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Is the default discard mode to ignore?



virt-manager 'Discard Mode" defaults to "Hypervisor default" but I'm
not sure if there's ever a case where this could translate into
on/unmap?

What problems could result if the guest OS file system mount option
were to default to 'discard'? Is it reasonable to expect discards will
be inhibited by default because of Discard Mode's default setting?

What I'm concerned about is the use case of either raw or qcow2
backing files that have been fully preallocated. If discards pass
through to the backing file, they become sparse files, which may be
suboptimal due to ensuing fragmentation. Of course it's a trade off,
you also gain efficient storage of the backing file - it consumes only
what the guest uses. But I think the more compatible behavior is to
not do discards out of the box. Someone who wants sparse backing
files, I expect, will (a) create sparse backing files and (b) set
discard mode to unmap.

Background and genesis of all of this:
RFE: kickstart option to control discard configuration
https://bugzilla.redhat.com/show_bug.cgi?id=1860720


Thanks,


-- 
Chris Murphy


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