An online upgrade method applied to a distributed storage system containing storage nodes, comprising: obtaining, by a monitoring node, identifiers of N to-be-upgraded nodes, wherein N ≥ 3; setting, by the monitoring node, an upgrade sequence of the N to-be-upgraded nodes according to the identifiers of the N to-be-upgraded nodes, wherein the N to-be-upgraded nodes have different ranks in the upgrade sequence; determining, by the monitoring node, a backup node of each to-be-upgraded node in the N to-be-upgraded nodes according to the upgrade sequence of the N to-be-upgraded nodes, to obtain node backup information, wherein the node backup information comprises a correspondence between an identifier of each to-be-upgraded node and an identifier of a backup node of the to-be-upgraded node; and sending, by the monitoring node, the node backup information to the N to-be-upgraded nodes, so that during an online upgrade period of any to-be-upgraded node in the N to-be-upgraded nodes, service data that points to be written onto the to-be-upgraded node is stored onto a backup node of the to-be-upgraded node according to the node backup information, wherein the service data is data written into the distributed storage system by a user; whereina backup node of the X th to-be-upgraded node is at least one node in a set comprising a to-be-upgraded node prior to the X th to-be-upgraded node in the upgrade sequence and a to-be-upgraded node following the (X+[Z+1]) th to-be-upgraded node in the upgrade sequence, Z is a ratio of a time required by the X th to-be-upgraded node for migrating back service data to an online upgrade time of the X th to-be-upgraded node, and 1 ≤ X ≤ N. obtaining, by a monitoring node, identifiers of N to-be-upgraded nodes, wherein N ≥ 3; setting, by the monitoring node, an upgrade sequence of the N to-be-upgraded nodes according to the identifiers of the N to-be-upgraded nodes, wherein the N to-be-upgraded nodes have different ranks in the upgrade sequence; determining, by the monitoring node, a backup node of each to-be-upgraded node in the N to-be-upgraded nodes according to the upgrade sequence of the N to-be-upgraded nodes, to obtain node backup information, wherein the node backup information comprises a correspondence between an identifier of each to-be-upgraded node and an identifier of a backup node of the to-be-upgraded node; and sending, by the monitoring node, the node backup information to the N to-be-upgraded nodes, so that during an online upgrade period of any to-be-upgraded node in the N to-be-upgraded nodes, service data that points to be written onto the to-be-upgraded node is stored onto a backup node of the to-be-upgraded node according to the node backup information, wherein the service data is data written into the distributed storage system by a user; whereina backup node of the X th to-be-upgraded node is at least one node in a set comprising a to-be-upgraded node prior to the X th to-be-upgraded node in the upgrade sequence and a to-be-upgraded node following the (X+[Z+1]) th to-be-upgraded node in the upgrade sequence, Z is a ratio of a time required by the X th to-be-upgraded node for migrating back service data to an online upgrade time of the X th to-be-upgraded node, and 1 ≤ X ≤ N.