[pve-devel] Default cache mode for VM hard drives

[Stanislav German-Evtushenko]

On Fri, May 29, 2015 at 1:21 PM, Dietmar Maurer <dietmar at proxmox.com> wrote:
>> > It is also quite curious that DRBD layer has checks that detect this
>> > problem (but doesn't attempt to fix it).
>> >
>>
>> DRBD does not catch this by default but:
>> - it detects checksum mismatch if data-integrity-alg is enabled
>> (disabled by default and recommended to be disabled in production)
>> - the issue can be discovered during drbdadm verify
>
> I can see that it is bad if RAID devices contains different data (not
> really acceptable for me). But in this case, it is due to wrong behavior
> inside the application, so the application cannot expect to read back
> "correct" data. So my question is what errors/problems do you get with
> those VMs? RAID data is inconsistent, but does those VMs show any errors
> or problems?

The are two big issues when we use DRBD:
1) proper online migration is not possible (it the best case it just
craches and in the worst we have data loss)
2) if we shutdown both hosts and then start again the data can be
synchronized in a wrong way (from the bad host to the good one)