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 <johann.lombardi@xxxxxxxx> -- 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 - To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
