Re: Unused blocks and fstrim

[Tim Woodall <debianuser@woodall.me.uk>]

On Mon, 23 Sep 2024, Steve Keller wrote:

> Tim Woodall <debianuser@woodall.me.uk> writes:

> > The raid rebuild is a particular pain point IMO. It's important to do a
> > discard after a failed disk rebuild otherwise every block is 'in use' on
> > the underlying storage.
>
> Hmm, does a RAID rebuild really always copy the whole new disk, even
> the unused space?  But what kind of info is then kept in the first
> 128 MiB of /dev/md0, if not a flag for every block telling whether it's
> used or not?
>
> > After a rebuild I always create a LV with all the free space and then
> > discard it.
>
> :(
>
> I currently have RAID only on a server with HDDs which don't support
> TRIM anyway.  I have only needed twice to rebuild the RAID-1 with 2
> disks and I seem to remember that not the whole disk was copied, but I
> might be wrong on that.

I think the bitmaps are for dirty blocks - so a resynch after a power
failure is quick, not for a failed disk replacement rebuild.

But perhaps there's a config option somewhere so that the md device can
track discards in a bitmap.

My guess is most people run at 90% capacity so it's not that useful...