We have observed noticeable performance improvement by using large single
command data transfers (up to 4MB) with some storage subsystems such as DDN's.
The patch below adds a new config option (CONFIG_SCSI_LARGE_SG) which allows to
increase SCSI_MAX_PHYS_SEGMENTS. When enabled, we should get 4MB transfers even
if the buffers are completely fragmented in physical memory.
Thanks in advance for any comments.
Signed-off-by: Johann Lombardi <[EMAIL PROTECTED]>
--
Index: linux-2.6.20/drivers/scsi/Kconfig
===================================================================
--- linux-2.6.20.orig/drivers/scsi/Kconfig 2007-02-04 19:44:54.000000000
+0100
+++ linux-2.6.20/drivers/scsi/Kconfig 2007-02-26 10:25:54.000000000 +0100
@@ -240,6 +240,15 @@ config SCSI_SCAN_ASYNC
You can override this choice by specifying scsi_mod.scan="sync"
or "async" on the kernel's command line.
+config SCSI_LARGE_SG
+ bool "Support for large SCSI scatter/gather list"
+ depends on SCSI
+ help
+ This option increases the scatter/gather list maximum length to
+ allow larger single command data transfers.
+
+ If unsure, say N.
+
menu "SCSI Transports"
depends on SCSI
Index: linux-2.6.20/drivers/scsi/scsi_lib.c
===================================================================
--- linux-2.6.20.orig/drivers/scsi/scsi_lib.c 2007-02-04 19:44:54.000000000
+0100
+++ linux-2.6.20/drivers/scsi/scsi_lib.c 2007-02-26 10:20:04.000000000
+0100
@@ -56,11 +56,17 @@ static struct scsi_host_sg_pool scsi_sg_
#if (SCSI_MAX_PHYS_SEGMENTS > 128)
SP(256),
#if (SCSI_MAX_PHYS_SEGMENTS > 256)
+ SP(512),
+#if (SCSI_MAX_PHYS_SEGMENTS > 512)
+ SP(1024),
+#if (SCSI_MAX_PHYS_SEGMENTS > 1024)
#error SCSI_MAX_PHYS_SEGMENTS is too large
#endif
#endif
#endif
#endif
+#endif
+#endif
};
#undef SP
@@ -731,6 +737,16 @@ struct scatterlist *scsi_alloc_sgtable(s
case 129 ... 256:
cmd->sglist_len = 5;
break;
+#if (SCSI_MAX_PHYS_SEGMENTS > 256)
+ case 257 ... 512:
+ cmd->sglist_len = 6;
+ break;
+#if (SCSI_MAX_PHYS_SEGMENTS > 512)
+ case 513 ... 1024:
+ cmd->sglist_len = 7;
+ break;
+#endif
+#endif
#endif
#endif
#endif
Index: linux-2.6.20/include/scsi/scsi.h
===================================================================
--- linux-2.6.20.orig/include/scsi/scsi.h 2007-02-04 19:44:54.000000000
+0100
+++ linux-2.6.20/include/scsi/scsi.h 2007-02-26 10:27:26.000000000 +0100
@@ -12,10 +12,14 @@
/*
* The maximum sg list length SCSI can cope with
- * (currently must be a power of 2 between 32 and 256)
+ * (currently must be a power of 2 between 32 and 1024)
*/
-#define SCSI_MAX_PHYS_SEGMENTS MAX_PHYS_SEGMENTS
+#ifdef CONFIG_SCSI_LARGE_SG
+#define SCSI_MAX_PHYS_SEGMENTS 1024
+#else
+#define SCSI_MAX_PHYS_SEGMENTS MAX_PHYS_SEGMENTS
+#endif
/*
* SCSI command lengths
-
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