Tag Archives: 8b10b

One rotten apple spoils the bunch – 4

Config Guide Troubleshooting No Comments

Credit – who doesn’t want to have lots of it at the bank

According to Wikipedia the short definition in finance terms is :

Credit is the trust which allows one party to provide resources to another party where that second party does not reimburse the first party immediately (thereby generating a debt, but instead arranges either to repay or return those resources (or other materials of equal value) at a later date


In Fibre Channel it’s not much different. During initialization of a port they both provide the remote party a certain amount of resources (buffer credits) which tell the remote party it is allowed to send this x amount of frames. If this credit is used up the sending port is no longer allowed to send anymore frames. The receiving port get the frame and when the first couple of bytes have been read it will send a so called R_RDY to the sending port telling him to increase his credit by one. This part I described before.

A short side note is that this is normal operation in a class 3 FC network. A class 1 network uses End-to-End credits but this is hardly ever used. 

Taking the above into account it shows that all links in a FC network use this method for flow control. This is thus not only restricted to HBA, Disk, Tape and switchports but also internally within switches this method is used. The most explanatory way of showing this is when you have a blade based chassis where ports in one blade need to transfer frame to a port on another blade. Basically this means that the frame will traverse two more hops before it reaches that port. If you have a small switch with only one ASIC all frames will be short routed in that same ASIC.

As an example.
A Brocade 5100 has a single 40-port Condor2 ASIC which means all 40 front-end ports are connected to this same chip.



 

 

Any frame traversing from port 1 to port 9 will be switched inside that same chip. Sounds obvious. This also means there are only two points of B2B flow control and that is between the devices connected to both port 1 and 9.

When looking at a Brocade 5300 there is completely different architecture.

 

 

 

This switch has 5 GoldenEye2 ASICS which serve 80 front-end ports (16 each) and 4 back-end ASIC which serve as interconnect between those 5 front-end ASICs. Each GoldenEye2 chips has 32 8G ports and each front-end to back-end link is connected with a single 4-port trunk which allows for any-to-any 1:1 subscription ratio.

If we look at this picture and have an HBA connected to port 1 and a storage device connected to port 45 you’ll see that the frame has to traverse 2 additional hops from ASIC 1 to a back-end ASIC and from there onward to ASIC 5. (Internal links are not counted as an official “hop-count” so it is not calculated on fabric parameters.) As you have seen in my previous post when a link error between an end-device and a switch port surfaces causing credit depletion the switch or device will reset the link, bring back the credit count to login values and the upper layer protocol (ie SCSI)  has to retry the IO. There is no such mechanism on back-end ports. You might argue that given the fact this is all inside the switch and no vulnerable components like optical cables can cause corrupt primitives hence lost R_RDY’s are impossible. Unfortunately this is not entirely true. There might be circumstances where front-end problems might propagate to the back-end. This is often seen during very high traffic times and problematic front-end ports. The result is that one or more of those back-end links have a credit stuck at zero which basically means the front-end port is no longer allowed to send frames to the back-end therefore causing similar problems with high latency and elongated IO recovery times. The REALLY bad news is that there is (now : “was”) no recovery possible besides bouncing the entire switch. (By bouncing I mean a reboot and not throwing it on the floor hoping it will return in your hands. Believe me, it wont. At least not in one piece)

All the Brocade OEM’s have run into situations like this with their customers and especially on larger fabrics with multiple blade based chassis with hundreds of ports connected you can imagine this was not a good position to be in.

In order to fix this Brocade has implemented some new logic, albeit proprietary and not FC-FS standard, which allows you to enable a feature to turn on back-end credit checking. In short what it does is that it monitors the number of credits on each of these back-end links, if the credit counter stays at less than the number of credits negotiated during login for the E_D_TOV timeframe and no frames has been sent during that timeframe, the credit recovery feature will reset the link for you in a similar fashion as the front-end port do, and it will resume normal operation.

The way turn on this feature is also part of the bottleneckmon command:

bottleneckmon –cfgcredittools -intport -recover onLrOnly

To be able to use this command you have to be at least at FOS level 6.3.2d, 6.4.2a or higher.
The latest FOS releases also have a manual check in case you might suspect a stuck credit condition.

As soon as you’ve enabled this feature (which I suggest you do immediately) you might run into some new errorcodes in the eventlog. When a lost credit condition is observed you will see a C2-1012 which shows something similar like this:

Message , [C2-1012], ,, WARNING, ,S,P(): Link Timeout on internal portftx= tov= (>)vc_no= crd(s)lost= complete_loss:.  (s)>

If this happens due to a problem on the back-end whereby a physical issue might be the problem the cause is most likely an increased bit error rate causing the same encoding/decoding errors. As shown before this will also corrupt R_RDY primitives. In addition to the C2-1012 you will also see a C2-1006 sometimes followed by a C2-1010

Message , [C2-1006], ,, WARNING, ,S,C: Internal link errors reported, no hardware faultsidentified, continuing monitoring: fault1:, fault2:thresh1:0x.  

Message , [C2-1010], ,, CRITICAL, ,S, C: Internal monitoring has identified suspecthardware, blade may need to be reset or replaced: fault1:,fault2: th2:0x.  

There are some more errorcodes which outline specific conditions but I refer to the Brocade Message Reference guide for more info.

We talked a bit of frameflow and latency in the previous articles. Besides corruption due to a high bit-error rate you might be running into a high latency device which basically is a device which is slow in returning credits. Sometimes this is by design as firmware engineers might use this as a way to throttle incoming traffic which they are unable to offload onto the PCI bus or there is some other problem inside the system itself which causes elongated handling of data by which the HBA cannot send the data quick enough to memory via RDMA or to the CPU for further handling.
Although it is a choice of the firmware and design engineers normally the response time to send back  buffercredits should be virtually instantaneous.

To to give you a feeling of the timing an average R_RDY reponse time is around 1 to 5us. If you have a older network with low performance links this might increase to around 50us or sometimes higher.

Sorry, here should be a picture of a FC trace snippet
FC trace

As you can see (i hope you have good eyesight) the R_RDY is returned within 2us of the dataframe being sent. This includes the reception of the first 24 bytes of the frame and the routing decision time-span the switchport has made. So all in all pretty quick.

On somewhat slower equipment it looks a bit like this:

(The time between the frame and R_RDY shows a delta of 10.5us)

When you have a latency problem where the device is realy slow in returning credits this time is far greater than shown above.

The above picture come from a pretty expensive FC analyser and I don’t expect you to buy one (although it’s a pretty cool toy to get to the nitty gritty of things.) If you run into a performance problem where you don’t see any obvious physical issue you might have ran into a slow drain device. With the later FOS versions from Brocade there is a counter called er_tx_c3_timeout. This counters shows how many time a frame has been discarded due to upstream credit shortages. If this is an F-port than the device connected to this port is a very candid suspect of mucking up your storage network. If this is an ISL then you will need to look at devices that are more upstream of this port connected to other switches, routers or other equipment.

As always be aware that all these counters are cumulative and will just wrap after a while. You will have to establish a new baseline and then monitor for any counter to increase. The counter I mentioned above is also part of the porterrshow output since FOS version 7 which makes it very easy to determine such a condition.

I hope the blog posts in this series have helped a bit to explain how FC flow control work, how it operates in normal environments and also what might happen if it doesn’t go as planned plus the ways to prevent and solve such conditions.

Let me know if you want to know about this or any other topic and I’ll see what I can produce.

Regards,
Erwin

One rotten apple spoils the bunch – 1

Config Guide Troubleshooting 7 Comments

Last week I had another one. A rotten apple that spoiled the bunch or, in storage terms, a slow drain device causing havoc in a fabric.

This time it was a blade-center server with a dubious HBA connection to the blade-center switch which caused link errors and thus corrupt frames, encoding errors and credit depletion. This, being a blade connected to a blade-switch, also propagated the credit depletion back into the overall SAN fabric and thus the entire fabric suffered significantly from this single problem device.

“Now how does this work” you’ll say. Well, it has everything to do with the flow-control methodology used in FC fabrics. In contrast to the Ethernet and TCP/IP world we, the storage guys, expect a device to behave correctly, as gentleman usually do. That being said, as with everything in life, there are always moment in time when nasty things happen and in the case of the “rotten apple” one storage device being an HBA, tape drive, or storage array may be doing nasty things.

Let’s take a look how this normally should work.

FC devices run on a buffer-to-buffer credit model. This means the device reserves an certain amount of buffers on the FC port itself. This amount of buffers is then communicated to the remote device as credits. So basically devices a gives the remote device permission to use X amount of credits. Each credit is around 2112 bytes (A full 2K data payload plus frame header and footer)

The number of credits each device can handle are “negotiated” during fabric login (FLOGI). On the left a snippet from a FLOGI frame were you see the number of credits in hex.

So what happens after the FLOGI. As an example we use a connection that has negotiated 8 credits either way. If the HBA sends a frame (eg. a SCSI read request) it knows it only has 7 credits left. As soon as the switch port receives the frame it has to make a decision where to send this frame to. It does this based on routing tables, zoning configuration and some other rules, and if everything is correct it will route the frame to the next destination. Meanwhile it simultaneously sends back a, so called, R_RDY primitive. This R_RDY tells the HBA that it can increase the credit counter back by one. So if the current credit counter was 5 it can now bump it back up to 6. (A “primitive” lives only between two directly connected ports and as such it will never traverse a switch or router. A frame can, and will, be switched/routed over one or more links)

Below is a very simplistic overview of two ports on a FC link. On the left we have an HBA and on the right we have a switch port. The blue lines represent the data frames and the red lines the R_RDY primitives.

As I said, it’s pretty simplistic. In theory the HBA on the left could send up to 8 frames before it has to wait for an R_RDY to be returned.

So far all looks good but what if the path from the switch back to the device is broken? Either due to a crack in the cable, unclean connectors, broken lasers etc. The first problem we often see is that bits get flipped on a link which in turn causes encoding errors. FC up to 8G uses a 8b10b encoding decoding mechanism. According to this algorithm the normal 8 data bits are converted to a, so called, 10-bit word or transmission character. These 10 bits are the actual ones that travel over the wire. The remote side uses this same algorithm to revert the 10-bits back into the original 8 data bits. This assures bit level integrity and DC balance on a link. However when a link has a problem as described above, chances are that one or more of these 10-bits flip from a 0 to 1 or vice-versa. The recipient detects this problem however since it is unaware of which bit got corrupted it will discard the entire transmission character. This means that if such a corruption is detected it will discard en entire primitive, or, if the corrupted piece was part of a data frame, this entire frame will be dropped.

A primitive (including the R_RDY) consists of 4 words. (4 * 10 bits). The first word is always a control character (K28.5) and it is followed by three data words (Dxx.x). 

0011111010 1010100010 0101010101 0101010101 (-K28.5 +D21.4  D10.2  D10.2 )

I will not go further into this since its beyond the scope of the article.

So if this R_RDY is discarded the HBA does not know that the switch port has indeed free-ed up the buffer and still think it can only send N-1 frames. The below depicts such a scenario:

As you can see when an R_RDY is lost at some point in time it will become 0 meaning the HBA is unable to send any frames. When this happens an error recovery mechanism kicks in which basically resets the link, clearing all buffers on both side of that link and start from scratch. The upper layers of the FC protocol stack (SCSI-FCP, IPFC etc) have to make sure that any outstanding frame have either to be re-transmitted or the entire IO needs to be aborted in which case this IO in it’s entirety needs to be re-executed. As you can see this will cause a problem on this link since a lot of things are going on except actually making sure your data frames are transmitted. If you think this will not have such an impact be aware that the above sequence might run in less than one tenth of a second and thus the credit depletion can be reached within less than a second. So how does this influence the rest of the fabric since this all seems to be pretty confined within the space of this particular link.

Let broaden the scope a bit from an architectural perspective. Below you see a relatively simple, though architecturally often implemented, core-edge fabric.

Each HBA stands for one server (Green, Blue,Red and Orange), each mapped to a port on a storage array.
Now lets say server Red is a slow drain device or has a problem with its direct link to the switch. It is very intermittently returning credits due to the above explained encoding errors or it is very slow in returning credits due to a driver/firmware timing issue. The HBA sends a read request for an IO of 64K data. This means that 32 data frames (normally FC uses a 2K frame size) will be sent back from the array to the Red server. Meanwhile the other 3 servers and the two storage arrays are also sending and receiving data. If the number of credits negotiated between the HBA’s and the servers is 8 you can see that after the first 16K of that 64K request will be send to Red server however the remaining 48K still is either in transit from the array to the HBA or it is still in some outbound queue in the array. Since the edge switch (on the left) is unable to send frames to the Red server the remaining data frames (from ALL SERVERS) will stack up on the incoming ISL port (bright red). This in turn causes the outbound ISL port on the core switch (the one on the right) to deplete its credits which means that at some point in time no frames are able to traverse the ISL therefore causing most traffic to come to a standstill.

You’ll probably ask “So how do we recover from this?”. Well, basically the port on the edge switch to the Red server will send a LR (Link Reset) after the agreed “hold-time”. The hold time is a calculated period in which the switch will hold frames in its buffers. In most fabrics this is 500ms. So if the switch has had zero credit available during the entire hold period and it has had at least 1 frame in its output buffer it will send a LR to the HBA. This causes both the switch and HBA buffer to clear and the number of credits will return to the value that was negotiated during FLOGI.

If you don’t fix the underlying problem this process will go on forever and, as you’ve seen, will severely impact your entire storage environment.

“OK, so the problem is clear, how do I fix it?”

There are two ways to tackle the problem, the good and the bad way.

The good way is to monitor and manage your fabrics and link for such a behavior. If you see any error counter increasing verify all connections, cables, sfp’s, patch-panels and other hardware sitting in between the two devices. Clean connectors, replace cables and make sure these hardware problems do not re-surface again. My advice is if you see any link behaving like this DISABLE IT IMMEDIATELY !!!! No questions asked.

The bad way is to stick your head in the sand and hope for it go away. I’ve seen many of such issues crippling entire fabrics and due strictly enforced change control severe outages occurred and elongated recovery (very often multiple days) was needed to get things back to normal again. Make sure you implement emergency procedures which allow you to bypass these operational guidelines. It will save you a lot of problems.

Regards,
Erwin van Londen