--- title: "The fastest hash is the one you never compute · DuplicateDuster" description: "How it stays fast on big drives, and what 'secure erase' does and does not guarantee." url: "https://royalsoftworks.com/products/duplicate-duster/how-it-works/hashing/" source: "https://royalsoftworks.com" format: "markdown" note: "Markdown rendering of the HTML page at `url`. Same content, same canonical URL." --- How it works · 3 of 3 # The fastest hash is the one you never compute. How it stays fast on big drives, and what 'secure erase' does and does not guarantee. Built for real drives ## The fastest hash is the one you never compute. A file whose exact byte size appears nowhere on the other side cannot be a duplicate of anything there — so it is never read at all. On a typical drive that removes the overwhelming majority of the bytes before hashing even starts. Size first then hash Both sides are indexed by exact size. Only files whose size occurs on the other side are ever opened, which is what makes a whole-drive scan finish in minutes rather than hours. Every core you can spare Hashing runs across a worker pool sized at three quarters of your cores, capped at sixteen. Work is handed out in batches as threads free up, so one thread drawing a 4 GB video doesn't leave the rest idle. Never stuck never silent An unreadable file is recorded as a warning and the scan carries on. If every hashing thread dies, the remaining files are hashed on the dispatcher instead — a scan always reaches a finish. ### Cancel actually cancels The stop signal is shared memory, not a message — so it reaches threads that are mid-read, in the middle of a batch, with no event loop turn to spare. ### Filters before the walk Minimum and maximum size, an include or exclude list of extensions, skip dot-files, skip system folders. A scan you narrowed is a scan that finishes sooner. ### Spinning disks respected Parallel reads lose on mechanical drives, where seeking between eight files costs more than the extra cores win. An environment variable set before launch pins the thread count to one. ### Nothing blocks the window The whole scan lives on worker threads. The interface stays responsive while a million files are being indexed behind it. ## See exactly what 'secure erase' guarantees. And what it doesn't — stated plainly, including the SSD caveat most tools leave out. [Security](https://royalsoftworks.com/products/duplicate-duster/security/) [Read the whitepaper](https://royalsoftworks.com/products/duplicate-duster/whitepaper/) Free to download, no account · Pro €20 once, VAT included · Windows · [Download free →](https://royalsoftworks.com/download/#duplicate-duster) [Previous Four ways to get the gigabytes back.](https://royalsoftworks.com/products/duplicate-duster/how-it-works/reclaim/) [All of DuplicateDuster how it works](https://royalsoftworks.com/products/duplicate-duster/how-it-works/)