VSAN-Troubleshooting-Reference-Manual

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Diagnostics and Troubleshooting Reference Manual – Virtual SAN Anatomy of a write in hybrid configurations When a VM issue a write, and that write is acknowledged as having completed successfully to the VM, the user is guaranteed that the data just written is persisted according to the policy specified. For example, if “failures to tolerate” was set to 1 and there are 2 mirror copies of the VMDK on different hosts, Virtual SAN guarantees that the data has been written to both mirrors. Lets take the example where there is a 3 node Virtual SAN cluster. There is a VM running on host H1 and it is using a VM Storage Policy which has the capability “Number Of Failures To Tolerate = 1”. This means that the disk object has two mirrors, one on host H1 and the other on host H2. Let’s now examine the steps involved in a write operation: 1. Guest OS issues write operation to virtual disk 2. Owner clones write operation 3. In parallel: sends “prepare” operation to H1 (locally) and H2 4. H1, H2 persist write operation to Flash (log) 5. H1, H2 Acknowledge prepare operation to owner 6. Owner waits for ACK from both ‘prepares’ and completes I/O. 7. Later, the owner commits a batch of writes to hard disk or flash used as capacity. V M W A R E S T O R A G E B U D O C U M E N T A T I O N / 2 4 4
Diagnostics and Troubleshooting Reference Manual – Virtual SAN Destaging writes from flash to magnetic disk in hybrid configurations After the previous write operation, we are now left the newly written data residing on flash on two different ESXi hosts. How is this data now destaged to the capacity layer, which is version 5.5, is always made up of magnetic disks? Remember, magnetic disks are good with sequential workloads. So Virtual SAN tries to make things optimal for these magnetic disks. Virtual SAN uses a self-tuning algorithm that decides how often writes on the flash cache device (e.g. SSD) de-stage to magnetic disk. The heuristics used for this are sophisticated and take into account many parameters such as rate of incoming I/O, queues, disk utilization, and optimal batching. As we have seen, Virtual SAN accumulates written data in the write buffer of the flash device. It has an elevator algorithm that runs independently on each disk group and decides (locally) whether to move any data to the magnetic disk and when should it do it. It juggles multiple criteria to reach this decision; most importantly of all, it batches together big chunks of data that are physically proximal on a magnetic disk and retires them altogether. Therefore, writes to the magnetic disk are largely sequential. But it is also conservative. It does not rush to move data if there is plenty of space in the Write Buffer because data that is written tends to be overwritten multiple times within a short period of time. So Virtual SAN avoids writing the same block over and over to the magnetic disk. V M W A R E S T O R A G E B U D O C U M E N T A T I O N / 2 4 5
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