On 08/17/2010 05:54 PM, Anthony Liguori wrote:
This is simply unrealistic. O_SYNC might force data to be on a platter when using a directly attached disk but many NAS's actually do writeback caching and relying on having an UPS to preserve data integrity. There's really no way in the general case to ensure that data is actually on a platter once you've involved a complex storage setup or you assume FUA
That's fine. Memory backed up by a UPS is a disk platter as far as the user is concerned, if the NAS is reliable.
Let me put it another way. If an admin knows the disks on a machine have battery backed cache, he's likely to leave writeback caching enabled.
In this case, as far as the host is concerned, there is no cache. Data written is guaranteed to reach the disk eventually even without a flush. Hopefully the disk advertises itself as not having a volatile cache.
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