onstat -g rah command: Section 5 Partition ReadAhead Statistics

This section displays cumulative information for all partitions involved with read-ahead operations. It consists of seven sub-sections.

Figure 5. onstat -g rah command output, section 5, Buffer subsection
Partition ReadAhead Statistics
           Buffer              hit    …
partnum    bfcread  dskreads   ratio  …
0x200005   278628   5573       98     …
0x20008e   3814566  228874     94     …
0x300345   22       4          81     …
0x6000b3   60517    13314      78     …
0x60030a   66       26         60     …

Output description

partnum: The partition number of the table or index.
bfcread: Number of buffer reads for this partition.
dskreads: Number of disk reads for this partition.
hit ratio: A measure of how often we are reading from the cache vs. performing I/O.

Figure 6. onstat -g rah command output, section 5, Data subsection
… Data …
… #reqs pagecnt nios    eff …
… 1001  1000248 1000248 100 …
… 1     1       0       0   …
… 58    1102    25      2   …
… 2     2       0       0   …

Output description

#reqs: Number of data page read-ahead requests.
pagecnt: Number of pages read ahead for these requests.
nios: Number of I/Os done for these requests.
eff: Effectiveness of the read-ahead requests. This value is a bit counter-intuitive because it is higher the more I/O that occurs. When a data page read-ahead is requested and the page is discovered in the buffer pool, that is considered a read-ahead "miss", because unnecessary work was done. A request that results in an I/O is deemed a "hit", because the read-ahead was proven necessary and the requestor likely benefited from the daemon's work. The closer the ‘nios’ value is to the ‘pagecnt’ value, the more effective the read-ahead requests are judged to be.

Figure 7. onstat -g rah command output, section 5, Index subsection
… Index …
… #reqs pagecnt nios eff  …
… 14    173     170  98   …
… 7     1       0    0    …
… 10    5       1    20   …
… 3     36      25   69   …

Output description

#reqs: Number of leaf page read-ahead requests.
pagecnt: Number of pages read ahead for these requests.
nios: Number of I/Os done for these requests.
eff: Effectiveness of the read-ahead requests.

Figure 8. onstat -g rah command output, section 5, Index/Data subsection
… I/D …
… #reqs pagecnt nios eff …
… 8     562     539  96  …
… 1     1       0    0   …
… 0     0       0    0   …
… 20    3683    2595 70  …

Output description

#reqs: Number of index/data read-ahead requests. These are data page read-aheads triggered by leaf scans. As batches of rowids are gathered from index items, RA requests are made for the associated data pages.
pagecnt: Number of pages read ahead for these requests.
nios: Number of I/Os done for these requests.
eff: Effectiveness of the read-ahead requests.

Figure 9. onstat -g rah command output, section 5, LogRec/PageList subsection
… LogRec/PageList  …
… npages nios eff  …
… 92     539  96   …
… 30     0    0    …
… 2345   2177 93   …
… 1      1    100  …

Output description

npages: Number of pages read ahead for this request, which was made by a thread performing a rollback. While undoing a long series of operations for a transaction we modify all types of pages, and if a rollback finds it is waiting on I/O for these pages it will request “page list" read-aheads.
nios: Number of I/Os done for this request.
eff: Effectiveness of the read-ahead requests.

Figure 10. onstat -g rah command output, section 5, Last Committed subsection
… Last Committed …
… #reqs #used #resched #fail eff  …
… 5     5     0        0     100  …
… 1     1     0        0     100  …
… 0     0     0        0     0    …
… 10    9     0        0     90   …

Output description

#reqs: Number of read-ahead requests triggered by a last-committed read.
#used: Number of pages that were used by the last-committed read.
#resched: Number of times a request was rescheduled due to contention.
#fail: Number of times a last-committed read failed.
eff: Effectiveness of the read-ahead requests (#used/#reqs)*100.

Figure 11. onstat -g rah command output, section 5, Partn_Pages subsection
… Partn_Pages            Total RA Pages
… #reqs pagecnt nios eff read used
… 1     495     481  97  523  443
… 1     98      33   33  111  86
… 1     197     85   43  345  301
… 1     97      32   32  134  98

Output description

#reqs: Number of read-ahead requests for partition pages. These requests are made at the beginning of an archive, when all partition pages in the relevant DBspaces must be read in order to build the list of used extents. In this case the partnum shown will be for the partition partition being scanned.
pagecnt: Number of pages read by these requests.
nios: Number of I/Os done for this request.
eff: Effectiveness of the read-ahead requests.
read: Total pages read ahead for this partition.
used: Total pages that were read ahead and actually used by scanners.