How to slow down plex resync or other VERITAS Volume Manager copy operations

The speeds of a plex resync and other VERITAS Volume Manager (VxVM) copy operations like vxrelayout are governed by two parameters:  iosize and slow.

The optional parameter iosize defines the size of data blocks to be copied [default=32k]. Theoretically, the greater the block size the faster the copy process. For large values of iosize it can have a detrimental effect on other processes doing I/O.

The optional parameter 'slow'  defines the number of milliseconds the copy process will pause until the next data block of size iosize gets processed[default=0].

The most effective way to lower the resync speed is to set the optional slow parameter:   slow=iodelay


CaseA:  lower the resync speed of a running process:

1. Check throttle setting [parameter slow]:

# vxtask -l list
Task:      179 RUNNING (Parent:178)
Type:      ATCOPY
Operation: PLXATT Vol opt Plexopt-02
Started:   Tue Mar 12 16:20:342002
Throttle:  0
Progress:  5.84% 612560 of 10487232Blocks
Work time: 38 seconds (10:12 remaining)
#

2. Increase the throttle setting:

# vxtask set slow=50 179

# vxtask -l list 179
Task:      179 RUNNING (Parent:178)
Type:      ATCOPY
Operation: PLXATT Vol opt Plexopt-02
Started:   Tue Mar 12 16:20:342002
Throttle:  50
Progress:  36.20% 3796880 of 10487232Blocks
Work time: 3 minutes, 46 seconds (44:38remaining)
#



Case B:   start a new VxVM copy process with a lowered resync speed:

e.g: vxrecover -o slow=50 -sb
    vxplex -g <DG> -o slow=30 att opt opt-02
    vxassist -g <DG> -o slow=10 mirror opt

Check the new throttle setting [parameter slow] with vxtask -l list.

The optional parameter slow is accepted for a wide range of commands:  
vxvol, vxplex, vxrelayout, vxassist,vxrecover.

If the resync speed of a process was reduced by too much it can always be set to a more appropriate level using the vxtask command as shown in case A.