Ah, I see, the point is that even if you turn off the power *and pull the disk* halfway through a write, the disk state is still consistent? Yeah, battery-backed cache alone obviously can't ensure that.
No one said anything about pulling a disk. I did mention pulling a power cord. I meant the power cord that supplies the RAID enclosure (storage server).
A RAID enclosure with a battery inside that powers only the memory can keep the data consistent in the face of a power cord pull, but fails the persistence test, because the battery eventually dies. I think when people think persistent, they think indefinite. High end storage servers do in fact let you pull the power cord and not plug it in again for years and still be able to read back all the data that was completely written to the server before the pull. Some do it by powering disk drives (not necessarily the ones that normally hold the data) for a few seconds on battery.
Also, I think some people expect of persistence that you can take the machine, once powered down, apart and put it back together and the data will still be there. Battery backed memory probably fails that test.
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