VM

The User File (VM) Caching System Statistics report shows the performance of the zFS user file cache. The Sysplex Client Caching System Statistics Report shows the performance of the zFS client file cache. The size of both of these caches is controlled by the IOEFSPRM user_cache_size or client_cache_size configuration options or the zfsadm config command.

The zFS user file cache and the client file cache data are stored in a collection of dataspaces. zFS prefers to use multiple dataspaces rather than one large dataspace when it can to reduce lock contention (as shown in this example). zFS has a structure for each file that is cached. The client cache breaks the cached file into 32 K segments and the user cache breaks the cached file into 64 K segments. In both cases, each segment is broken into 4 K pages. A segment is assigned to a dataspace, hence the pages for any given segment belong only to one dataspace. A file's pages can be scattered throughout multiple segments.

At any given time, a file need not (and for large files often might not) have all of its segments in the cache. Furthermore, any segment need not (and often might not) have all of its pages in the cache. Reuse of pages and segments is done in an least-recently used (LRU) fashion.

The cache provides asynchronous read-ahead and write-behind of large files when access is considered sequential. Read-ahead and write-behind for a file is performed by reading/writing segments (up to 64 K).

Following is a sample VM report.

IOEZ00438I Starting Query Command VM.                             
                   User File (VM) Caching System Statistics           
                   ----------------------------------------           
                                                                      
External Requests:                                                    
------------------                                                    
Reads        5107802     Fsyncs          1990     Schedules     366517
Writes       5503223     Setattrs        7091     Unmaps        349352
Asy Reads    3809619     Getattrs     1219759     Flushes            0
                                                                      
File System Reads:                                                    
------------------                                                    
Reads Faulted      164132     (Fault Ratio    3.213%)                 
Writes Faulted     167756     (Fault Ratio    3.048%                  
Read Waits              0     (Wait Ratio    0.000%)                   
Total Reads        357542                                             
                                                                      
File System Writes:                                                   
-------------------                                                   
Scheduled Writes        119608     Sync  Waits        22706           
Error Writes                 0     Error Waits            0          
Scheduled deletes            0                                  
Page Reclaim Writes      37315     Reclaim Waits      19392  
Write Waits               5454     (Wait Ratio    0.099%    
                                                                      
Page Management (Segment Size = 64K) ) (Page Size = 4K)               
----------------------------------------------------                  
Total Pages       12800     Free              12800                   
Segments           2373                                               
Steal Invocations      24146     Waits for Reclaim       62038        
                                                                      
Number of dataspaces used:     1  Pages per dataspace:       12800    
                                                                      
Dataspace   Allocated     Free                                        
Name        Segments      Pages                                       
--------    ----------    ----------                                  
ZFSUCD00             0         12800                                  
                   Sysplex Client Caching System Statistics           
                   ----------------------------------------           
                                                                      
External Requests:                                                    
------------------                                                    
Reads              0     Fsyncs             0     Schedules          1
Writes         46737     Setattrs           0     Unmaps             0
Asy Reads          0     Getattrs           0     Flushes            0
                                                                      
File System Reads:                                                    
------------------                                                    
Reads Faulted           0     (Fault Ratio    0.000%)                  
Writes Faulted          0     (Fault Ratio    0.000%)                  
Read Waits              0     (Wait Ratio     0.000%)                  
Total Reads             0                                                                                                                   
File System Writes:                                                   
-------------------                                                   
Scheduled Writes          1462     Sync  Waits            0           
Error Writes               106     Error Waits            0           
Scheduled deletes            0                                        
Page Reclaim Writes          0     Reclaim Waits          0           
Write Waits                  0     (Wait Ratio    0.000%)              
                                                                      
Page Management (Segment Size = 32K) ) (Page Size = 4K)               
----------------------------------------------------                  
Total Pages       32768     Free              32263                   
Segments          16384                                               
Steal Invocations          0     Waits for Reclaim           0        
                                                                      
Number of dataspaces used:     2  Pages per dataspace:       16384    
                                                                      
Dataspace   Allocated     Free                                        
Name        Segments      Pages                                         
--------    ----------    ----------                                    
ZFSCCD00            96         15616                                    
ZFSCCD01            96         15621

Table 1 describes the fields of the User File (VM) Caching System Statistics report.

Table 1. User File (VM) Caching System Statistics report fields
Field name	Contents
External Requests:	Describes the requests made to the user file cache to perform operations as requested by applications. Reads, Writes How often the cache was called to read or write files. Asy Reads How often read-ahead is performed. Fsync How often applications requested that zFS sync a file's data to disk. Unmaps The count of file deletions.
File System Reads:	Shows how often the cache reads data from disk for a file. Cache misses and read I/Os degrade application response time and the goal is for these numbers to be as low as possible. Increasing the cache size is the typical method for lowering these numbers. Reads Faulted Count of read requests that needed to perform at least one I/O to read the requested portion of the file from disk. Writes Faulted Count of how often a write to a file needed to perform a read from disk. If a write only updates a portion of a page of a file on disk and that page is not in memory, then the page must be read in (the zFS I/O driver can only perform I/O in whole pages) before the new data is written to the in-memory page. Read Waits How often a read had to wait for a pending I/O. For example, how often a read of a file found that the range of the file is pending read (probably because of asynchronous read ahead). Total Reads Total number of file system reads made for any reason.
File System Writes:	Shows how often the cache wrote the data to disk. In general, it is desirable to minimize the Page Reclaim Writes and Reclaim Waits. If these occur often, relative to the external zFS request rate (shown in the KN report), then the cache might be too small. Scheduled Writes Count of how often the cache wrote out dirty segments for a file. Segments are written as soon as every page becomes dirty (segments are said to be dirty if they contain live blocks). When a file is closed all of its dirty segments are scheduled asynchronously and segments are also written asynchronously during file system syncs through the zFS sync daemon (which by default runs every 30 seconds). Sync Waits Count of how often a fsync request that is needed to wait on pending I/O for dirty segments. Error Writes and Error Waits Count of the error handling paths and should almost always be 0 unless a disk hardware error occurs. Whenever an unexpected error occurs for a file, all of its dirty segments are written and synced to disk. (A file system that is running out of space is not an error condition that causes the cache to sync a file, the cache reserves storage for files as they are written which ensures no unexpected out of space conditions arise). Scheduled Deletes Count of times a pending I/O was canceled because a file was being deleted. In this case, the data is not appropriate to be on disk (because the file is 0 link count). Therefore, canceling the I/O is done to avoid an I/O wait. This is a performance optimization for file remove. Page Reclaim Writes Count of times that a segment had to be written to DASD to reclaim space in the cache Page Reclaim Waits Count of times that the reclaim function waited on pending I/O to reclaim segment pages. Write Waits Count of times a write occurred to a page that was already pending I/O. In this case, the I/O must be waited upon before the page is updated with the new data.
Page Management:	Shows how storage in the user file cache and client file cache is used. It is generally desirable to minimize the number of steal invocations (reclaims). This can typically be done by increasing the size of the cache. Performance is increased as more dataspaces are used. Total pages The number of 4 K pages in the cache. That is, (user_cache_size / 4K) or (client_cache_size / 4K). Free The number of available 4 K pages in the cache. Segments The number of 64 K sections that was referenced in a file. The number of segments starts out as half of vnode_cache_size and is allocated as needed, similar to vnodes. Steal Invocations The number of times 4 K pages were reclaimed from the cache. Waits for Reclaim The number of times 4 K pages were reclaimed from the cache. Number of dataspaces The number of dataspaces used to hold the 4 K pages in the cache. The pages are spread evenly across the dataspaces to allow for better performance of the cache. The number of dataspaces used is approximately one per 16384 4 K pages, up to a maximum of 32. Pages per dataspace The number of 4 K pages that is assigned to each dataspace.