We're currently limited to 256 TAP interfaces preventing creation of guests above that limit.
Upstream commits from HP.
Created attachment 378785 [details]
Remove limit on number of tap devices
libvirt-0.6.3-26.el5 has been built into dist-5E-qu-candidate with the fix
The bug has been fixed in libvirt-0.6.3-26.el5
I booted thirty-two virtual machines, each with 8 virtual nics on a x86_64 system with libvirt-0.6.3-26.el5 version, there were 256 tap devices created in total, no problem found, when I booted another more one, that's ok also for creating the number of tap devices more than 256.
I did the same on libvirt-0.6.3-25.el5, when booting the 33th virtual machines, it reports error:
error: Failed to start domain guest21
error: internal error Failed to add tap interface 'vnet%d' to bridge 'virbr1' : Device or resource busy
virbr1 is a temporary bridge created for the bug verification.
An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on therefore solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.