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Advanced data-protection methods describes how ECS uses a combination of triple mirroring and erasure coding to write data in a distributed fashion to be resilient against various failure scenarios.
To ensure consistency of data, checksums are validated upon reads and by a consistency checker. The consistency checker is a background process that periodically performs checksum verification over the entire dataset. Read requests also run checksum verification.
If a read request is missing a fragment, due to a drive not responding, or if checksum verification fails, a notification is sent to chunk manager. Chunk manager initiates a reconstruction of the missing fragment or fragments by using either the remaining erasure coded data and parity fragments or the replica copies. It then updates the chunk information. Once the fragments have been re-created, any outstanding or new read requests use the updated chunk information to request the data fragment or fragments and service the read request.
ECS nodes are constantly performing health checks on disks directly attached to them. If a disk becomes unresponsive, the ECS node notifies chunk manager to stop including the disk in new write operations. If the disk remains unresponsive after a period of time (default is 60 minutes), a notification is sent to chunk manager to re-create the data from the failed drive. The ECS node identifies which chunks have blocks on the failed drive and need to be recovered. It sends the information to chunk manager, which starts parallel recovery of all chunk fragments stored on the failed drive. The chunk fragments are recovered onto other disks using the remaining erasure coded fragments or replica copies. As new fragments are written, the associated chunk tables are updated with this information. If possible, chunk manager also deletes the fragments from the failed drive.
If this drive comes online after it has been removed, it is added as a new drive, and chunk manager includes it in new write operations.