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Re: [libvirt] [PATCH] qemu: Migrate at unlimited speed by default

On 08/03/2012 12:16 PM, Jiri Denemark wrote:
> Previously, qemu did not respond to monitor commands during migration if
> the limit was too high. This prevented us from raising the limit
> earlier. The qemu issue seems to be fixed (according to my testing) and
> we may remove the 32Mb/s limit.
> ---
>  src/qemu/qemu_domain.c    | 2 +-
>  src/qemu/qemu_domain.h    | 5 ++---
>  src/qemu/qemu_migration.c | 4 ++--
>  3 files changed, 5 insertions(+), 6 deletions(-)
> diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c
> index 86f0265..a54df2f 100644
> --- a/src/qemu/qemu_domain.c
> +++ b/src/qemu/qemu_domain.c
> @@ -217,7 +217,7 @@ static void *qemuDomainObjPrivateAlloc(void)
>      if (!(priv->cons = virConsoleAlloc()))
>          goto error;
> -    priv->migMaxBandwidth = QEMU_DOMAIN_DEFAULT_MIG_BANDWIDTH_MAX;
> +    priv->migMaxBandwidth = QEMU_DOMAIN_MIG_BANDWIDTH_MAX;

This says that new libvirt will unconditionally try to use unlimited
bandwidth, instead of the qemu default of 32Mbps-limited bandwidth, in
all cases where the user does not additionally provide their own limiting.

Is that a backwards-incompatible change?  Could there be an existing
client relying on the default limiting that will now be swamped by
libvirt being faster?  Or is it a bug fix, where things will now just
operate faster by default, and people that care about limiting are not
impacted because they already had an explicit limit?

I think it is the latter (a bug fix - no one that cares about limiting
should be relying on defaults), so after the discussion we've had in
this thread, I'm okay giving:


Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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