I don't think fiemap was ever intended as anything more than an optimization to permit a user to avoid transferring unnecessary zeroes. SeaStore will probably not track sparseness at more than a 4k granularity. I don't think the EC implementation is clever about sparse reads/writes at all since that information would probably need to be duplicated above the objectstore in the object_info. -Sam On Mon, May 2, 2022 at 7:47 AM Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > On Mon, 2022-05-02 at 16:41 +0200, Ilya Dryomov wrote: > > On Mon, May 2, 2022 at 4:22 PM Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > > > > > (sorry for the resend, but the first message got rejected by the list because it was from an unsubscribed address) > > > > > > On Mon, 2022-05-02 at 14:05 +0200, Ilya Dryomov wrote: > > > > Hi Sam, > > > > > > > > I wanted to clarify ObjectStore::fiemap API and sparse-read OSD op > > > > guarantees as this came up in Jeff's fscrypt work and just recently in > > > > RBD as well. > > > > > > > > In fscrypt for kcephfs, Jeff has opted to use sparse-read to ensure > > > > that file holes (which must contain all zeroes logically) don't get > > > > "decrypted" into seemingly random junk. (Unlike ecryptfs, fscrypt > > > > framework doesn't attempt to protect the information about existence > > > > and location of holes in files, so logical holes generally correspond > > > > to physical holes.) > > > > > > > > > > The fscrypt client infrastructure generally prevents you from reading a > > > file when you don't have the key, but you could always analyze the > > > backing device and determine where the holes are. The situation with > > > cephfs is analogous. > > > > Yup. > > > > > > > > I imagine this is the same with ecryptfs though. I don't believe it > > > fills in the holes when you do a write past the EOF either. Were you > > > thinking of LUKS? That operates at the device level, so finding holes > > > there is a much different matter. > > > > I'm pretty sure ecryptfs always fills holes by encrypting logical zeroes and > > writing the resulting ciphertext out to the backing filesystem. Quoting the > > FAQ: > > > > eCryptfs does not currently support sparse files. Sequences of encrypted > > extents with all 0's could be interpreted as sparse regions in eCryptfs > > without too much implementation complexity. However, this would open up > > a possible attack vector, since the fact that certain segments of data are > > all 0's could betray strategic information that the user does not > > necessarily want to reveal to an attacker. For instance, if the attacker > > knows that a certain database file with patient medical data keeps > > information about viral infections in one region of the file and > > information about diabetes in another section of the file, then the very > > fact that the segment for viral infection data is populated with data at > > all would reveal that the patient has a viral infection. > > > > I stand corrected then! That tends to be pretty horrible for performance > though. Prepare to wait for a while if you do create a file and then > start writing at the 2G offset. > > In principle, we could also have the client fill in holes instead. It > may be worthwhile to have a mode where it does that. That might alsogive > us a way to support this on non-bluestore pools if it's not feasible to > allow for sparseness there). > -- > Jeff Layton <jlayton@xxxxxxxxxx> > _______________________________________________ Dev mailing list -- dev@xxxxxxx To unsubscribe send an email to dev-leave@xxxxxxx
