Disk options
1. Disk options
At this point you've chosen an operating system, but that operating system is going to be included as part of some kind of disk. So let's look at the disk options. Every single VM comes with a single root persistent disk, because you're choosing a base image to have that loaded on. This image is bootable in that you can attach it to a VM and boot from it, and it is durable in that it can survive if the VM terminates. To have a boot disk survive a VM deletion, you need to disable the "Delete boot disk when instance is deleted" option in the instance's properties. As discussed earlier, there are different types of disks. Let's explore these in more detail. The first disk that we create is what we call a persistent disk. That means it's going to be attached to the VM through the network interface. Even though it's persistent, it's not physically attached to the machine. This separation of disk and compute allows the disk to survive if the VM terminates. You can also perform snapshots of these disks, which are incremental backups that we'll discuss later. The choice between HDD and SSD disks comes down to cost and performance. To learn more about disk performance and how it scales with disk size, refer to the link in the course resources for this module. Another cool feature of persistent disks is that you can dynamically resize them, even while they are running and attached to a VM. You can also attach a disk in read-only mode to multiple VMs. This allows you to share static data between multiple instances, which is cheaper than replicating your data to unique disks for individual instances. Zonal persistent disks offer efficient, reliable block storage. Regional persistent disks provide active-active disk replication across two zones in the same region. Regional persistent disks deliver durable storage that is synchronously replicated across zones and are a great option for high-performance databases and enterprise applications that also require high availability. When you configure a zonal or regional persistent disk, you can select one of the following disk types: Standard persistent disks are backed by standard hard disk drives and are suitable for large data processing workloads that primarily use sequential I/Os. Performance SSD persistent disks are backed by solid-state drives and are suitable for enterprise applications and high-performance databases that require lower latency and more IOPS than standard persistent disks provide. Balanced persistent disks are also backed by solid-state drives. They are an alternative to SSD persistent disks that balance performance and cost. These disks have the same maximum IOPS as SSD persistent disks and lower IOPS per gigabyte. For most VM shapes, except very large ones, this disk type offers performance levels suitable for most general-purpose applications at a price point between that of standard and performance persistent disks. Extreme persistent disks are zonal persistent disks also backed by solid-state drives. Extreme persistent disks are designed for high-end database workloads, providing consistently high performance for both random access workloads and bulk throughput. Unlike other disk types, you can provision your desired IOPS. By default, Compute Engine encrypts all data at rest. Google Cloud handles and manages this encryption for you without any additional actions on your part. However, if you wanted to control and manage this encryption yourself, you can either use Cloud Key Management Service to create and manage key encryption keys (which is known as customer-managed encryption keys) or create and manage your own key encryption keys (known as customer-supplied encryption keys). Now, local SSDs are different from persistent disks in that they are physically attached to the virtual machine. Therefore, these disks are ephemeral but provide very high IOPS. For up-to-date numbers, I recommend referring to the documentation in the course resources for this module. Currently, local SSDs are 375 GB in size and you can attach up to to 24 local SSD partitions for a total of 9 TB per instance. Data on these disks will survive a reset but not a VM stop or terminate, because these disks cannot be reattached to a different VM. You also have the option of using a RAM disk. You can simply use tmpfs if you want to store data in memory. This will be the fastest type of performance available if you need small data structures. I recommend a high-memory virtual machine if you need to take advantage of such features, along with a persistent disk to back up the RAM disk data. In summary, you have several different disk options. Persistent disks can be rebooted and snapshotted, but local SSDs and RAM disks are ephemeral. I recommend choosing a persistent HDD disk when you don't need performance but just need capacity. If you have high performance needs, start looking at the SSD options. The persistent disks offer data redundancy because the data on each persistent disk is distributed across several physical disks. Local SSDs provide even higher performance, but without the data redundancy. Finally, RAM disks are very volatile but they provide the highest performance. Now, just as there is a limit on how many Local SSDs you can attach to a VM, there is also a limit on how many persistent disks you can attach to a VM. As illustrated in this table, this limit depends on the machine type. For the Shared-core machine type, you can attach up to 16 disks. For the Standard, High Memory, High-CPU, Memory-optimized, and Compute-optimized machine types, you can attach up to 128 disks. So you can create massive amounts of capacity for a single host. Now remember that little nuance when I told you about how throughput is limited by the number of cores that you have? That throughput also shares the same bandwidth with Disk I/O. So if you plan on having a large amount of Disk I/O throughput, it will also compete with any network egress or ingress throughput. So remember that, especially if you will be increasing the number of drives attached to a virtual machine. There are many differences between a physical hard disk in a computer and a persistent disk, which is essentially a virtual networked device. First of all, if you remember with normal computer hardware disks, you have to partition them. Essentially, you have a drive and you're carving up a section for the operating system to get its own capacity. If you want to grow it, you have to repartition, and if you want to make changes you might even have to reformat. If you want redundancy, you might create a redundant disk array, and if you want encryption, you need to encrypt files before writing them to the disk. With cloud persistent disks, things are very different because all that management is handled for you on the backend. You can simply grow disks and resize the file system because disks are virtual networked devices. Redundancy and snapshot services are built in and disks are automatically encrypted. You can even use your own keys, and that will ensure that no party can get to the data except you.2. Let's practice!
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