问题
Google start to use The High Replication datastore (HRD) as the default for new applications.
HR from the docs:
The HRD is a highly available, highly reliable storage solution. It remains available for reads and writes during planned downtime and is extremely resilient in the face of catastrophic failure—but it costs more than the master/slave option.
M/S from the docs:
your data may be temporarily unavailable during data center issues or planned downtime
Now, have you ever expirienced downtime? If this "downtime disclaimer" is just something theorical and doesn't happen frecuently I would use the M/S becouse it's cheaper.
What are the numbers that Google handle to say "downtime"? maybe their downtime is just a few seconds in a year, something totaly acceptable for some kind of apps.
Would love answers from experienced AppEngine developers.
回答1:
Downtime isn't theoretical - it happens in any distributed system. There are two types, roughly speaking: localized and global. Localized issues occur when a particular machine has trouble and can't serve requests; global downtime happens when something happens to the service as a whole.
Both can occur on App Engine: the former due to localized hardware failure, and the latter generally only due to planned maintenance that requires setting the master-slave datastore read-only for a brief period. The HR datastore handles both more robustly than the MS datastore, and doesn't require a read-only period during maintenance windows.
Once the new pricing scheme comes into effect, both datastores will be charged at the same rate.
For these and many other reasons, you should always use the HR datastore in new apps.
回答2:
I would recommend you to use HRD as Google said they will make M/S more expensive than HRD until the end of the year and even remove the M/S option as they are looking to "force" the businesses & developers to take advantage of all HRD goodies. The real reason is that maintaining a single type of infrastructure is cheaper than to maintain the both HRD and M/S so Google picks HRD. Source : Google I/O 2011
来源：https://stackoverflow.com/questions/6467345/have-you-experienced-datastore-downtime-in-appengine-what-are-the-odds