[dm-devel] Enabling/disabling BIO_RW_FAILFAST at load time

[Kumar, Vijay (STSD)]

Hi,
 
One Typo below . Its as  
 Flag can be set On/Off as follows 

        modprobe dm-multipath fastfailbit=0             //seting it off 
        modprobe dm-multipath fastfailbit=1             //seting it on 

Thanks, 

Vijay Kumar 

[Kumar, Vijay (STSD)] 
mailto: vijay.kumar7 at hp.com 


________________________________

	From: dm-devel-bounces at redhat.com
[mailto:dm-devel-bounces at redhat.com] On Behalf Of Kumar, Vijay (STSD)
	Sent: Wednesday, September 26, 2007 12:18 AM
	To: device-mapper development; christophe.varoqui at free.fr
	Subject: [dm-devel] Enabling/disabling BIO_RW_FAILFAST at load
time
	
	

	Hi Christophe and All, 
	During I/O on a path, the paths are marked as failed at DM
Multipath, when the I/O returns with Error status as 0x20000 (host code
0x2 denotes DID_BUS_BUSY). What we understand here is, since I/O on
multipath devices have BIO_RW_FAILFAST set(hence REQ_FASTFAIL ), retries
are not allowed at SCSI mid layer for errors such as QUEUEFULL,
UNDERRUN....(as captured in the below syslog snippet)

	syslog snippet 
	******************* 
	May 23 16:51:49 lnx kernel: sd 2:0:3:8: SCSI error: return code
= 0x20000
	May 23 16:51:49 lnx kernel: end_request: I/O error, dev sdfg,
sector 4188002
	May 23 16:51:49 lnx kernel: end_request: I/O error, dev sdfg,
sector 4188008
	May 23 16:51:49 lnx kernel: device-mapper: dm-multipath: Failing
path 130:32.
	May 23 16:51:49 lnx kernel: sd 2:0:3:8: SCSI error: return code
= 0x20000
	May 23 16:51:49 lnx kernel: end_request: I/O error, dev sdfg,
sector 4188302
	May 23 16:51:49 lnx kernel: end_request: I/O error, dev sdfg,
sector 4188303
	May 23 16:52:00 lnx kernel: sd 1:0:2:12: SCSI error: return code
= 0x20000 

	As an option we can have user configurable BIO_RW_FAILFAST for
enabling and disabling which allows scsi mid layer to retry for I/0 .
Below is the patch for passing parameter at load time with DM multipath
module for enabling/disabling BIO_RW_FAILFAST flag. 

	* Flag can be set On/Off as follows 

	        modprobe dm-multipath fastfailbit=0             //seting
it off 
	        modprobe dm-multipath fastfailbit=0             //seting
it on 

	* By default the flag is kept on. 

	It can also be set from modprobe.conf as : 

	        options dm_multipath fastfailbit=0 


	Please comment !! 

	*************************************** 
	diff -Naur linux-2.6.18.x86_64-orig/drivers/md/dm-mpath.c linux-

	2.6.18.x86_64/drivers/md/dm-mpath.c 
	--- linux-2.6.18.x86_64-orig/drivers/md/dm-mpath.c
2007-08-27 
	02:18:51.000000000 +0530 
	+++ linux-2.6.18.x86_64/drivers/md/dm-mpath.c   2007-09-12
16:02:29.000000000 
	+0530 
	@@ -19,6 +19,7 @@ 
	 #include <linux/slab.h> 
	 #include <linux/time.h> 
	 #include <linux/workqueue.h> 
	+#include <linux/moduleparam.h> 
	 #include <asm/atomic.h> 

	 #define DM_MSG_PREFIX "multipath" 
	@@ -107,6 +108,7 @@ 
	 static void process_queued_ios(void *data); 
	 static void trigger_event(void *data); 

	+static int fastfailbit = 1;   /*fastfail bit set on by
default*/ 

	 /*----------------------------------------------- 
	  * Allocation routines 
	@@ -800,7 +802,7 @@ 
	        dm_bio_record(&mpio->details, bio); 

	        map_context->ptr = mpio; 
	-       bio->bi_rw |= (1 << BIO_RW_FAILFAST); 
	+       bio->bi_rw |= (fastfailbit << BIO_RW_FAILFAST); 
	        r = map_io(m, bio, mpio, 0); 
	        if (r < 0 || r == DM_MAPIO_REQUEUE) 
	                mempool_free(mpio, m->mpio_pool); 
	@@ -1340,6 +1342,10 @@ 
	 { 
	        int r; 

	+       /* validating fastfailbit */ 
	+        if(fastfailbit) 
	+                fastfailbit=1; 
	+ 
	        /* allocate a slab for the dm_ios */ 
	        _mpio_cache = kmem_cache_create("dm_mpath",
sizeof(struct mpath_io), 
	                                        0, 0, NULL, NULL); 
	@@ -1387,6 +1393,8 @@ 
	 module_init(dm_multipath_init); 
	 module_exit(dm_multipath_exit); 

	+module_param(fastfailbit, int, 0); 
	+MODULE_PARM_DESC(fastfailbit, "FASTFAIL bit flag "); 
	 MODULE_DESCRIPTION(DM_NAME " multipath target"); 
	 MODULE_AUTHOR("Sistina Software <dm-devel at redhat.com
<mailto:dm-devel at redhat.com> >"); 
	 MODULE_LICENSE("GPL"); 

	****************************** 
	  
	Thanks, 

	Vijay Kumar 

	[Kumar, Vijay (STSD)] 
	mailto: vijay.kumar7 at hp.com 

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