[Libguestfs] [libguestfs] Options for hotplugging

[Matthew Booth]

On 20/08/12 22:07, Richard W.M. Jones wrote:
> (1) The "raw libvirt" option
>
> In this one we'd simply provide thin wrappers around
> virDomainAttachDevice and virDomainDetactDevice, and leave it up to
> the user to know what they're doing.
>
> The problem with this is the hidden appliance disk.  We certainly
> don't want the user to accidentally detach that(!)  It's also
> undesirable for there to be a "hole" in the naming scheme so that
> you'd have:
>
>    /dev/sda <- your normal drives
>    /dev/sdb <-
>   [/dev/sdc # sorry, you can't use this, we won't tell you why]
>    /dev/sdd <- your first hotplugged device
>
> As far as I know, the kernel assigns /dev/sdX names on a first-free
> basis, so there's no way to permanently put the appliance at
> /dev/sdzzz (if there is, please let me know!)

There are numerous reasons not to like this. If there were any reason to 
want to expose the underlying libvirt api directly I'd suggest it only 
be a debug option.

> (2) The "slots" option
>
> In this option you'd have to use null devices to reserve the maximum
> number of drive slots that you're going to use in the libguestfs
> handle before launch.  Then after launching you'd be allowed to
> hotplug only those slots.
>
> So for example:
>
>    guestfs_add_drive (g, "/dev/null"); # reserves /dev/sda
>    guestfs_add_drive (g, "/dev/null"); # reserves /dev/sdb
>    guestfs_add_drive (g, "/dev/null"); # reserves /dev/sdc
>    guestfs_launch (g);
>    guestfs_hotplug (g, 1, "/tmp/foo"); # replaces index 1 == /dev/sdb
>    guestfs_hotplug (g, 3, "/tmp/foo"); # error!
>
> Although ugly, in some ways this is quite attractive.  It maps easily
> into guestfish scripts.  You have contiguous device naming.  You often
> know how many drives you'll need in advance, and if you don't then you
> can reserve up to max_disks-1.

Echo Dan's general dislike of this.

> (3) The "serial numbers" option
>
> This was Dan's suggestion.  Hotplugged drives are known only by their
> serial number.  ie. We hotplug them via libvirt using the <serial/>
> field, and then they are accessed using /dev/disk/by-id/serial.
>
> This is tempting, but unfortunately it doesn't quite work in stock
> udev, because the actual name used is:
>
>    /dev/disk/by-id/scsi-0QEMU_QEMU_HARDDISK_SERIAL
>
> We could add a custom udev rule to get the path we wanted.

Presumably you would specify then serial number when adding the drive? 
I'm not opposed to this, but it's not the simplest.

> (4) The "rewriting device names" option
>
> Since we already have the infrastructure to rewrite device names, we
> could do some complicated and hairy device name rewriting to make
> names appear continguous, even though there's an hidden appliance
> drive.
>
> This is my least favourite option, mainly because of the complexity,
> and complexity is bound to lead to bugs.

I don't think this rewriting is required. Having a hole in the drive 
letters isn't a big deal. In fact, I suspect it would be simpler in most 
code to use returned rather than calculated device names.

I'd suggest a very simple api:

char * guestfs_hotplug_drive(g, path, <opts>)

This does the same as add_drive, except that it works after launch and 
returns the api name of the newly added drive. list_devices will return 
a list with a hole in it. If it isn't already there, we can add some 
generic code to methods taking a Device parameter to guard against 
passing in the root device.

Matt
-- 
Matthew Booth, RHCA, RHCSS
Red Hat Engineering, Virtualisation Team

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