Comparison of block based volumes with ongoing inputs and outputs

Abstract

A process compares respectively corresponding data blocks in redundant data storage systems having ongoing I/O activity, to detect discrepancies between blocks that are supposed to be identical. Storage systems have replica copies of the data. A hashing algorithm processes a first data block from to produce a first hash. The hashing algorithm processes the second data block (a replica of the first data block) to produce a second hash. The first and second hashes are compared to determine whether or not the underlying data blocks match each other. If a write command directed to the data blocks is received during the overall comparison process, the comparison process is halted to permit the write command to proceed without adverse impact from the comparison process. The comparison process is then restarted.

What is claimed is: 
     
         1 . A method for comparing storage volumes having ongoing input/output (I/O) activity, comprising:
 receiving, by a local system, and from a requestor, a request to conduct a comparison process to determine whether or not a local data block on a local volume is identical to a remote data block on a remote volume;   responsive to receiving the request to conduct the comparison process, initiating the comparison process;   computing a first local hash based on the local data block;   receiving a host write command, with respect to the local data block and the remote data block, where the host write command is received during execution of the comparison process, and where the host write command has a higher execution priority than has the comparison process;   responsive to receiving the host write command, modifying the local data block in accordance with the host write command, to produce a modified local data block, and sending the host write command to a remote system;   re-initiating the comparison process;   computing a second local hash based on the modified local data block;   sending, to a remote computer system, a request to compare a remote hash against the second local hash, where the request includes the second local hash and an identifier to identify the remote data block;   receiving, from the remote computer system, a write completion message, indicating that the host write command has been processed at the remote computer system to produce a modified remote data block;   receiving, from the remote computer system, a comparison result, wherein the comparison result includes information to indicate whether or not the second local hash and the remote hash are identical;   receiving, by the remote system and from the local system, the host write command;   receiving, by the remote system and from the local system, the request to compare the remote hash against the second local hash;   modifying the remote data block in accordance with the host write command, to produce a modified remote data block;   sending the write completion message to the local system;   computing the remote hash based on the modified remote data block;   comparing the remote hash against the second local hash to produce the comparison result;   sending the comparison result to the local system; and   sending the comparison result to the requestor;   wherein:
 the remote data block is intended to be an identical copy of the local data block, 
 the host write command is directed to both the local data block and the remote data block, 
 the comparison result includes information indicating whether the second local hash is identical to the remote hash, 
 the second local hash is computed from the modified local data block, by application of a hashing algorithm to the modified local data block, and 
 the remote hash is computed from the modified remote data block, by application of the hashing algorithm to the modified remote data block.