Re: [Qemu-devel] blobstore disk format (was Re: Design of the blobstore)

[Stefan Berger]

On 09/15/2011 08:28 AM, Michael S. Tsirkin wrote:
> So the below is a proposal for a directory scheme
> for storing (optionally multiple) nvram images,
> along with any metadata.
> Data is encoded using BER:
> http://en.wikipedia.org/wiki/Basic_Encoding_Rules
> Specifically, we mostly use the subsets.
Would it change anything if we were to think of the NVRAM image as 
another piece of metadata?

I am also wondering whether each device shouldn't just handle the 
metadata itself, so generate a blob from data structures containing all 
the metadata it needs, arranging attribute and value pairs itself (maybe 
using some convenience function for serialization/deserialization) and 
let the NVRAM layer not handle the metadata at all but only blobs, their 
maximum sizes, actual sizes encryption, integrity value (crc32 or sha1) 
and so on. What metadata should there be that really need to be handled 
on the NVRAM API and below level rather than on the device-specific code 
level?

> We use a directory as a SET in a CER format.
> This allows generating directory online without scanning
> the entries beforehand.
I guess it is the 'unknown' for me... but what is the advantage of using 
ASN1 for this rather than just writing out packed and 
endianess-normalized data structures (with revision value), having them 
crc32-protected to have some sanity checking in place?

    Stefan
> The rest of the encoding uses a DER format.
> This makes for fast parsing as entries are easy to skip.
>
> Each entry is encoded in DER format.
> Each entry is a SEQUENCE with two objects:
> 1. nvram
> 2. optional name - a UTF8String
>
> Binary data is stored as OCTET-STRING values on disk.
> Any RW metadata is stored as OCTET-STRING value as well.
> Any RO metadata is stored in appropriate universal encoding,
> by type.
>
> On the context below, an attribute is either a IA5String or a SEQUENCE.
> If IA5String, this is the attribute name, and it has no value.
> If SEQUENCE, the first entry in the sequence is an
> IA5String, it is the attribute name. The rest of the entries
> represent the attribute value.
>
> Mandatory/optional attributes: depends on type.
> tpm will have realsize as RW mandatory attribute.
>
> Each nvram is built as a SEQUENCE including 4 objects
> 1. type - an IA5String. downstreams can use other types such as
>                       UUIDs instead to ensure no conflicts with upstream
> 2. SET of mandatory attributes
> 3. SET of optional attributes
> 4. data - a RW OCTET-STRING
>
> It is envisioned that attributes won't be too large,
> so they can easily be kept in memory.