Hi Douglas, Ravi,
According to the SAS specs, (ISO/IEC 14776-152:200x, sas2r15.pdf), on
page 45, they state that that a port is formed by a unique tuple of
the SAS Phy address and the attached SAS Phy address.
For instance, if you take 2 * 2 phy wide ports, where all 4 phys from
these two ports have the same sas address, let's call it "A" and
connect them each to another port that each has a different address,
"B" and "C", they state that two ports will be formed, one connecting
"A" to "B" and one connecting "A" to "C".
This is what Douglas is saying with the SAS disks for instance, that
are typically given two separate SAS addresses to avoid forming a wide
port with the expander (since the expander will have the same sas
address on all phys), and to allow for dual expander multiplexing for
redundancy.
But what I don't understand is that, in the context of two HBAs
connected together, things seem to be different:
I configured a 9200-8e HBA (8Phys) and changed all its SAS phys
addresses from being the same to being incremental, therefore the last
byte of each SAS phy address changed from:
0 1 2 3 4 5 6 7
b0 b0 b0 b0 b0 b0 b0 b0
to:
b0 b1 b2 b3 b4 b5 b6 b7
I also changed the "ports" setup from "Auto" to "Wide", making two
4*phys ports:
Port 0 | Port 1
b0 b1 b2 b3 | b4 b5 b6 b7
I also set all these ports to Target.
I then connected this HBA to another 9200-8e HBA, which was left setup
as default:
Auto
Initiator
0 1 2 3 4 5 6 7
10 10 10 10 10 10 10 10
However, when I looked up the SAS topology on either side in LSIUtil,
I saw that there was two ports connected on each HBAs, one connected
on phy 0 and one on phy 4.
On the second (Initiator) HBA, the two ports appeared as b0 and b4,
with two separate handles.
On the first (Target) HBA, both ports appeared as 10, with two
separate handles.
What I don't understand above, is since all phys on the Target HBAs
have a different SAS address, and all the ones on the Initiator one
have the same, 8 narrow ports should have been created there.
However, there is a separate notion of "port" in LSIUtil, does that
mean that agglomerating 4 phys with different SAS addresses in a
logical LSIUtil "port" forces the HBA FW to transmit the same sas
address on these 4 Phys, to make them look like a single port? Or is
there an extra separate notion of "port", that does not rely on the
phy SAS address and its attached SAS address?
I guess my question is: Is there an extra information ontop of phy sas
address and phy id that is transmitted in SAS, like a "port" id or a
handle?
Also, in the above case, if we assume that the HBA FW was transmitting
the same phys for phy 0-3 and phy 4-8 on the Target HBA, it would make
sense that we have two ports, since there is two pairs of SAS
addresses / attached SAS addresses here.
Ben,
Port 0 | Port 1
b0 b1 b2 b3 | b4 b5 b6 b7
In above configuration you are assigning different SAS Address for each
PHY but over riding with WIDE port clause. After
individual PHY are reset, it transmits IDENTIFICATION address frames as
part of identification sequence so down stream devices
know the attributes of the attached devices.
PHY 0-3: Transmit Identification frame with SAS address xxxxxxxxb0
PHY 4-7: Transmit identification frame with SAS address xxxxxxxxb4
The second HBA (with SAS address xxxxxxxx10 with Auto mode) receives
above identification frame on PHY 0-3 and 4-7 respectively. So this
essentially forms x4 wide port instead of narrow as you expected.
As far I know there are no port id or handle transmitted on the fabric.
Couple of interesting question regarding wide port. From bandwidth
perspective, x4 port are termed as 24 Gb/sec ( 6 Gb/s * 4). But do we
really get 24 Gb/sec bandwidth ?. I see questions being raised that SSD
disks need wide ports for bandwidth aggregation. SAS protocol
and Expander has following limitation which could be problematic
depending on topology.
1) Unlike FC, SAS is a connection oriented protocol (full duplex Class 1
vs Class 3 FC)
2) Flow control primitives (K words) are transmitted inside connection
(without being packetized).
3) When connecting through Expanders, typically only x4 or x8 physical
links are used. If there are hundreds of Initiator/Target exist in such
fabric, the number of active I/O transfers across devices are
limited to number of links between Expanders (due to Class 1 protocol).
My understanding for SSD disks with wide ports the HBA and Disks can
queue several commands using Tagged Queuing. This way we can
maximize number of commands and data frames across devices.
Wondering has anyone measured performance under such scenario ?. It
would be great to see Expanders terminating SSP frames to over come
some of above limitation. Links between HBA and Expander and Expander to
Disk can be still Class 1.
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