A GitHub account is the identity that every repository, commit, and collaboration is attached to. The signup itself takes minutes, but the profile setup is what makes the account useful: a verified email is required before you can create a repository, and a profile README is what visitors see before they see any of your…
Git is a version control system. GitHub is a hosting platform built on top of Git. The distinction sounds trivial until you try to use one without the other: Git works entirely on your local machine, and GitHub does nothing without a Git repository to host. The two are related the way a document and…
Configuring swap is the part of memory management that the administrator controls directly. The concepts from the previous chapter—inactive pages, the OOM killer, the three forms—become commands in this one. The sequence is consistent across all three forms: create the block device or file, format it with mkswap, protect it with permissions, activate it with…
Swap space is the disk area that the kernel uses to store memory pages that it cannot keep in physical RAM. It is not a filesystem, it is not mounted, and it is not a backup. It is a temporary holding area for pages that the kernel has decided are not needed right now, so…
An LVM snapshot is a point-in-time view of a Logical Volume. It is not a copy; it is a mapping that records the original state of the blocks that have changed since the snapshot was taken. When the origin is written to, the old data is preserved in the snapshot’s exception store, and the snapshot…
Reducing a Logical Volume is the operation that LVM permits and the filesystem may forbid. The LV can always be shrunk; the filesystem on it cannot. ext4 supports shrinking, but only offline. XFS does not support shrinking at all. The workflow therefore begins with the filesystem type, not with lvreduce, and the order of operations…
Extending an LVM volume is the operation that justifies the entire abstraction. A partition cannot grow while mounted; an LVM Logical Volume can. The workflow is two commands in the simple case and three when the Volume Group has no free space. The critical detail is that lvextend changes the block device’s size, not the…
Rust has exactly one kind of subtyping: lifetime subtyping. A reference with a longer lifetime is a subtype of a reference with a shorter lifetime, and that single rule propagates through every generic type according to its variance. The interaction between subtyping and lifetime bounds—the ‘a: ‘b syntax and the implicit bounds that the compiler…
Variance is the rule that determines whether a generic type preserves, reverses, or ignores the subtyping relationship of its arguments. Rust’s only subtyping is lifetime-based: a reference with a longer lifetime can be used where a shorter lifetime is expected. Variance answers the question that follows: if &’long T is a subtype of &’short T,…
The typestate pattern moves a state machine from the runtime to the type system. Instead of a single Order type with a state: OrderState field, the pattern produces Order<Created>, Order<Paid>, and Order<Shipped> as distinct types, each with only the methods that are legal in that state. Calling a method that does not exist on the…