On Tue, Apr 6, 2021 at 12:14 AM antlists <antlists@xxxxxxxxxxxxxxx> wrote: > (I don't understand it, but if a system crashes in the middle > of a raid-5 write it can apparently mess things up something horrid). Short version is that the disks making up a RAID stripe don't always get written simultaneously. If things crash just wrong, you can get half the stripe with old data, half the stripe with new data, and no way to tell which is which. A journal fixes this by writing the data twice: first to the journal, then to the array. If the system crashes while writing to the journal, you've still got the entire old data on the array; if it crashes while writing to the array, you've got the entire new data on the journal. You're never in an inconsistent half-and-half situation. -- Mark
