>>> On Jul 15, 2018, at 5:47 AM, Richard W.M. Jones <rjo...@redhat.com> wrote:
>>>
>>> On Fri, Jul 13, 2018 at 04:05:42PM +0200, Mikolaj Izdebski wrote:
>>> On 07/12/2018 10:17 PM, Richard W.M. Jones wrote:
>>> Does each build start with its own fresh VM?  Do you care about the
>>> data in that build VM if either qemu or the host crashes?  If the
>>> answers are 'Yes' and 'No' respectively to these questions then IMHO
>>> this is the ideal situation for cache=unsafe.
>>
>> The answers are 'No' and 'Not much'.
>>
>> 1. VMs are installed once and are running for week/months until they are
>> reinstalled. In the meantime guests and hosts are rebooted during
>> routine maintenance, to apply updates.
>
> In this case my preferred advice would be: DO NOT use cache=unsafe.
>
> We've only tested scenarios for very short-lived build or temporary
> VMs (for example when I was building RISC-V packages before we had
> Koji, I used a script which created a VM per build and there it made
> sense to use cache=unsafe).
>
> I do not think it's a good idea to be using this for VMs which are in
> any way long-lived as there could be unforeseen side effects which I'm
> not aware of and certainly have never tested.
>
>> 2. There would be no data loss in case of host or hypervisor crash.
>> Worst case, if guest operating system was corrupted sysadmins would need
>> to trigger VM install.
>
> Host crash => yes you'd definitely need to reinstall that VM.
>
> It's not a worst case, a host crash would near-definitely corrupt a VM
> that was ignoring flush requests.  It might even corrupt in an
> undetectable way (eg. throwing away data while leaving metadata
> intact).

Would it make sense to boot the builders with -snapshot and
cache=unsafe?  After all, during normal operation, they don’t need to
persist anything.

It might even be reasonable to reboot the VMs after every single build.
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