virt-sparsify is a tool for trimming free space in virtual disk
images. The new implementation uses vfs/kernel/qemu discard support.
Essentially it does:
for each filesystem:
mount -o discard $fs /mnt
sync
fstrim /mnt
umount /mnt
sync
# qemu is killed after sync returns
Although typing these commands by hand works fine, when you run them
from a program the fstrim doesn't happen all the way down the stack
reliably. Mostly it works, but sometimes it only trims some space
from the host file.
It appears that when the host is slow / under load, the problem
happens more frequently. Also it may happen more frequently on i686
than on x86-64 (possibly also due to speed of host).
The question is: What can I do to make sure the trim happens reliably,
all the way down the stack, before qemu is killed?
I am testing this using the latest upstream kernel & qemu.
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine. Supports Linux and Windows.
http://people.redhat.com/~rjones/virt-df/
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