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Chapter 3. Hardware Monitoring
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## Chapter 3. Hardware Monitoring

While software component monitoring is central to insuring that high availability service levels are met, hardware monitoring, fault isolation, and ultimately the replacement of a failed component and how to recover from that failure are equally important. The following sections cover guidelines on what to monitor and how to detect potential hardware failures. It also discusses the replacement procedures of replacing failed hardware components and how to bring the Oracle NoSQL Database components (that were utilizing the components that were replaced) back online .

## Monitoring for Hardware Faults

There are several different hardware scenarios/failures that are considered when monitoring the environment for Oracle NoSQL Database. The sections below cover the monitoring of network, disk, and machine failures as well as the correlation of these failures with log events in the Oracle NoSQL Database. Finally, it discusses how to recover from these failure scenarios.

### The Network

Monitoring packet loss, round trip average latencies, and network utilization provides a glimpse into critical network activity that can affect the performance as well as the ongoing functioning of the Oracle NoSQL Database. There are two critical types of network activity in the Oracle NoSQL Database. The client driver will utilize Java RMI over TCP/IP to communicate between the machine running the application, and the machines running the nodes of the NoSQL Database cluster. Secondly, each node in the cluster must be able to communicate with each other. Replication nodes will utilize Java RMI over TCP/IP and will also utilize streams based communication over TCP/IP. Administrative nodes and storage node agents will only utilize RMI over TCP/IP. The key issue in insuring an operational store that is able to maintain predictable latencies and throughput is to monitor the health of the network through which all of these nodes communicate.

The following tools are recommended for monitoring the health of the network interfaces that the Oracle NoSQL Database relies on:

- Sar, ping, iptraf – These operating system tools display critical network statistics such as # of packets lost, round trip latency, and network utilization. It is recommended to use `ping` in a scripted fashion to monitor round trip latency as well as packet loss and use either `sar` or `iptraf` in a scripted fashion to monitor network utilization. A good rule of thumb is to raise an alert if network utilization goes above 80%.

- Oracle NoSQL Ping command – The Oracle NoSQL Administrative console contains a ping command that attempts to contact each node of the cluster. Directions on how to run and script this command can be found here: http://docs.oracle.com/cd/NOSQL/html/AdminGuide/cli_command_reference.html#commands_subcommands

#### Correlating Network Failure to NoSQL Log Events

Network failures that affect the runtime operation of NoSQL Database is ultimately logged as instances of Java runtime exceptions. Using log file monitoring, the following exception strings are added to a list of regular expressions that are recognized as critical events. Correlating the timestamps of these events with the timestamps of whatever network monitoring tool is being utilized.

### Note

While searching the log file for any of the exceptions stated below, the log level must also be checked such that only log levels of SEVERE is considered. These exceptions are logged at a level of INFO which indicates no errors will be encountered by the application.

- UnknownHostException – A DNS lookup of a node in the NoSQL Database failed due to either a misconfigured NoSQL Database or a DNS error. Encountering this error after a NoSQL cluster has been operational for some time indicates a network failure between the application and the DNS server.

- ConnectException – The client driver cannot open a connection to the NoSQL Database node. Either the node is not listening on the port being contacted or the port is blocked by a firewall.

- ConnectIOException – Indicates a possible handshake error between the client and the server or an I/O error from the network layer.

- MarshalException – Indicates a possible I/O error from the network layer.

- UnmarshalException – Indicates a possible I/O error from the network layer.

- NoSuchObjectException – Indicates a possible I/O error from the network layer.

- RemoteException – Indicates a possible I/O error from the network layer.

#### Recovering from Network Failure

In general, the NoSQL Database will retry and recover from network failures and no intervention at the database level is necessary. It is possible that a degraded level of service is encountered due to the network failure; however, the failure of network partitions will not cause the NoSQL Database to fail.

### Persistent Storage

One of the most common failure scenarios you can expect to encounter while managing a deployed Oracle NoSQL Database instance (sometimes referred to as KVStore) is a disk that fails and needs to be replaced; where the disk is typically a hard disk drive (HDD), or a solid state drive (SSD). Because HDDs employ many moving parts that are continuously in action when the store performs numerous writes and reads, moving huge numbers of bytes on and off the disk, parts of the disk can easily wear out and fail. With respect to SSDs, although the absence of moving parts makes SSDs a bit less failure prone than HDDs, when placed under very heavy load, SSDs will also generally fail with regularity. As a matter of fact, when such stores scale to a very large number of nodes (machines), a point can be reached where disk failure is virtually guaranteed; much more than other hardware components making up a node. For example, disks associated with such systems generally fail much more frequently than the system's mother board, memory chips, or even the network interface cards (NICs).

Since disk failures are so common, a well-defined procedure is provided for replacing a failed disk while the store continues to run; providing data availability.

#### Detecting and Correlating Persistent Storage Failures to NoSQL Log Events

There are many vendor specific tools for detecting the failure of persistent storage devices as well as SNMP monitoring agents that perform similar functionality. It is beyond the scope of this book to recommend any vendor specific mechanism or SNMP based monitoring mechanism. There are however, some general things that can be done to identify a failed persistent storage device;

### Note

Using log file monitoring, the following exception string is to a list of regular expressions that should be recognized as critical events. Correlating the timestamps of these events with the timestamps of whatever storage device monitoring tool is being utilized. When searching the log file for any of the exception stated below, the log level must also be checked such that only log levels of SEVERE is considered.

- I/O errors in /var/log/messages – Monitoring /var/log/messages for I/O errors indicate that something is wrong with the device and it may be failing.

- Smartctl – If available, the smartctl tool detects a failure with a persistent storage device and displays the serial number of the specific device that is failing.

- EnvironmentFailureException – The storage layer of NoSQL Database (Berkeley DB Java Edition) converts Java IOExceptions detected from the storage device into an EnvironmentFailureException and this exception is written to the log file.

#### Resolving Storage Device Failures

The sections below describe that procedure for two common machine configurations.

In order to understand how a failed disk can be replaced while the KVStore is running, review what and where data is stored by the KVStore; which is dependent on each machine's disk configuration, as well as how the store's capacity and storage directory location is configured. Suppose a KVStore is distributed among 3 machines – or storage nodes (SNs) — and is configured with replication factor (RF) equal to 3, each SN's capacity equal to 2, KVROOT equal to /opt/ondb/var/kvroot, and store name equal to "store-name". Since the capacity or each SN is 2, each machine will host 2 replication nodes (RNs). That is, each SN will execute 2 Java VMs and each run a software service (an RN service) responsible for storing and retrieving a replicated instance of the key/value data maintained by the store.

Suppose in one deployment, the machines themselves (the SNs) are each configured with 3 disks; whereas in another deployment, the SNs each have only a single disk on which to write and read data. Although the second (single disk) scenario is fine for experimentation and "tire kicking", that configuration is strongly discouraged for production environments, where it is likely to have disk failure and replacement. In particular, one rule deployers are encouraged to follow in production environments is that multiple RN services should never be configured to write data to the same disk. That said, there may be some uncommon circumstances in which a deployer may choose to violate this rule. For example, in addition to being extremely reliable (for example, a RAID device), the disk may be a device with such high performance and large capacity that a single RN service would never be able to make use of the disk without exceeding the recommended 32GB heap limit. Thus, unless the environment consists of disks that satisfy such uncommon criteria, deployers always prefer environments that allow them to configure each RN service with its own disk; separate from all configuration and administration information, as well as the data stored by any other RN services running on the system.

As explained below, to configure a KVStore use multiple disks on each SN, the storagedir parameter must be employed to exploit the separate media that is available. In addition to encouraging deployers to use the storagedir parameter in the multi-disk scenario, this note is also biased toward the use of that parameter when discussing the single disk scenario; even though the use of that parameter in the single disk case provides no substantial benefit over using the default location (other than the ability to develop common deployment scripts). To understand this, first compare the implications of using the default storage location with a non-default location specified with the storagedir parameter.

Thus, suppose the KVStore is deployed – in either the multi-disk scenario or the single disk scenario – using the default location; that is, the storagedir parameter is left unspecified. This means that data will be stored in either scenario under the KVROOT; which is `/opt/ondb/var/kvroot` in the examples below. For either scenario, a directory structure like the following is created and populated:

     - Machine 1 (SN1) -     - Machine 2 (SN2) -    - Machine 3 (SN3) -
    /opt/ondb/var/kvroot   /opt/ondb/var/kvroot  /opt/ondb/var/kvroot
      log files             log files             log files
      /store-name           /store-name           /store-name
        /log                   /log                  /log
        /sn1                   /sn2                  /sn3
          config.xml             config.xml            config.xml
          /admin1                /admin2               /admin3
            /env                   /env                  /env

      /rg1-rn1                 /rg1-rn2                /rg1-rn3
        /env                     /env                    /env

      /rg2-rn1                 /rg2-rn2                /rg2-rn3
        /env                     /env                    /env

Compare this with the structure that is created when a KVStore is deployed to the multi-disk machines; where each machine's 3 disks are named /opt, /disk1, and/disk2. Assume that the makebootconfig utility (described in Chapter 2 of the Oracle NoSQL Database Administrator's Guide, section, "Installation Configuration") is used to create an initial boot config with parameters such as the following:

    > java -jar KVHOME/lib/kvstore.jar makebootconfig \
           -root /opt/ondb/var/kvroot \
           -port 5000  \
           -admin 5001 \
           -host <host-ip>
           -harange 5010,5020 \
           -num_cpus 0  \
           -memory_mb 0 \
           -capacity 2  \
           -storagedir /disk1/ondb/data \
           -storagedir /disk2/ondb/data

With a boot config such as that shown above, the directory structure that is created and populated on each machine would then be:

     - Machine 1 (SN1) -     - Machine 2 (SN2) -    - Machine 3 (SN3) -
    /opt/ondb/var/kvroot   /opt/ondb/var/kvroot  /opt/ondb/var/kvroot
      log files             log files             log files
      /store-name           /store-name           /store-name
        /log                   /log                  /log
        /sn1                   /sn2                  /sn3
          config.xml             config.xml            config.xml
          /admin1                /admin2               /admin3
            /env                   /env                  /env

    /disk1/ondb/data         /disk1/ondb/data        /disk1/ondb/data
      /rg1-rn1                 /rg1-rn2                /rg1-rn3
        /env                     /env                    /env

    /disk2/ondb/data         /disk2/ondb/data        /disk2/ondb/data
      /rg2-rn1                 /rg2-rn2                /rg2-rn3
        /env                     /env                    /env

In this case, the configuration information and administrative data is stored in a location that is separate from all of the replication data. Furthermore, the replication data itself is stored by each distinct RN service on separate, physical media as well. That is, the data stored by a given member of each replication group (or shard) is stored on a disk that separate from the disks employed by the other members of the group.

### Note

Storing the data in these different locations as described above, provides for failure isolation and will typically make disk replacement less complicated and less time consuming. That is, by using a larger number of smaller disks, it is possible to recover much more quickly from a single disk failure because of the reduced amount of time it will take to repopulate the smaller disk. This is why both this note and Chapter 2 of the Oracle NoSQL Database Administrator's Guide, section, "Installation Configuration" strongly encourage configurations like that shown above; configurations that exploit separate physical media or disk partitions.

Even when a machine has only a single disk, nothing prevents the deployer from using the storagedir parameter in a manner similar to the multi-disk case; storing the configuration and administrative data under a parent directory that is different than the parent(s) under which the replicated data is stored. Since this non-default strategy may allow to create deployment scripts that can be more easily shared between single disk and multi-disk systems, some may prefer this strategy over using the default location (KVROOT); or may simply view it as a good habit to follow. Employing this non-default strategy is simply a matter of taste, and provides no additional benefit other than uniformity with the multi-disk case.

Hence, such a strategy applied to a single disk system will not necessarily make disk replacement less complicated; because, if that single disk fails and needs to be replaced, not only is all the data written by the RN(s) unavailable, but the configuration (and admin) data is also unavailable. As a result, since the configuration information is needed during the (RN) recovery process after the disk has been replaced, that data must be restored from a previously captured backup; which can make the disk replacement process much more complicated. This is why multi-disk systems are generally preferred in production environments; where, because of sheer use, the data disks are far more likely to fail than the disk holding only the configuration and other system data.

#### Procedure for Replacing a Failed Persistent Storage Device

Suppose a KVStore has been deployed to a set of machines, each with 3 disks, using the 'storagedir' parameter as described above. Suppose that disk2 on SN3 fails and needs to be replaced. In this case, the administrator would do the following:

1. Execute the KVStore administrative command line interface (CLI), connecting via one of the healthy admin services.

2. From the CLI, execute the following command:

       kv-> plan stop-service-service rg2-rn3

   This stops the service so that attempts by the system to communicate with that particular service are no longer necessary; resulting in a reduction in the amount of error output related to a failure the administrator is already aware of.

3. Remove disk2, using whatever procedure is dictated by the OS, the disk manufacture, and/or the hardware platform.

4. Install a new disk using the appropriate procedures.

5. Format the new disk to have the same storage directory as before; that is, `/disk2/ondb/var/kvroot`

6. From the CLI, execute the following commands; where the `verify configuration` command simply verifies that the desired RN is now up and running:

       kv-> plan start-service -service rg2-rn3 -wait
       kv-> verify configuration

7. Verify that the recovered RN data file(s) have the expected content; that is, `/disk2/ondb/var/kvroot/rg2-rn3/env/*.jdb`

In step 2, the RN service with id equal to 3, belonging to the replication group with id2, is stopped (rg2-rn3). To determine which specific RN service to stop when using the procedure outlined above, the administrator combines knowledge of which disk has failed on which machine with knowledge about the directory structure created during deployment of the KVStore. For this particular case, the administrator has first used standard system monitoring and management mechanisms to determine that disk2 has failed on the machine corresponding to the SN with id equal to 3 and needs to be replaced. Then, given the directory structure shown previously, the administrator knows that – for this deployment – the store writes replicated data to disk2 on the SN3 machine using files located under, `/disk2b/data/rg2-rn3/en`. As a result, the administrator determined that the RN service with name equal to rg2-rn3 must be stopped before replacing the failed disk.

In step 6, if the RN service that was previously stopped has successfully restarted when the `verify configuration` command is executed, and although the command's output indicates that the service is up and healthy, it is not necessary that the restarted RN has completely repopulated the new disk with that RN's data. This is because, it could take a considerable amount of time for the disk to recover all its data; depending on the amount of data that previously resided on the disk before failure. The system may encounter additional network traffic and load while the new disk is being repopulated.

Finally, it should be noted that step 7 is just a sanity check, and therefore optional. That is, if the RN service is successfully restarted and the `verify configuration` command reports RN as healthy, the results of that command is viewed as sufficient evidence for declaring the disk replacement a success. As indicated above, even if some data is not yet available on the new disk, that data will continue to be available via the other members of the recovering RN's replication group (shard), and will eventually be replicated to, and available from, the new disk as expected.

#### Example

Below, an example is presented that allows you to gain some practical experience with the disk replacement steps presented above. This example is intended to simulate the multi-disk scenario using a single machine with a single disk. Thus, no disks will actually fail or be physically replaced. But you should still feel how the data is automatically recovered when a disk is replaced.

For simplicity, assume that the KVStore is installed under `/opt/ondb/kv`; that is, `KVHOME=/opt/ondb/kv`, and that `KVROOT=/opt/ondb/var/kvroot`; that is, if you have not done so already, create the directory:

    > mkdir -p /opt/ondb/var/kvroot

To simulate the data disks, create the following directories:

    > mkdir -p /tmp/sn1/disk1/ondb/data
    > mkdir -p /tmp/sn1/disk2/ondb/data

    > mkdir -p /tmp/sn2/disk1/ondb/data
    > mkdir -p /tmp/sn2/disk2/ondb/data

    > mkdir -p /tmp/sn3/disk1/ondb/data
    > mkdir -p /tmp/sn3/disk2/ondb/data

Next, open 3 windows; Win_A, Win_B, and Win_C, which will represent the 3 machines (SNs). In each window, execute the `makebootconfig` command, creating a different, but similar, boot config for each SN that will be configured.

On Win_A

    java -jar /opt/ondb/kv/lib/kvstore.jar makebootconfig \
         -root /opt/ondb/var/kvroot \
         -host <host-ip> \
         -config config1.xml \
         -port 13230 \
         -harange 13232,13235 \
         -admin 13231 \
         -memory_mb 100 \
         -capacity 2 \
         -storagedir /tmp/sn1/disk1/ondb/data \
         -storagedir /tmp/sn1/disk2/ondb/data

On Win_B

    java -jar /opt/ondb/kv/lib/kvstore.jar makebootconfig \
         -root /opt/ondb/var/kvroot \
         -host <host-ip> \
         -config config2.xml \
         -port 13240 \
         -harange 13242,13245 \
         -admin 13241 \
         -memory_mb 100 \
         -capacity 2 \
         -storagedir /tmp/sn2/disk1/ondb/data \
         -storagedir /tmp/sn2/disk2/ondb/data

On Win_C

    java -jar /opt/ondb/kv/lib/kvstore.jar makebootconfig \
         -root /opt/ondb/var/kvroot \
         -host <host-ip> \
         -config config3.xml \
         -port 13250 \
         -harange 13252,13255 \
         -admin 13251 \
         -memory_mb 100 \
         -capacity 2    \
         -storagedir /tmp/sn3/disk1/ondb/data \
         -storagedir /tmp/sn3/disk2/ondb/data

This will produce 3 configuration files:

    /opt/ondb/var/kvroot
         /config1.xml
         /config2.xml
         /config3.xml

Using the different configurations just generated, start a corresponding instance of the KVStore storage node agent (SNA) from each window.

On Win_A

    > nohup java -jar /opt/ondb/kv/lib/kvstore.jar start \
                 -root /opt/ondb/var/kvroot -config config1.xml &

On Win_B

    > nohup java -jar /opt/ondb/kv/lib/kvstore.jar start \
                 -root /opt/ondb/var/kvroot -config config2.xml &

On Win_C

    > nohup java -jar /opt/ondb/kv/lib/kvstore.jar start \
                 -root /opt/ondb/var/kvroot -config config3.xml &

Finally, from any window (Win_A, Win_B, Win_C, or a new window), use the KVStore administrative CLI to configure and deploy the store.

To start the administrative CLI, execute the following command:

    > java -jar /opt/ondb/kv/lib/kvstore.jar runadmin \
           -host <host-ip> -port 13230

To configure and deploy the store, type the following commands from the administrative CLI prompt (remembering to substitute the actual IP address or hostname for the string <host-ip>):

    configure -name store-name
    plan deploy-zone -name Zone1 -rf 3 -wait
    plan deploy-sn -zn 1 -host <host-ip> -port 13230 -wait
    plan deploy-admin -sn 1 -port 13231 -wait
    pool create -name snpool
    pool join -name snpool -sn sn1
    plan deploy-sn -zn 1 -host <host-ip> -port 13240 -wait
    plan deploy-admin -sn 2 -port 13241 -wait
    pool join -name snpool -sn sn2
    plan deploy-sn -zn 1 -host <host-ip> -port 13250 -wait
    plan deploy-admin -sn 3 -port 13251 -wait
    pool join -name snpool -sn sn3
    change-policy -params "loggingConfigProps=oracle.kv.level=INFO;"
    change-policy -params cacheSize=10000000
    topology create -name store-layout -pool snpool -partitions 100
    plan deploy-topology -name store-layout -plan-name RepNode-Deploy -wait

### Note

The CLI command prompt (`kv->`) was excluded from the list of commands above to facilitate cutting and pasting the commands into a CLI load script.

When the above commands complete (use `show plans`), the store is up and running and ready for data to be written to it. Before proceeding, verify that a directory like that shown above for the multi-disk scenario has been laid out. That is:

       - Win_A -                 - Win_B -                - Win_C -

    /opt/ondb/var/kvroot      /opt/ondb/var/kvroot      /opt/ondb/var/kvroot
      log files                 log files                 log files
      /example-store            /example-store            /example-store
        /log                      /log                      /log
        /sn1                      /sn2                      /sn3
          config.xml                config.xml                config.xml
          /admin1                   /admin2                   /admin3
            /env                      /env                      /env
    /tmp/sn1/disk1/ondb/data  /tmp/sn2/disk1/ondb/data /tmp/sn3/disk1/ondb/data
      /rg1-rn1                  /rg1-rn2                  /rg1-rn3
        /env                      /env                      /env
          00000000.jdb              00000000.jdb              00000000.jdb

When a key/value pair is written to the store, it is stored in each of the (rf=3) files named, 00000000.jdb that belong to a given replication group (shard); for example, when a single key/value pair is written to the store, that pair would be stored in either these files:

    /tmp/sn1/disk2/ondb/data/rg2-rn1/env/00000000.jdb
    /tmp/sn2/disk2/ondb/data/rg2-rn2/env/00000000.jdb
    /tmp/sn3/disk2/ondb/data/rg2-rn3/env/00000000.jdb

Or in these files:

    /tmp/sn1/disk1/ondb/data/rg1-rn1/env/00000000.jdb
    /tmp/sn2/disk1/ondb/data/rg1-rn2/env/00000000.jdb
    /tmp/sn3/disk1/ondb/data/rg1-rn3/env/00000000.jdb

At this point, each file should contain no key/value pairs. Data can be written to the store in the most convenient way. But a utility that is quite handy for doing this is the KVStore client shell utility; which is a process that connects to the desired store and then presents a command line interface that takes interactive commands for putting and getting key/value pairs. To start the KVStore shell, type the following from a window:

    > java -jar /opt/ondb/kv/lib/kvstore.jar runadmin\
           -host <host-ip> -port 13230 -store store-name

    kv-> get -all
      0 Record returned.

    kv-> put -key /FIRST_KEY -value "HELLO WORLD"
      Put OK, inserted.

    kv-> get -all
      /FIRST_KEY
      HELLO WORLD

A quick way to determine which files the key/value pair was stored in is to simply grep for the string "HELLO WORLD"; which should work with binary files on most linux systems. Using the `grep` command in this way is practical for examples that consist of only a small amount of data.

    > grep "HELLO WORLD" /tmp/sn1/disk1/ondb/data/rg1-rn1/env/00000000.jdb
    > grep "HELLO WORLD" /tmp/sn2/disk1/ondb/data/rg1-rn2/env/00000000.jdb
    > grep "HELLO WORLD" /tmp/sn3/disk1/ondb/data/rg1-rn3/env/00000000.jdb

    > grep "HELLO WORLD" /tmp/sn1/disk2/ondb/data/rg2-rn1/env/00000000.jdb
      Binary file /tmp/sn1/disk2/ondb/data/rg2-rn1/env/00000000.jdb matches
    > grep "HELLO WORLD" /tmp/sn2/disk2/ondb/data/rg2-rn2/env/00000000.jdb
      Binary file /tmp/sn2/disk2/ondb/data/rg2-rn2/env/00000000.jdb matches
    > grep "HELLO WORLD" /tmp/sn3/disk2/ondb/data/rg2-rn3/env/00000000.jdb
      Binary file /tmp/sn3/disk2/ondb/data/rg2-rn3/env/00000000.jdb matches

In the example above, the key/value pair that was written to the store was stored by each RN belonging to the second shard; that is, each RN is a member of the replication group with id equal to 2 (rg2-rn1, rg2-rn2, and rg2-rn3).

### Note

With which shard a particular key is associated depends on the key's value (specifically, the hash of the key's value) as well as the number of shards maintained by the store. It is also worth noting that although this example shows log files with the name 00000000.jdb, those files are only the first of possibly many such log files containing data written by the corresponding RN service.

As the current log file reaches its maximum capacity, a new file is created to receive all new data written. That new file's name is derived from the previous file by incrementing the prefix of the previous file. For example, you might see files with names such as, "..., 00000997.jdb, 00000998.jdb, 00000999.jdb, 00001000.jdb,00001001.jdb, ...".

After the data has been written to the store, a failed disk can be simulated, and the disk replacement process can be performed. To simulate a failed disk, pick one of the storage directories where the key/value pair was written and, from a command window, delete the storage directory. For example:

    > rm -rf /tmp/sn3/disk2

At this point, if the log file for SN3 is examined, you should see repeated exceptions being logged. That is:

    > tail /opt/ondb/var/kvroot/store-name/log/sn3_0.log

    rg2-rn3: ProcessMonitor: java.lang.IllegalStateException: Error occurred
    accessing statistic log file
      /tmp/sn3/disk2/ondb/data/rg2-rn3/env/je.stat.csv.
    .......

But if the client shell is used to retrieve the previously stored key/value pair, the store is still operational, and the data that was written is still available. That is:

    kvshell-> get -all
      /FIRST_KEY
      HELLO WORLD

The disk replacement process can now be performed. From the command window in which the KVStore administrative CLI is running, execute the following (step 2 from above):

    kv-> plan stop-service -service rg2-rn3
      Executed plan 9, waiting for completion...
      Plan 9 ended successfully

    kv-> verify configuration
      .........
      Rep Node [rg2-rn3] Status: UNREACHABLE

If you attempt to restart the RN service that was just stopped, the attempt would not succeed. This can be seen via the contents of SN3's log `file/opt/ondb/var/kvroot/store-name/log/sn3_0.log`. The contents of that file indicate repeated attempts to restart the service, but due to the missing directory – that is, because of the "failed" disk – each attempt to start the service fails, until the process reaches an ERROR state; for example:

    kv-> show plans
      1 Deploy Zone (1) SUCCEEDED
      ......
      9 Stop RepNodes (9) SUCCEEDED
      10 Start RepNodes (10) ERROR

Now, the disk should be "replaced". To simulate disk replacement, we must create the original parent directory of rg2-rn3; which is intended to be analogous to installing and formatting the replacement disk:

    > mkdir -p /tmp/sn3/disk2/ondb/data

From the administrative CLI, attempt to restart the RN service should succeed since the disk has been "replaced".

    kv-> plan start-service -service rg2-rn3 -wait
      Executed plan 11, waiting for completion...
      Plan 11 ended successfully

    kv-> verify configuration
      .........
      Rep Node [rg2-rn3] Status: RUNNING,REPLICA at sequence
      number 327 haPort:13254

To verify that the data has been recovered as expected, grep for "HELLO WORLD" again.

    > grep "HELLO WORLD" /tmp/sn3/disk2/ondb/data/rg2-rn3/env/00000000.jdb
      Binary file /tmp/sn3/disk2/ondb/data/rg2-rn3/env/00000000.jdb matches

To see why the disk replacement process outlined above might be more complicated for the default – and by extension, the single disk – case than it is for the multi-disk case, try running the example above using default storage directories; that is, remove the storagedir parameters from the invocation of the `makebootconfig` command above. This will result in a directory structure such as:

    /opt/ondb/var/kvroot   /opt/ondb/var/kvroot  /opt/ondb/var/kvroot
      log files              log files             log files
      /store-name            /store-name           /store-name
        /log                   /log                  /log
        /sn1                   /sn2                  /sn3
          config.xml             config.xml            config.xml
          /admin1                /admin2               /admin3
          /rg1-rn1               /rg1-rn2              /rg1-rn3
          /rg2-rn1               /rg2-rn2              /rg2-rn3

In a similar example, to simulate a failed disk in this case, you would delete the directory `/opt/ondb/var/kvroot/sn3` ; which is the parent of the `/admin3` database, the `/rg1-rn3` database, and the `/rg2-rn3` database.

It is important to note that the directory also contains the configuration for SN3. Since SN3's configuration information is contained under the same parent – which is analogous to that information stored on the same disk – as the replication node databases; when the "failed" disk is "replaced" as it was in the previous example, the step where the RN service(s) are restarted will fail because SN3's configuration is no longer available. While the replicated data can be automatically recovered from the other nodes in the system when a disk is replaced, the SN's configuration information cannot. That data must be manually restored from a previously backed up copy. This extends to the non-default, single disk case in which different storagedir parameters are used to separate the KVROOT location from the location of each RN database. In that case, even though the replicated data is stored in separate locations, that data is still stored on the same physical disk. Therefore, if that disk fails, the configuration information is still not available on restart, unless it has been manually reinstalled on the replacement disk.

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