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 <ebl...@redhat.com>
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. Kevin