Thu Aug 21 17:24:06 2014
WARNING: Waited 15 secs for write IO to PST disk 0 in group 1.
WARNING: Waited 15 secs for write IO to PST disk 0 in group 1.
WARNING: Waited 15 secs for write IO to PST disk 0 in group 2.
WARNING: Waited 15 secs for write IO to PST disk 0 in group 2.
WARNING: Waited 15 secs for write IO to PST disk 0 in group 3.
WARNING: Waited 15 secs for write IO to PST disk 1 in group 3.
WARNING: Waited 15 secs for write IO to PST disk 2 in group 3.
WARNING: Waited 15 secs for write IO to PST disk 3 in group 3.
WARNING: Waited 15 secs for write IO to PST disk 4 in group 3.
WARNING: Waited 15 secs for write IO to PST disk 0 in group 3.
WARNING: Waited 15 secs for write IO to PST disk 1 in group 3.
WARNING: Waited 15 secs for write IO to PST disk 2 in group 3.
WARNING: Waited 15 secs for write IO to PST disk 3 in group 3.
WARNING: Waited 15 secs for write IO to PST disk 4 in group 3.
Thu Aug 21 17:24:06 2014
NOTE: process _b000_+asm1 (24903780) initiating offline of disk 0.2095165706 (GRID_0000) with mask 0x7e in group 3
NOTE: process _b000_+asm1 (24903780) initiating offline of disk 1.2095165707 (GRID_0001) with mask 0x7e in group 3
NOTE: process _b000_+asm1 (24903780) initiating offline of disk 2.2095165708 (GRID_0002) with mask 0x7e in group 3
NOTE: process _b000_+asm1 (24903780) initiating offline of disk 3.2095165709 (GRID_0003) with mask 0x7e in group 3
NOTE: process _b000_+asm1 (24903780) initiating offline of disk 4.2095165710 (GRID_0004) with mask 0x7e in group 3
NOTE: checking PST: grp = 3
GMON checking disk modes for group 3 at 10 for pid 35, osid 24903780
ERROR: no read quorum in group: required 3, found 0 disks
NOTE: checking PST for grp 3 done.

....

NOTE: initiating PST update: grp = 3, dsk = 0/0x7ce1b10a, mask = 0x6a, op = clear

2014-08-21 20:36:04.495: [  OCRRAW][9264]proprior: Retrying buffer read from another mirror for disk group [+GRID] for block at offset [6909952]
2014-08-21 20:36:04.495: [  OCRASM][9264]proprasmres: Total 0 mirrors detected
2014-08-21 20:36:04.495: [  OCRASM][9264]proprasmres: Only 1 mirror found in this disk group.
2014-08-21 20:36:04.495: [  OCRASM][9264]proprasmres: Need to invoke checkdg. Mirror #0 has an invalid buffer.
2014-08-21 20:36:04.595: [  OCRASM][9264]proprasmres: kgfoControl returned error [8]
[  OCRASM][9264]SLOS : SLOS: cat=8, opn=kgfoCkDG01, dep=15032, loc=kgfokge

2014-08-21 20:36:04.595: [  OCRASM][9264]ASM Error Stack : ORA-27091: unable to queue I/O
ORA-15078: ASM diskgroup was forcibly dismounted
ORA-06512: at line 4

2014-08-21 20:36:04.595: [  OCRRAW][9264]proprior: ASM re silver returned [22]
2014-08-21 20:36:04.597: [  OCRRAW][9264]fkce:2: problem [22] reading the tnode 6909952

Symptoms
Normal or high redundancy diskgroup is dismounted with these WARNING messages.

//ASM alert.log

Mon Jul 01 09:10:47 2013
WARNING: Waited 15 secs for write IO to PST disk 1 in group 6.
WARNING: Waited 15 secs for write IO to PST disk 4 in group 6.
WARNING: Waited 15 secs for write IO to PST disk 1 in group 6.
WARNING: Waited 15 secs for write IO to PST disk 4 in group 6.
....
GMON dismounting group 6 at 72 for pid 44, osid 8782162

Cause
Generally this kind messages comes in ASM alertlog file on below situations,
Delayed ASM PST heart beats on ASM disks in normal or high redundancy diskgroup,
thus the ASM instance dismount the diskgroup.By default, it is 15 seconds.
By the way the heart beat delays are sort of ignored for external redundancy diskgroup.
ASM instance stop issuing more PST heart beat until it succeeds PST revalidation,
but the heart beat delays do not dismount external redundancy diskgroup directly.

The ASM disk could go into unresponsiveness, normally in the following scenarios:

+    Some of the paths of the physical paths of the multipath device are offline or lost
+    During path 'failover' in a multipath set up
+    Server load, or any sort of storage/multipath/OS maintenance
The Doc ID 10109915.8  briefs about Bug 10109915(this fix introduce this underscore parameter). And the issue is with no OS/Storage tunable timeout mechanism in a case of a Hung NFS Server/Filer. And then  _asm_hbeatiowait  helps in setting the time out.

Solution
1]    Check with OS and Storage admin that there is disk unresponsiveness.

2]    Possibly keep the disk responsiveness to below 15 seconds.

This will depend on various factors like
+    Operating System
+    Presence of Multipath ( and Multipath Type )
+    Any kernel parameter

So you need to find out, what is the 'maximum' possible disk unresponsiveness for your set up.
For example, on AIX  rw_timeout  setting affects this and defaults to 30 seconds.
Another example is Linux with native multipathing. In such set up, number of physical paths and  polling_interval value in multipath.conf file, will dictate this maximum disk unresponsiveness.
So for your set up ( combination of OS / multipath / storage ), you need to find out this.

3]If you can not keep the disk unresponsiveness to below 15 seconds, then the below parameter can be set in the ASM instance ( on all the Nodes of RAC ):

_asm_hbeatiowait
Set it to 200.

Run below in asm instance to set desired value for _asm_hbeatiowait

alter system set "_asm_hbeatiowait"=<value> scope=spfile sid='*';