Edge: HA, failure and the moving of volumes explained

edge HA FailoverOpen vStorage is designed to be rock solid and survive failures. These failures can come in many forms and shapes: nodes might die, network connections might get interrupted, … Let’s give an overview of the different tactics that are used by Open vStorage when disaster strikes by going over some possible use cases where the new edge plays a role.

Use case 1: A hypervisor fails

In case the hypervisor fails, the hypervisor management (OpenStack, vCenter, …) will detect the failure and restart the VM on another hypervisor. Since the VM is started on another hypervisor, the VM will talk to the edge client on the new hypervisor. The edge client will connect to a volume driver in the vPool and enquire which volume driver owns the disks of the VM. The volume driver responds who is the owner and the edge connects to the volume driver owning the volume. This all happens almost instantaneously and in the background so the the IO of the VM isn’t affected.

Use case 2: A Storage Router fails

In case a Storage Router and hence the volume driver on it die, the edge client automatically detects that the connection to the volume driver is lost. Luckily the edge keeps a list of volume drivers which also serve the vPool and it connects to one of the remaining volume drivers in the vPool. It is clear that the edge prefers to fail-over to a volume driver which is close-by f.e. within the same datacenter. The new volume driver to which the edge connects detects that it isn’t the owner of the volume. As the old volume driver is no longer online, the new volume driver steals the ownership of the VMs volume. Stealing is allowed in this case as the old volume driver is down. Once the new volume driver becomes the owner of the volumes, the edge client can start serving IO. This whole process process happens in the background and halts the IO of the VM for a fraction of a second.

Use case 3: Network issues

In some exceptional cases it isn’t the hypervisor or the storage router that fails but the network in between. This is an administrator’s worst nightmare as it might lead to split brain scenarios. Even in this case the edge is able to outlive the disaster. As the network connection between the edge and the volume driver is lost, the edge will assume the volume driver is dead. Hence, as in use case 2 the edge connects to another volume driver in the same vPool. The volume driver first tries to contact the old volume driver.

Now there are 2 options:

  • The new volume driver can contact the old volume driver. After some IO is exchanged the new volume driver asks the old volume driver to hand over the volume. This handover doesn’t impact the edge.
  • The new volume driver can also not contact the old volume driver. In that case old volume driver steals the volume from the old volume driver. It does this by updating the ownership of the volume in the distributed DB and by uploading a new key to the backend. As the ALBA backend uses a conditional write approach, it only writes the IO to disks of the backend if the accompanying key is valid, it can ensure only the new volume driver is allowed to write to the backend. If the old volume driver would still be online (split brain) and try to update the backend, the write would fail as it is using an outdated key.

Fargo: the updated Open vStorage Architecture

With the Fargo release of Open vStorage we are focussing even more on the Open vStorage sweet spot: multi-petabyte, multi-datacenter storage clusters which offer super-fast block storage.
In order to achieve this we had to significantly change the architecture for the Fargo release. Eugene, the version before Fargo, already had the Shared Memory Server (SHM) in its code base but its wasn’t activated by default. The Fargo release now primarily uses the SHM approach. To make even more use of it, we created the Open vStorage Edge. The Edge is a lightweight block storage driver which can be installed on Linux servers (hosts running the hypervisor or inside the VM) and talks across the network to the Shared Memory of a remote Volume Driver. Both TCP/IP and the low latency RDMA protocol can be used to connect the Edge with the Volume Driver. Northbound the Edge has an iSCSI, Blktap and QEMU interface. Additional interfaces such as iSER and FCoE are planned. Next to the new Edge interface, the slower Virtual Machine interface which exposes a Virtual File System (NFS, FUSE), is still supported.

Architecture

The Volume Driver has also been optimized for performance. The locks in the write path have been revised in order to minimize their impact. More radical is the decision to remove the deduplication functionality from the Volume Driver in order to keep the size of the metadata of the volumes to a strict minimum. By removing the bytes reserved for the hash, we are capable of keeping all the metadata in RAM and push the performance across 1 million IOPS per host on decent hardware. For those who absolutely need deduplication there is still a version available of the Volume Driver which has support for deduplication.

With the breakthrough of RDMA, the network bottleneck is removed and network latency is brought down to a couple of microseconds. Open vStorage makes use of the possibilities RDMA offers to implement a shared cache layer. To achieve this it is now possible to create an ALBA backend out of NVMe or SSD devices. This layer acts as a local, within a single datacenter, cache layer in front of an SATA ALBA backend, the capacity tier, which is spread across multiple datacenters.
This means all SSDs in a single datacenter devise a shared cache for the data of that datacenter. This minimizes the impact of an SSD failure and removes the cold cache effect when moving a volume between hosts. In order to minimize the impact of a single disk failure we introduced the NC-ECC (Network and Clustered Error Correction Codes) algorithm. This algorithm can be compared with solving a Sudoku puzzle. Each SCO, a collection of consecutive writes, is chopped up in chunks. All these chunks are distributed across all the nodes and datacenters in the cluster. The total amount of chunks can be configured but allows for example to recover from a multi node failure or a complete datacenter loss. A failure, whether it is a disk, node or datacenter will cross out some numbers from the complete Sudoku puzzle but as long as you have enough numbers left, you can still solve the puzzle. The same goes for data stored with Open vStorage: as long as you have enough chunks (disk, nodes or datacenters) left, you can always recover the data. The NC-ECC algorithm is based on forward error correction codes and is further optimized for usage within a multi-datacenter approach. When there is a disk or node failure, additional chunks will be created using only data from within the same datacenter. This ensures the bandwidth between datacenters isn’t stressed in case of a simple disk failure.

By splitting up the Edge, the Volume Driver, the cache layer and the capacity tier, you have the ultimate flexibility to build the storage cluster of your needs. You can run everything on the same server, hyperconverged, or you can install each component on a dedicated server to maximize scalability and performance.

The first alpha version of Fargo is now available on the repo.