Re: Unused blocks and fstrim

From	Tim Woodall <debianuser@woodall.me.uk>
Newsgroups	linux.debian.user
Subject	Re: Unused blocks and fstrim
Date	2024-09-23 16:40 +0200
Message-ID	<JpSoh-eiCc-1@gated-at.bofh.it> (permalink)
References	<JoI7D-dAku-3@gated-at.bofh.it> <JoIKl-dANq-9@gated-at.bofh.it> <JpRix-ei0s-3@gated-at.bofh.it>
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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...

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Thread

Unused blocks and fstrim Steve Keller <keller.steve@gmx.de> - 2024-09-20 11:30 +0200
  Re: Unused blocks and fstrim Tim Woodall <debianuser@woodall.me.uk> - 2024-09-20 12:10 +0200
    Re: Unused blocks and fstrim Michael Kjörling <c9bc136c6063@ewoof.net> - 2024-09-20 13:00 +0200
    Re: Unused blocks and fstrim Steve Keller <keller.steve@gmx.de> - 2024-09-23 15:30 +0200
      Re: Unused blocks and fstrim Tim Woodall <debianuser@woodall.me.uk> - 2024-09-23 16:40 +0200

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