Hello everyone. We are exploring a possibility to enhance the subject. Unfortunately many serious encryption modes are "atomic". That said, cipher transform (encryption/decryption) is going on by "atoms", i.e. blocks of size more than 1 byte. In particular this means that we need to read from disk before performing a partial write. This creates problems specific for stackable file system: our local copy of data can get obsolete, if another process modifies the same block, so that we'll end with loss of operations. Example: Suppose 2 processes overwrite a large file encrypted in atomic cipher mode with atom size 8 bytes (e.g. DES-ECB). Process A wants to overwrite one byte at offset #0, and process B wants to overwrite one byte at offset #7, i.e. the same block. Process A puts a respective "atom" of plain text (the first 8 bytes of the file) to his stack; Process B puts the same block of plain text to his stack; Process B modifies its local copy of plain text (overwrite the byte #7), encrypt the block and write the result to disk. Process A modifies its local copy of plain text (overwrite the byte #0), encrypt the block and write the result to disk. Here we've lost the modification performed by B, since A were not aware about those modification. The straightforward solution is to serialize read-modify-writes. I wonder if GlusterFS has any per-file serialization means, that would allow to resolve this problem. Or maybe there are possibilities to create such means. Any hints would be highly appreciated. Thank you in advance, Edward.
