![]() Imaging the mess if half the data blocks that make up a busy payroll table were out of sync by a few seconds. If you decide to take a snapshot of the RAID set, you need all the snapshots of the individual disks to be in sync so that when you restore the snapshots all of the chunks match up and there are no holes. There are options to make RAID sets more fault tolerant, but not to the extent that it would compensate for out-of-sync snapshots. In this simple form if one of those chunks got lost, you could not reassemble the original data (it would have holes). To achieve this the Operating System cuts the data up into chunks (4 to 128kB depending on the service) and spreads the write and read actions over the set of disks in parallel. The core concept of RAID is that disk access is basically slow, by spreading the packets across multiple disks, we can achieve reads and writes in parallel, which is faster. In the context of EC2 we are only considering Operating System managed RAID, and only mirrored or striped sets in-line with AWS recommendations. All enterprise storage makes use of one form of RAID or another. This can be done in different ways to achieve different results: faster I/O, a larger volume, or increased fault tolerance (but never all three at once). A RAID set is a group of disks (2 to 18+) that are controlled and addressed so that they appear to the operating system as a Volume. Below I hope to address some of the issues raised. ![]() We have had a number of questions about the snapshot process on EC2 RAID disks.
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