To sort out these troubles, gstat is your friend. Log as root via ssh and execute gsstat with a short inverval (gstat -I1s). You can see if one of the disks is being much slower than the others, which would give you a clue. In my never polished "devilator" (a FreeBSD data collector for Orca) I graph disk delay for that reason.
Second: for flashed LSI2008 cards there is a way that will help you identify a disk slot. There is a program called sas2ircu (you can download it from the LSI website) and there's even a FreeBSD port for it. Even though it's intended for the "Megaraid" cards, a simple "sas2ircu 0 display" will give you a nice listing of SAS devices _including_ the backplane slot number.
At least in my case I have found that the identification works, it doesn't get fooled by LSI's odd reordering of SAS target numbers. Sample output:
Code:
# sas2ircu 0 display
LSI Corporation SAS2 IR Configuration Utility.
Version 18.00.00.00 (2013.11.18)
Copyright (c) 2009-2013 LSI Corporation. All rights reserved.
Read configuration has been initiated for controller 0
------------------------------------------------------------------------
Controller information
------------------------------------------------------------------------
Controller type : SAS2008
BIOS version : 7.37.00.00
Firmware version : 19.00.00.00
Channel description : 1 Serial Attached SCSI
Initiator ID : 0
Maximum physical devices : 255
Concurrent commands supported : 3432
Slot : Unknown
Segment : 0
Bus : 17
Device : 0
Function : 0
RAID Support : No
------------------------------------------------------------------------
IR Volume information
------------------------------------------------------------------------
------------------------------------------------------------------------
Physical device information
------------------------------------------------------------------------
Initiator at ID #0
Device is a Hard disk
Enclosure # : 2
Slot # : 16
SAS Address : 5005076-0-3e8e-81a1
State : Ready (RDY)
Size (in MB)/(in sectors) : 953869/1953525167
Manufacturer : ATA
Model Number : Samsung SSD 840
Firmware Revision : BB0Q
Serial No :NI
GUID : N/A
Protocol : SATA
Drive Type : SATA_SSD
Device is a Hard disk
Enclosure # : 2
Slot # : 17
SAS Address : 5005076-0-3e8e-81a2
State : Ready (RDY)
Size (in MB)/(in sectors) : 953869/1953525167
Manufacturer : ATA
Model Number : Samsung SSD 840
Firmware Revision : BB0Q
Serial No : NINI
GUID : N/A
Protocol : SATA
Drive Type : SATA_SSD
Device is a Hard disk
Enclosure # : 2
Slot # : 18
SAS Address : 5005076-0-3e8e-81a3
State : Ready (RDY)
Size (in MB)/(in sectors) : 953869/1953525167
Manufacturer : ATA
Model Number : Samsung SSD 840
Firmware Revision : BB0Q
Serial No : NININI
GUID : N/A
Protocol : SATA
Drive Type : SATA_SSD
Device is a Hard disk
Enclosure # : 2
Slot # : 19
SAS Address : 5005076-0-3e8e-81a4
State : Ready (RDY)
Size (in MB)/(in sectors) : 953869/1953525167
Manufacturer : ATA
Model Number : Samsung SSD 840
Firmware Revision : BB0Q
Serial No : S1D9NEADA08547X
GUID : N/A
Protocol : SATA
Drive Type : SATA_SSD
Device is a Hard disk
Enclosure # : 2
Slot # : 20
SAS Address : 5005076-0-3e8e-81a5
State : Ready (RDY)
Size (in MB)/(in sectors) : 953869/1953525167
Manufacturer : ATA
Model Number : Samsung SSD 840
Firmware Revision : BB0Q
Serial No : NINININI
GUID : N/A
Protocol : SATA
Drive Type : SATA_SSD
Device is a Hard disk
Enclosure # : 2
Slot # : 21
SAS Address : 5005076-0-3e8e-81a6
State : Ready (RDY)
Size (in MB)/(in sectors) : 953869/1953525167
Manufacturer : ATA
Model Number : Samsung SSD 840
Firmware Revision : BB0Q
Serial No : NININININI
GUID : N/A
Protocol : SATA
Drive Type : SATA_SSD
Device is a Hard disk
Enclosure # : 2
Slot # : 22
SAS Address : 5005076-0-3e8e-81a7
State : Ready (RDY)
Size (in MB)/(in sectors) : 953869/1953525167
Manufacturer : ATA
Model Number : Samsung SSD 840
Firmware Revision : BB0Q
Serial No : S1D9NEADA08568E
GUID : N/A
Protocol : SATA
Drive Type : SATA_SSD
Device is a Enclosure services device
Enclosure # : 2
Slot # : 255
SAS Address : 5005076-0-3e8e-81b9
State : Standby (SBY)
Manufacturer : IBM-ESXS
Model Number : SAS EXP BP
Firmware Revision : 61A6
Serial No : 00000006
GUID : N/A
Protocol : SAS
Device Type : Enclosure services device
Device is a Hard disk
Enclosure # : 3
Slot # : 12
SAS Address : 5005076-0-3e8e-86e5
State : Ready (RDY)
Size (in MB)/(in sectors) : 953869/1953525167
Manufacturer : ATA
Model Number : Samsung SSD 840
Firmware Revision : BB0Q
Serial No : NINININININI
GUID : N/A
Protocol : SATA
Drive Type : SATA_SSD
Device is a Hard disk
Enclosure # : 3
Slot # : 13
SAS Address : 5005076-0-3e8e-86e6
State : Ready (RDY)
Size (in MB)/(in sectors) : 953869/1953525167
Manufacturer : ATA
Model Number : Samsung SSD 840
Firmware Revision : BB0Q
Serial No : NINININININININI
GUID : N/A
Protocol : SATA
Drive Type : SATA_SSD
Device is a Hard disk
Enclosure # : 3
Slot # : 14
SAS Address : 5005076-0-3e8e-86e7
State : Ready (RDY)
Size (in MB)/(in sectors) : 953869/1953525167
Manufacturer : ATA
Model Number : Samsung SSD 840
Firmware Revision : AB0Q
Serial No : NININININININININI
GUID : N/A
Protocol : SATA
Drive Type : SATA_SSD
Device is a Hard disk
Enclosure # : 3
Slot # : 15
SAS Address : 5005076-0-3e8e-86e8
State : Ready (RDY)
Size (in MB)/(in sectors) : 953869/1953525167
Manufacturer : ATA
Model Number : Samsung SSD 840
Firmware Revision : BB0Q
Serial No : NINININININININININI
GUID : N/A
Protocol : SATA
Drive Type : SATA_SSD
Device is a Enclosure services device
Enclosure # : 3
Slot # : 255
SAS Address : 5005076-0-3e8e-86f9
State : Standby (SBY)
Manufacturer : IBM-ESXS
Model Number : SAS EXP BP
Firmware Revision : 61A6
Serial No : 00000006
GUID : N/A
Protocol : SAS
Device Type : Enclosure services device
------------------------------------------------------------------------
Enclosure information
------------------------------------------------------------------------
Enclosure# : 1
Logical ID : 500605b0:07ba2100
Numslots : 8
StartSlot : 0
Enclosure# : 2
Logical ID : 50050760:3e8e81a0
Numslots : 25
StartSlot : 0
Enclosure# : 3
Logical ID : 50050760:3e8e86e0
Numslots : 25
StartSlot : 0
------------------------------------------------------------------------
SAS2IRCU: Command DISPLAY Completed Successfully.
SAS2IRCU: Utility Completed Successfully.
Of course, always _do_ verify that it really identifies your slots accurately. At least in my case it works with IBM and Dell backplanes. And yes, the output is unsorted but the slot number is correct.