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Subject[PATCH] aic94xx: Increase can_queue and cmds_per_lun
Hi all,

Below is a patch that sets cmd_per_lun and can_queue in the aic94xx
driver's scsi_host_template to better performing values than what's
there currently. The cmd_per_lun setting is stolen straight out of the
adp94xx source, and can_queue is derived from the max_scb value that we
calculate in asd_init_hw. To the best of my (admittedly limited)
knowledge, this method provides the correct values (can_queue = 443 in
both adp94xx and aic94xx on my 9405W) but if anybody knows better,
please enlighten me. :)

That said, the effect of leaving these values set to 1 is terrible
performance in the case of either (a) certain Maxtor SAS drives flying
solo or (b) flooding several disks with I/O simultaneously (md-raid).
There may be more scenarios where we see similar problems that I
haven't uncovered.

Just for grins, I ran bogodisk (an O_DIRECT-enabled read speed test)
against three different scenarios:

1) adp94xx 1.0.8-6, pounded into 2.6.18-rc4 [green]
2) aic94xx 1.0.2, without this patch [red]
3) aic94xx 1.0.2, with this [blue]

...with these results:

As you can see, the read performance is cut in half by the aic94xx
driver not getting a chance to have multiple I/Os in flight at any given
time. With the patch, the two drivers are fairly close bandwidth-wise.
Also thanks to Mike Anderson for helping me figure this out.


Signed-off-by: Darrick J. Wong <>

diff --git a/drivers/scsi/aic94xx/aic94xx_hwi.h b/drivers/scsi/aic94xx/aic94xx_hwi.h
index c7d5053..a2d8881 100644
--- a/drivers/scsi/aic94xx/aic94xx_hwi.h
+++ b/drivers/scsi/aic94xx/aic94xx_hwi.h
@@ -36,6 +36,9 @@
#include "aic94xx.h"
#include "aic94xx_sas.h"

+/* Leave a few empty data buffers. */
+#define ASD_FREE_SCBS 3
/* Define ASD_MAX_PHYS to the maximum phys ever. Currently 8. */
#define ASD_MAX_PHYS 8
#define ASD_PCBA_SN_SIZE 12
diff --git a/drivers/scsi/aic94xx/aic94xx_init.c b/drivers/scsi/aic94xx/aic94xx_init.c
index 3ec2e46..34a7b4b 100644
--- a/drivers/scsi/aic94xx/aic94xx_init.c
+++ b/drivers/scsi/aic94xx/aic94xx_init.c
@@ -71,7 +72,7 @@ static struct scsi_host_template aic94xx
.change_queue_type = sas_change_queue_type,
.bios_param = sas_bios_param,
.can_queue = 1,
- .cmd_per_lun = 1,
+ .cmd_per_lun = 2,
.this_id = -1,
.sg_tablesize = SG_ALL,
@@ -612,13 +616,17 @@ static int __devinit asd_pci_probe(struc
goto Err_free_cache;

asd_printk("device %s: SAS addr %llx, PCBA SN %s, %d phys, %d enabled "
- "phys, flash %s, BIOS %s%d\n",
+ "phys, flash %s, BIOS %s%d, SCBs %d\n",
pci_name(dev), SAS_ADDR(asd_ha->hw_prof.sas_addr),
asd_ha->hw_prof.pcba_sn, asd_ha->hw_prof.max_phys,
asd_ha->hw_prof.flash.present ? "present" : "not present",
asd_ha->hw_prof.bios.present ? "build " : "not present",
- asd_ha->hw_prof.bios.bld);
+ asd_ha->hw_prof.bios.bld,
+ asd_ha->hw_prof.max_scbs);
+ aic94xx_sht.can_queue = asd_ha->hw_prof.max_scbs - ASD_FREE_SCBS;
+ shost->can_queue = aic94xx_sht.can_queue;

if (use_msi)
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