Size of the DSTORAGE_REQUEST_OPTIONS

[hasenbanck]

While porting the DirectStorage API to Rust, we found the following curriosity and would like to as if it's working as designed:

The DSTORAGE_REQUEST_OPTIONS is defines as a bitfield and it seems that the design goal was that all fields are packed into a single unsigned 64 bit integer field.

But the current implementation is not of byte size 8, but of size 16, since MSVC is only packing fields with the same type into the same backing field. Since DSTORAGE_COMPRESSION_FORMAT is an UINT8 and not an UINT64 like the other fields, it will be stored by MSVC into it's own backing field. Allignment and padding then will grow the size of DSTORAGE_REQUEST_SOURCE_TYPE to 16 bytes.

[damyanp]

Yes, we noticed this recently as well. Unfortunately, it would be a breaking change to fix this to what was intended so we need to be very careful about how and when we do it - we'll probably try and line it up with some other breaking change.

One possible short-term fix we've considered is to update the struct definition so that it more obviously reflects the actual layout of the struct so it isn't so surprising.

[damyanp]

Note: although version 1.2.0 of DirectStorage updated the struct definition to make it more obvious, keeping this issue open to track fixing the options field to what was originally intended.

[osor-io]

Also porting this to another language. I found it strange that the last member, Reserved, is set to occupy 48 bits, which leaves still one byte at the end of the struct unnacounted for as far as I can tell. Should that last member have been UINT64 Reserved : 56?

It doesn't effectively matter, since it'll be padded to 16 bytes. but if the definition update was intended to make the distribution of the member more obvious this might be worth changing.