Re: RAID-6 and write hole with write-intent bitmap

[Nix <nix@esperi.org.uk>]

On 24 Nov 2020, Wols Lists told this:

> On 24/11/20 07:20, Mukund Sivaraman wrote:
>> * With RAID-6 with write-intent bitmap in use, ext4 in ordered mode, no
>>   disk failures, and abrupt power loss, is there any chance of data loss
>>   in files other than those being written to just before the power loss?
>
> Probably. Sod's law, you will have other files on the same stripe and
> things could go wrong ... Plus I believe some file systems (including
> ext4?) store small files in the directory, not as their own i-node, so

ext4 can store small files in the *inode*, not in the containing
directory: that's impossible, since an inode can appear in many
directories at the same time.

... but, of course, inodes on many filesystems are packed into big
tables of inodes, and if you have a write hole hitting an inode write,
you've probably buggered up a bunch of other inodes too. And inodes are
more or less unordered, so that's just smashed a random spray of things
on the disk... it is quite posible that a lot of them are in directories
close in space or in time-of-write to the one you were updating, but not
necessarily. Reach for backups time.

> RULE 0: RAID IS NO SUBSTITUTE FOR BACKUPS.

But backups are a substitute for lack of sleep (for me anyway, because
if I don't have good backups, I can't sodding sleep).