6.3.9.3 Monitoring Cell Disk I/O Using Exadata Metrics

Cell disk metrics provide information about the I/O load for cell disks, such as the number of large blocks read from a cell disk.

Cell disk metrics are identified in the METRICCURRENT, METRICDEFINITION, and METRICHISTORY objects as having objectType=CELLDISK.

The cell disk for the metric is specified by the metricObjectName attribute of the METRICCURRENT and METRICHISTORY objects.

For cumulative metrics, the metric value for a specific time period is determined by subtracting values from different collectionTime periods. For rate metrics, the time period for the metric value is over the previous minute.

Example 6-12 Displaying Cell Disk Metric Definitions

This example shows how to display the cell disk metric definitions that are available in the Oracle Exadata System Software.

CellCLI> LIST METRICDEFINITION ATTRIBUTES NAME,DESCRIPTION WHERE OBJECTTYPE = CELLDISK
         CD_BY_FC_DIRTY          "Number of unflushed megabytes cached in FLASHCACHE on a cell disk"
         CD_IO_BY_R_LG           "Number of megabytes read in large blocks from a cell disk"
         CD_IO_BY_R_LG_SEC       "Number of megabytes read in large blocks per second from a cell disk"
         CD_IO_BY_R_SCRUB        "Number of megabytes read from a cell disk by the scrubbing job"
         CD_IO_BY_R_SCRUB_SEC    "Number of megabytes read per second from a cell disk by the scrubbing job"
         CD_IO_BY_R_SM           "Number of megabytes read in small blocks from a cell disk"
         CD_IO_BY_R_SM_SEC       "Number of megabytes read in small blocks per second from a cell disk"
         CD_IO_BY_W_LG           "Number of megabytes written in large blocks to a cell disk"
         CD_IO_BY_W_LG_SEC       "Number of megabytes written in large blocks per second to a cell disk"
         CD_IO_BY_W_SM           "Number of megabytes written in small blocks to a cell disk"
         CD_IO_BY_W_SM_SEC       "Number of megabytes written in small blocks per second to a cell disk"
         CD_IO_ERRS              "Number of IO errors on a cell disk"
         CD_IO_ERRS_MIN          "Number of IO errors on a cell disk per minute"
         CD_IO_ERRS_SCRUB        "Number of IO errors hit by the scrubbing job on a cell disk"
         CD_IO_LOAD              "Average I/O load for the cell disk"
         CD_IO_RQ_R_LG           "Number of requests to read large blocks from a cell disk"
         CD_IO_RQ_R_LG_SEC       "Number of requests to read large blocks per second from a cell disk"
         CD_IO_RQ_R_SCRUB        "Number of requests to read from a cell disk by the scrubbing job"
         CD_IO_RQ_R_SCRUB_SEC    "Number of requests to read per second from a cell disk by the scrubbing job"
         CD_IO_RQ_R_SM           "Number of requests to read small blocks from a cell disk"
         CD_IO_RQ_R_SM_SEC       "Number of requests to read small blocks per second from a cell disk"
         CD_IO_RQ_W_LG           "Number of requests to write large blocks to a cell disk"
         CD_IO_RQ_W_LG_SEC       "Number of requests to write large blocks per second to a cell disk"
         CD_IO_RQ_W_SM           "Number of requests to write small blocks to a cell disk"
         CD_IO_RQ_W_SM_SEC       "Number of requests to write small blocks per second to a cell disk"
         CD_IO_ST_RQ             "Average service time per request for small IO requests to a cell disk"
         CD_IO_TM_R_LG           "Cumulative latency of reading large blocks from a cell disk"
         CD_IO_TM_R_LG_RQ        "Average latency of reading large blocks per request to a cell disk"
         CD_IO_TM_R_SM           "Cumulative latency of reading small blocks from a cell disk"
         CD_IO_TM_R_SM_RQ        "Average latency of reading small blocks per request from a cell disk"
         CD_IO_TM_W_LG           "Cumulative latency of writing large blocks to a cell disk"
         CD_IO_TM_W_LG_RQ        "Average latency of writing large blocks per request to a cell disk"
         CD_IO_TM_W_SM           "Cumulative latency of writing small blocks to a cell disk"
         CD_IO_TM_W_SM_RQ        "Average latency of writing small blocks per request to a cell disk"
         CD_IO_UTIL              "Percentage of disk resources utilized for the cell disk"
         CD_IO_UTIL_LG           "Percentage of disk resources utilized by large requests for the cell disk"
         CD_IO_UTIL_SM           "Percentage of disk resources utilized by small requests for the cell disk"

Note the following additional details:

• CD_IO_LOAD represents the average I/O load for the cell disk.

  I/O load specifies the length of the disk queue. It is similar to iostat's avgqu-sz, but I/O load is a weighted value depending on the type of disk:

  • For hard disks, a large I/O has three times the weight of a small I/O.
  • For flash disks, large and small I/Os have the same weight.

  Corresponding metrics are also available for each database (DB_IO_LOAD), pluggable database (PDB) (PDB_IO_LOAD), IORM category (CT_IO_LOAD), and consumer group (CG_IO_LOAD).

• CD_IO_ST_RQ cannot be used for flash devices, as explained in Using IOSTAT.

• CD_IO_UTIL is similar to %util of iostat for hard disks, but not for flash devices as mentioned in Using IOSTAT. For flash devices, it is a percentage of the maximum I/O bandwidth for your system, as specified in the product data sheet.

  Because this metric is computed by IORM, it is also available per database, PDB, and consumer group.

• The unit of measurement for the latency metrics is microseconds. The latency metrics have metric names starting with CD_IO_TM_.