Am 22.02.2018 um 16:59 hat Eric Blake geschrieben:
Although off_t permits up to 63 bits (8EB) of file offsets, in
practice, we're going to hit other limits first. Document some
of those limits in the qcow2 spec, and how choice of cluster size
can influence some of the limits.
While at it, notice that since we cannot map any virtual cluster
to any address higher than 64 PB (56 bits) (due to the L1/L2 field
encoding), it makes little sense to require the refcount table to
access host offsets beyond that point. Mark the upper bits of
the refcount table entries as reserved, with no ill effects, since
it is unlikely that there are any existing images larger than 64PB
in the first place, and thus all existing images already have those
bits as 0.
Signed-off-by: Eric Blake <address@hidden>
I think it would be good to mention the exact reason for the 56 bits in
the spec. Even this commit message is rather vague ('L1/L2 field
encoding'), so if at some point someone wonders, if we couldn't simply
extend the allowed range, they won't easily see that it's related to
compressed clusters.