Rules for max_coalesce and queue_depth for ssa RAID-5 disk array

max_coalesce 
 This is the maximum number of bytes which the SSA disk device
 driver attempts to transfer to or from an SSA logical disk in a
 single operation. The default value is appropriate for most
 environments. For applications that perform very long sequential
 write operations, there are performance benefits in writing data
 in blocks of 64KB times the number of disks in the array minus
 one (these are known as full-stride writes times the number of
 disks in the array minus one, or to some multiple of this number.

queue_depth 
 Specifies the maximum number of commands that the SSA disk device
 driver dispatches for a single disk drive for an hdisk. You can use
 the chdev command to modify this attribute. The default value is
 correct for normal operating conditions.

Rules 
max_coalesce is (N-1)*64k
queue_depth is from 2*N to 3*N

N = number of disk in the ssa raid-5 array
0x10000 is 64K


    N      max_coalesce                    queue_depth

 3 disks   0x20000                         6 to 9
 4 disks   0x30000                         8 to 12
 5 disks   0x40000                        10 to 15
 6 disks   0x50000                        12 to 18
 7 disks   0x60000                        14 to 21
 8 disks   0x70000                        16 to 24
 9 disks   0x80000                        18 to 27
10 disks   0x90000                        20 to 30
11 disks   0xA0000  ( not 0X100000)       22 to 33
12 disks   0xB0000  ( not 0X110000)       24 to 36
13 disks   0xC0000  ( not 0X120000)       26 to 39
14 disks   0xD0000  ( not 0X130000)       28 to 42
15 disks   0xE0000  ( not 0X140000)       30 to 45



To change max_coalesce or queue_depth:
1)  umount all filesystems on the raid-5 vg
2)  varyoffvg the volume group.
3)  Use  smitty chgssardsk or chdev
    Example: chdev -l hdisk2 -a max_coalesce=0xb0000
    Example: chdev -l hdisk2 -a queue_depth=3
4)  to verify the change:  lsattr -El hdiskX

Created: Nov 2003 by Bruno Croft