Stable Storage Implementation
We introduced the write-ahead log, which requires the availability of stable storage. By definition, information residing in stable storage is never lost. To implement such storage, we need to replicate the needed information on multiple storage devices (usually disks) with independent failure modes. We need to coordinate the writing of updates in a way that guarantees that a failure during an update will not leave all the copies in a damaged state and that, when we are recovering from a failure, we can force all copies to a consistent and correct value, even if another failure occurs during the recovery. In this section, we discuss how to meet these needs. A disk write results in one of three outcomes:
1. Successful completion. The data were written correctly on disk.
2. Partial failure. A failure occurred in the midst of transfer, so only some of the sectors were written with the new data, and the sector being written during the failure may have been corrupted.
3. Total failure. The failure occurred before the disk write started, so the previous data values on the disk remain intact. Whenever a failure occurs during writing of a block, the system needs to detect it and invoke a recovery procedure to restore the block to a consistent state.
To do that, the system must maintain two physical blocks for each logical block. An output operation is executed as follows:
1. Write the information onto the first physical block.
2. When the first write completes successfully, write the same information onto the second physical block,
3. Declare the operation complete only after the second write completes successfully. During recovery from a failure, each pair of physical blocks is examined. If both are the same and no detectable error exists, then no further action is necessary.
If one block contains a detectable error, then we replace its contents with the value of the other block. If neither block contains a detectable error, but the blocks differ in content, then w r e replace the content of the first block with that of the second. This recovery procedure ensures that a write to stable storage either succeeds completely or results in no change. We can extend this procedure easily to allow the use of an arbitrarily large number of copies of each block of stable storage.
Although having a large number of copies further reduces the probability of a failure, it is usually reasonable to simulate stable storage with only two copies. The data in stable storage are guaranteed to be safe unless a failure destroys all the copies. Because waiting for disk writes to complete (synchronous I/O) is time consuming, many storage arrays add NVRAM as a cache. Since the memory is nonvolatile (usually it has battery power as a backup to the unit's power), it can be trusted to store the data en route to the disks. It is thus considered part of the stable storage. Writes to it are much faster than to disk, so performance is greatly improved.
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