On Sat, 2024-12-21 at 10:34 +0800, Yu Kuai wrote: > Background > > We provide virtual machines for customers to use, which include an > important feature: in the initial state, the disks in the virtual > machine do not occupy actual storage space, and the data read by > users is all zeros until the user writes data for the first time. > This can save a large amount of storage. > > Problem > > However, after introducing dm-crypt, this feature has failed. Because > we expect the data read by users in the initial state to be zero, we > have to write all zeros from dm-crypt. Why do you expect the user to read all zeros? For DM crypt on a new physical disk, we don't set the disk contents to an initial value because there's no expectation on the part of the user that they can read a sector they haven't written and get a sensible result instead of the dm-crypt of whatever the sector contained. Why can't your cloud system just behave like a physical disk? Even for an unencrypted physical disk, there's no expectation of any particular value being in a sector, so why do your users have the expectation of all zeros? Regards, James
