Archive for February 27, 2014

Thursday, February 27, 2014 [Tweets] [Favorites]

bup

bup (via Charles Parnot):

Highly efficient file backup system based on the git packfile format. Capable of doing fast incremental backups of virtual machine images.

[…]

It uses a rolling checksum algorithm (similar to rsync) to split large files into chunks. The most useful result of this is you can backup huge virtual machine (VM) disk images, databases, and XML files incrementally, even though they’re typically all in one huge file, and not use tons of disk space for multiple versions.

It uses the packfile format from git (the open source version control system), so you can access the stored data even if you don’t like bup’s user interface.

Unlike git, it writes packfiles directly (instead of having a separate garbage collection / repacking stage) so it’s fast even with gratuitously huge amounts of data. bup’s improved index formats also allow you to track far more filenames than git (millions) and keep track of far more objects (hundreds or thousands of gigabytes).

But:

bup is overly optimistic about mmap. Right now bup just assumes that it can mmap as large a block as it likes, and that mmap will never fail.

[…]

Because of the way the packfile system works, backups become “entangled” in weird ways and it’s not actually possible to delete one pack (corresponding approximately to one backup) without risking screwing up other backups.

How to Succeed at Recursion Without Really Recursing

Mike Vanier (via @CompSciFact):

The Y combinator is a higher-order function. It takes a single argument, which is a function that isn't recursive. It returns a version of the function which is recursive. We will walk through this process of generating recursive functions from non-recursive ones using Y in great detail below, but that's the basic idea.

More generally, Y gives us a way to get recursion in a programming language that supports first-class functions but that doesn't have recursion built in to it. So what Y shows us is that such a language already allows us to define recursive functions, even though the language definition itself says nothing about recursion. This is a Beautiful Thing: it shows us that functional programming alone can allow us to do things that we would never expect to be able to do (and it's not the only example of this).