Re: mpt2sas: /sysfs sas_address entries do not show individual port sas addresses.

[Douglas Gilbert <dgilbert@interlog.com>]

On 11-08-19 03:06 PM, Ravi Shankar wrote:
>
>>
>> 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.

spl2r02.pdf section 6.18.2 [link layer, SSP, Full duplex]:
   "SSP is a full duplex protocol. An SSP phy may receive
    an SSP frame or primitive in a connection while it is
    transmitting an SSP frame or primitive in the same
    connection. A wide SSP port may send and/or receive
    SSP frames or primitives concurrently on different
    connections (i.e., on different phys)."

For a SCSI command like READ(10) a connection consumes
one initiator phy and one target phy plus the pathway
between them until it is closed. Typically a READ
would have two connections: one to send the CDB and a
second connection later to return the data and response
(SCSI status and possibly sense data). For a spinning
disk there could be milliseconds between those two
connections; with an SSD less (do they use only one
connection?).

Due to the full duplex nature of a connection, DATA
frames associated with a WRITE could overlap with DATA
frames associated with an READ CDB sent earlier.

In SAS-2, a single READ's maximum data rate is 6 Gbps.
If a 2-phy wide link is available (along the whole pathway
(see Figure 129 in spl2r02.pdf)) then two READs, sent one
after the other or concurrently, could have their DATA
frames returned concurrently. So the combined maximum
data rate of the two READs would be 12 Gbps.

Expanders don't change what is stated above. Pathways
become an interconnection of links. A small latency is
added to the opening of connections. And there is the
possibility that no links are available to establish a
connection (e.g. target to expander has available link(s)
but all expander to initiator links are occupied).

> 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.

Not sure I follow. Expanders come into play when
connections are being established.

Doug Gilbert