The compression process

The compression process uses zEDC Express. The average amount of disk space that is saved per file averages approximately 65%, depending on the type of data that is being compressed.

If you cancel a compression that is in progress, the zFS file system will remain partially compressed. In a partially compressed file system, new files may or may not be compressed. You can resume the compression later with another zfsadm compress command.

The compression process is not mandatory. If the compression of a file does not reduce space, the file is left in its uncompressed format.

Restrictions:

Do not enable compression for any file system until you migrate all of your systems to z/OS V2R3. All systems in a sysplex must be at least z/OS V2R3 before any file systems are compressed because compression is not supported prior to z/OS V2R3. Also, do not use compression until you know that no system will be regressed to a prior release. Compressed file systems cannot be mounted on a release prior to V2R3. Therefore, if there is no zFS system in the shared file system environment that is eligible to own a compressed file system, the file system will be inaccessible.
Decompression is supported if there are pre-V2R3 systems in the sysplex in order to allow the compression to be backed out.
Only files larger than 8 K can be compressed. Directories and other control information inside the zFS file system are not compressed.
You cannot compress or decompress an aggregate that is in a partially encrypted or partially decrypted state. In other words, if an encryption or decryption process was stopped for an aggregate, you cannot compress or decompress that aggregate until after the encryption or decryption is completed.

Defining a new file system that is always compressed

The IOEFSPRM configuration option format_compression=on indicates a global default that is used by all formatting methods when determining the default compression behavior while formatting a new file system. This global compression default can be overridden by specifying the -nocompress keyword.

If IOEFSPRM configuration option format_compression=off is specified, all formatting methods can explicitly specify the -compress keyword to format the file system with compression.

The following example is JCL for defining and compressing a new aggregate.

//ZDEFFMT  JOB ,'DEF FORMAT COMPRESS',
//             MSGCLASS=H,
//             CLASS=A,
//             TIME=(1440),MSGLEVEL=(1,1)
//*---------------------------------------------------
//*  DEFINE FORMAT COMPRESS 
//*---------------------------------------------------
//*
//DEFINE   EXEC PGM=IDCAMS
//SYSPRINT DD SYSOUT=H
//SYSUDUMP DD SYSOUT=H
//AMSDUMP  DD SYSOUT=H
//DASD0    DD DISP=OLD,UNIT=3390,VOL=SER=SMBRS3
//SYSIN    DD *
     DEFINE CLUSTER (NAME(SUIMGNS.HIGHRISK.TEST) -
     ZFS CYL(2 0) SHAREOPTIONS(3))
/*
//CREATE   EXEC PGM=IOEFSUTL,REGION=0M,
// PARM=('format -aggregate SUIMGNS.COMPRESS.TEST -compress')
//SYSPRINT DD SYSOUT=H
//STDOUT   DD SYSOUT=H
//STDERR   DD SYSOUT=H
//SYSUDUMP DD SYSOUT=H
//CEEDUMP  DD SYSOUT=H

The following example uses the zfsadm format command with the -compress option to compress the new file system.

zfsadm format -aggregate PLEX.DCEIMGNJ.ENC -compress
 IOEZ00077I HFS-compatibility aggregate PLEX.DCEIMGNJ.ENC was successfully created.

Compressing existing file system data

Use the zfsadm compress command to compress existing file system data. You can cancel compression with the -cancel option and reverse compression with the zfsadm decompress command.

Before file system data can be compressed, these requirements must be met:

The file system that contains the data to be compressed must be mounted in read/write mode.
To avoid performance issues when the file system data is compressed, ensure that the system has sufficient zEDC capacity. For more information about performance analysis, see z/OS RMF User's Guide.

Important: IBM highly recommends backing up file systems before you begin the compression process.

Tips to improve performance:

If you are compressing data in a zFS aggregate, fixing the user file cache with the edcfixed option often results in CPU savings, especially if enough real memory is available to support fixing the user file cache and compression is used with zFS. If you are not compressing data in a zFS aggregate, then the edcfixed option of the user file cache might slightly reduce the CPU.
The zEDC user cache limit that can be fixed with the edcfixed option is 14 G but might be less, depending on real memory. To determine how much of the user file cache is fixed, use F ZFS,QUERY,VM or zfsadm query -usercache.
For optimum performance, use the health check ZFS_VERIFY_COMPRESSION_HEALTH to determine whether compression is being used and all user cache pages are registered with zEDC Express.

The following example uses the zfsadm compress command to compress the data in an existing aggregate.

zfsadm compress -aggregate  PLEX.DCEIMGNJ.BIGENC
IOEZ00899I Aggregate PLEX.DCEIMGNJ.BIGENC is successfully compressed.

The following example shows a file that was compressed.

# zfsadm  fileinfo  -path  testmtpt/file4                                       
   path: /home/suimgju/C81500/testmtpt/file4                                    
   ***   global data   ***                                                      
   fid                    5,1           anode                  291,1524         
   length                 24960         format                 BLOCKED          
   1K blocks              8             permissions            755              
   uid,gid                0,10          access acl             0,0              
   dir model acl          na            file model acl         na               
   user audit             F,F,F         auditor audit          N,N,N            
   set sticky,uid,gid     0,0,0         seclabel               none             
   object type            FILE          object linkcount       1                
   object genvalue        0             dir version            na               
   dir name count         na            dir data version       na               
   dir tree status        na            dir conversion         na               
   file format bits       0x0,0,0       file charset id        0x0              
   file cver              none          charspec major,minor   na               
   direct blocks          0x00000007    0x80000401    0x80000000    0x80000000  
   indirect blocks        none                                                  
   mtime       Jan 19 12:27:56 2017     atime        Jan 19 12:27:56 2017       
   ctime       Jan 19 12:27:56 2017     create time  Jan 19 12:27:56 2017       
   reftime     none                                                             
   not encrypted                        compressed 24K saved

The following example uses the zfsadm compress command with the -cancel option to cancel a compression request.

zfsadm compress -aggregate PLEX.DCEIMGNJ.BIGENC -cancel           
IOEZ00903I Aggregate PLEX.DCEIMGNJ.BIGENC compress or decompress successfully canceled.

Then use zfsadm fsinfo to display the status:

Monitoring and displaying the compression status

Use the zfsadm fsinfo command to monitor the compression status. To display the compression status, use either zfsadm fileinfo or zfsadm fsinfo.

The following example uses zfsadm fsinfo to monitor the compression status.

zfsadm fsinfo -aggregate  PLEX.DCEIMGNJ.BIGENC

File System Name: PLEX.DCEIMGNJ.BIGENC

   *** owner information ***
   ..........
   Status:              RW,RS,NE,CI
   ...
   ...   
   Compress Progress: running, 48% started at Nov 21 16:34:40 2016 task 57F5E0
   ...

Legend: RW=Read-write, RS=Mounted RWSHARE, NE=Not encrypted
        CI=Partially compressed

The following example uses zfsadm fsinfo with the -basic option to display the compression status.

zfsadm fsinfo -aggregate PLEX.DCEIMGNJ.BIGENC  -basic
PLEX.DCEIMGNJ.BIGENC                         DCEIMGNJ RW,RS,EI,CO

Legend: RW=Read-write, RS=Mounted RWSHARE, EI=Partially Encrypted
        CO=Compressed

The following example uses zfsadm query with the -compress option to monitor the compression effectiveness and performance of zEDC services.

zfsadm query -compress

Compression calls:       246428  Avg. call time:               0.177
  KB input             13190960    KB output             1971456    
Decompression calls:     509140  Avg. call time:               0.154
  KB input              4073128    KB output            21406072

The zfsadm fileinfo command shows an exact count of kilobytes saved for a file that is compressed. The following example uses zfsadm fileinfo to display the compression status.

zfsadm fileinfo /tst/myfile
   path: /tst/myfile
   ***   global data   ***
   ...
   mtime       Nov  2 11:21:01 2015     atime        Nov  2 11:21:01 2015
   ctime       Nov  2 11:21:01 2015     create time  Nov  2 11:21:01 2015
   reftime     none
   not encrypted                        compressed 4762K saved