Example of zFS statistics

This blog post gives an example of zFS statistics, and my interpretation of what they mean.

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I IPLed my z/OS to give a clean system.

I used a batch job to read all of the files in a directory and throw away the output.

sh cat /usr/lpp/java/J8.0_64/lib/ext/* 1> /dev/null

The command

du -ka /usr/lpp/java/J8.0_64/lib/ext/

gave 16728 KB, and there were 30 files in the directory.

The interface layer

The command

query -knpfs


------------- ---------- ---------- ---------- ----------
Operation              Count      XCF req        Avg Time        Bytes 
-------------     ----------   ----------      ----------   ---------- 
zfs_opens                 37            0           0.053 
zfs_closes                37            0           0.024 
zfs_reads              4160            0           0.080      16.234M 
zfs_getattrs              86            0           0.036 
zfs_accesses             377            0           0.027 

There were 4160 read requests of 4096 bytes = 16MB

There were 30 opens one for each file.

There was an open for ‘/’, ‘/usr’ ‘/usr/lpp’ etc .. so 37 opens in total. At the end, each of these objects were closed.

The interface layer calls the buffer manager

The command

query -usercache

gave the User FIle (VM) Caching System Statistics report. It had

External requests
Reads     4160 Fsyncs     0 Schedules 0
Writes       0 Setattrs   0 Unmaps    0
Asy Reads 4126 Getattrs 153 Flushes   0

Which says there were 4160 read requests, which matches the zfs_reads request.

There were 4126 requests from the interface layer which had read-ahead set. This tells the buffer manager to get the pages. If they are not already in the buffer start reading them from disk. The Asy Reads does not give the reads from disk.

When I repeated the test I had: Reads 4160, Asy Reads 4120, with reads from disk 0 (as expected).

 File System Reads:
 Reads Faulted          34     (Fault Ratio    0.817%) 
 Writes Faulted          0     (Fault Ratio    0.000%) 
 Read Waits             34     (Wait Ratio     0.817%) 
 Total Reads           276 

This shows there were 276 reads from a file system, of which 34 requests had to wait for I/O.

I interpret this as saying there were 34 requests for get page which required disk I/O. The remained 276 – 34 caused I/O for read ahead so the application did not have to wait. I think the first page of each file was not in the cache, so there was an I/O to read the first segment(16 pages) of records in. There were 30 files, so 34 is close enough. The first request also started a Read Ahead to read the next segment in.

 Page Management (Segment Size = (64K) ) (Page Size = 8K) 
 Total Pages           121725     Free             118843 
 Segments                 395 
 Steal Invocations          0     Waits for Reclaim     0 

Before the test the free pages was 120933, so the delta is 2490 pages. Each page is 8KB, so the amount of storage used is 2490 * 8KB = 19.5 MB. The amount of data read from disk is 16.234MB so these numbers are comparable.

The Steal Invocations is the number of 64KB segments released to make space in the cache. In another test, I used a very small cache (10MB) and read 25636 KB of data in, and repeated the reads. Steal invocations was 404. 404 * 64 * 1024 = 25856 KB. This is close to the amount of data processed. Note: The documentation is incorrect,it says the value is the number of 4KB pages, not 64KB segments.

Data level

                   I/O Summary By Type 
 Count       Waits       Cancels     Merges      Type 
 ----------  ----------  ----------  ----------  ---------- 
         75          61           0           0  File System Metadata 
          0           0           0           0  Log File 
        276          51           0           0  User File Data 

This shows there were 75 I/O requests for meta information about the file, and 276 I/O requests to read the file itself. Reading the documentation I think the WAITS column indicates an I/O request was delayed before its I/O started, for example there was already an I/O outstanding.

                  zFS I/O by Currently Attached Aggregate 
 VOLSER IOs Mode  Reads  K bytes  Writes  K bytes  Dataset Name 
 ------ --- ----  -----  -------  ------  -------  ------------ 
 A4PRD3   1  R/O    302    16780       0        0  JVB800.ZFS 
 ------ --- ----  -----  -------  ------  -------  ------------ 
                    337    17104      14       56  *TOTALS* 

This shows there was I/O to the data set containing the Java file system. There were 302 reads, and it read 16780 KB of data.

I’ve omitted the other file systems which with 35 Reads, and 14 Writes.

These counts do not seem to tie up. There were 276 Reads to the User File Data, and 75 reads for File System Meta data, a total of 351. The zFS read count was 337.

3 thoughts on “Example of zFS statistics

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