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Marek Marczykowski-Górecki:
> On Wed, Oct 16, 2024 at 04:28:14PM +0000, Rusty Bird wrote:
> > Also, on file-reflink
> > systems, where the dom0 root filesystem is storing (possibly many
> > terabytes worth of) VM volumes, fstrim can take really long. E.g.
> > here on my main Btrfs system, which is otherwise quite fast:
> >
> > # time fstrim /var/tmp/
> > real 4m29.240s
>
> But that takes long only if there is really a lot of data to discard,
> no?
# for i in 1 2 3; do time fstrim /var/tmp/; done 2>&1 | grep real
real 4m24.308s
real 4m34.060s
real 4m29.806s
I don't see anything in Btrfs tracking which unused blocks it has
already issued discards for. Or in ext4, but it doesn't matter with
the small ext4 dom0 root fs in an LVM Thin installation. So a large fs
that's neither almost empty nor almost full has to at least generate a
gigantic list of (due to fragmentation) probably rather small ranges
of blocks to be discarded in response to every fstrim and forward it
through the block subsystem (which I don't think is keeping track
either?) to the drive. Only after all of that overhead, I guess the
drive might respond faster if it had already done most of the work
last time.
Rusty
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