’ll need to read the manual closely and experiment a bit.
If you ever used the old version of virt-v2v, our software that converts guests to run on KVM, then you probably found it slow, but worse still it was slow and could fail at the end of the conversion (after possibly an hour or more). No one liked that, least of all the developers and support people who had to help people use it.
A V2V conversion is intrinsically going to take a long time, because it always involves copying huge disk images around. These can be gigabytes or even terabytes in size.
My main aim with the rewrite was to do all the work up front (and if the conversion is going to fail, then fail early), and leave the huge copy to the last step. The second aim was to work much harder to minimize the amount of data that we need to copy, so the copy is quicker. I achieved both of these aims using a lot of new technology that we developed for qemu in RHEL 7.
Virt-v2v works (now) by putting an overlay on top of the source disk. This overlay protects the source disk from being modified. All the writes done to the source disk during conversion (eg. modifying config files and adding device drivers) are saved into the overlay. Then we qemu-img convert the overlay to the final target. Although this sounds simple and possibly obvious, none of this could have been done when we wrote old virt-v2v. It is possible now because:
- qcow2 overlays can now have virtual backing files that come from HTTPS or SSH sources. This allows us to place the overlay on top of (eg) a VMware vCenter Server source without having to copy the whole disk from the source first.
- qcow2 overlays can perform copy-on-read. This means you only need to read each block of data from the source once, and then it is cached in the overlay, making things much faster.
- qemu now has excellent discard and trim support. To minimize the amount of data that we copy, we first fstrim the filesystems. This causes the overlay to remember which bits of the filesystem are used and only copy those bits.
- I added support for fstrim to ntfs-3g so this works for Windows guests too.
- libguestfs has support for remote storage, cachemode, discard, copy-on-read and more, meaning we can use all these features in virt-v2v.
- We use OCaml — not C, and not type-unsafe languages — to ensure that the compiler is helping us to find bugs in the code that we write, and also to ensure that we end up with an optimized, standalone binary that requires no runtime support/interpreters and can be shipped everywhere.
There haven’t been too many updates around here for a while, and that’s for a very good reason: I’ve been “heads down” writing the new versions of virt-v2v and virt-p2v, our tools for converting VMware and Xen virtual machines, or physical machines, to run on KVM.
The new virt-v2v [manual page] can slurp in a guest from a local disk image, local Xen, VMware vCenter, or (soon) an OVA file — convert it to run on KVM — and write it out to RHEV-M, OpenStack Glance, local libvirt or as a plain disk image.
It’s easy to use too. Unlike the old virt-v2v there are no hairy configuration files to edit or complicated preparations. You simply do:
$ virt-v2v -i disk xen_disk.img -o local -os /tmp
That command (which doesn’t need root, naturally) takes the Xen disk image, which could be any supported Windows or Enterprise Linux distro, converts it to run on KVM (eg. installing virtio drivers, adjusting dozens of configuration files), and writes it out to
To connect to a VMware vCenter server, change the
-i options to:
$ virt-v2v -ic vpx://vcenter/Datacenter/esxi "esx guest name" [-o ...]
To output the converted disk image to OpenStack glance, change the
-o options to:
$ virt-v2v [-i ...] -o glance [-on glance_image_name]