Cases2 cases

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1. IL-0537 Sakura Cloud host servers repeatedly lost their storage connection, about five minutes per port switchover

   Sakura Internet2012

   Symptoms

   From mid-December 2011, after Sakura Cloud launched, traffic between the host servers and the storage device grew, and the network connection between the hosts that carry users' servers and the storage device was cut again and again. Each cut caused by a port switchover lasted about five minutes.

   Storage disconnected or stale mountsPacket loss

   Trigger

   Traffic between the host servers and the storage device grew from mid-December 2011, after the service launched on 15 November 2011.

   Root cause

   Capacity exhaustion

   Component

   Cloud block storageInfiniBand fabric

   Contributing factor

   Automation amplificationUnrepresentative stagingLimited debug access

   Remediation

   • Switched the network interface monitoring from ICMP to a LINK state check, so the device fails over only on a physical link down
   • Changed the InfiniBand Subnet Manager's check from normal mode to heavy sweep mode
   • Updated the storage device firmware on 19 March, which stopped the packet loss
   • Added a second storage device on 12 March and migrated servers to it to reduce and spread the load

   Action items

   • Replace the current storage with an entirely different new storage device
   • Test the new storage in an environment equal to its maximum capacity under real load, with a free beta before paid service
   • Use network interface drivers and similar parts equivalent to those already proven on the host servers
   • Develop the new storage with in-house engineers so Sakura Internet can manage and operate it itself

   Lessons

   Sakura Internet wrote that it could not test the storage device at its performance limit, because it could not prepare as many servers as the design capacity and used load generated from predictions, so it could not respond quickly and accurately to problems in real operation. Packet loss on the InfiniBand port kept the device below its expected performance and took three months to fix by firmware, and its engineers could not grasp the whole of the device's specification and behaviour. Sakura Internet concluded that its process for choosing and validating the storage device had serious problems.

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2. IL-0337 EBS volumes in one US East Availability Zone got stuck after a network change cut EBS nodes off from their replicas

   Amazon Web Services2011

   Symptoms

   From 12:47 AM PDT on 21 April 2011, a subset of Amazon EBS volumes in one Availability Zone of the US East Region could no longer serve reads or writes, and EC2 instances that tried to use them got stuck as well. AWS called these "stuck" volumes. EBS API calls across the whole US East Region also returned high error rates and latencies during two periods on the first day.

   Storage disconnected or stale mountsNetwork unreachableControl plane lagHigh error rateFailover failureData loss

   Trigger

   During a routine network change to upgrade the capacity of the primary EBS network in one Availability Zone, traffic was to be shifted off one of the redundant routers. The shift was executed incorrectly and sent the traffic onto the lower-capacity replication network instead of the other primary router, which could not carry it.

   Root cause

   Operator error

   Component

   Cloud block storageDatabase

   Contributing factor

   Automation amplificationRetry stormNo headroomExcessive blast radiusSignal buried in noise

   Remediation

   • Rolled back the incorrect traffic shift, which restored network connectivity
   • Disabled all new Create Volume requests in the affected Availability Zone at 2:40 AM PDT
   • Disabled all communication between the degraded EBS cluster and the EBS control plane from 8:20 AM PDT
   • Stopped EBS servers in the degraded cluster from futilely contacting other servers for free space
   • Changed the EBS control plane so new EBS-backed instance launches stopped failing
   • Moved spare server capacity physically from across the US East Region into the degraded cluster
   • Adjusted the negotiation throttles so the new servers could join without flooding the old ones
   • Built a separate EBS control plane partitioned to the affected zone and finer-grained throttles, then processed the backlog
   • Restored the remaining volumes from S3 snapshots taken early in the event
   • Restored volumes on failed machines that had no snapshot after forensic work
   • Intervened by hand on the RDS multi-AZ instances that did not fail over automatically

   Action items

   • Audit the change process and add automation to prevent the traffic shift mistake
   • Change capacity planning and alarming to keep the extra safety capacity needed for large recovery events
   • Change the retry logic in EBS nodes to back off harder and reconnect to previous replicas instead of searching for new nodes
   • Test and deploy the fix for the race condition that crashed EBS nodes
   • Fix the RDS bug that kept multi-AZ instances from failing over automatically
   • Improve the EBS control plane timeout logic to prevent thread exhaustion when one zone's cluster is slow
   • Make the EBS control plane aware of Availability Zones so it sheds load when over capacity
   • Move more EBS control plane functions into per-cluster services that run in the same zone as the cluster
   • Offer multiple Availability Zones to Amazon VPC customers
   • Provide better tools for building multi-AZ applications that survive the loss of a zone
   • Host free webinars and publish whitepapers on fault-tolerant architecture
   • Build the fine-grained control and throttling used in recovery into the EBS nodes and automate the recovery models
   • Look at changes that keep volumes working while a cluster is degraded, including the ability to take a snapshot of a stuck volume
   • Send more frequent and detailed customer updates and staff developer support more widely during events
   • Develop tools that show through the APIs whether instances are impaired

   Lessons

   AWS writes that with more spare capacity the cluster would have absorbed the re-mirroring requests without a storm, and that it now understands how much capacity large recovery events need. The event also showed that the EBS control plane let a failure in one Availability Zone affect the others, so more work is needed to make zones behave as if completely independent. AWS adds that some customers had critical components in a single zone, and that multi-AZ deployments need to become easier to design and run.

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