juicefs metadata structure stored in engine has not considered scalability

[liujiangang01]

What would you like to be added:
type Slice struct { id uint64 size uint32 off uint32 len uint32 pos uint32 }
Slice metadata structure has not considered scalability when been designed. If developer want to update Slice structure, it will cause a lot of trouble. I think adding a "version" field is needed

[SandyXSD]

For now there's no requirement to add more field.

[liujiangang01]

It has no problem now. But store format is designed for now and future, it is very normal. For example, if we want to support new requirement for switching storage, Slice structure may be changed. I think if we want to operate juicefs in a long time, compatibility should be considerd in design

[SandyXSD]

Yes, we may add version field in the future together with new features, but won't do it now to just add the version.

[davies]

For compatibility, newly added field will have a default value (0, for example), we can use that for backward compatibility.