[WIP] Change Segment, InodeMap, LFS

[travis1829]

• Suggestion for the in-memory cache of disk data
  • inode : on LFS::itable
  • inode data (direct/indirect...) : on Buf (VirtioDisk does this automatically)
  • inode map : on LFS::imap or somewhere else...
• Types
  • Superblock: Only hold constants
  • Checkpoint : Stores the last segment and the last segment block of it that is valid.

  • Tx : Update Checkpoint only after all locks on the Tx is released.

    Q. Change inode to store seg_no and seg_block_no instead of just block_no?

  • I think this is right.
  • However, it may be unnecessary. (Simple translation : seg_no = block_no / SEGSIZE, seg_block_no = block_no % SEGSIZE)

Q. Synchronization?

• We may need to add a lock to the Segment. (seems we need interior mutability anyway)
• Inode map : Share lock with Segment? (Only use inode map when have the Segment guard)

Q. TODO: delete bmap?

• What about revoking InodeGuard::{update, bmap_or_alloc, bmap, bmap_internal} from ufs and just tweaking them a little bit.

Q. When to actually write the checkpoint to the disk?

• Maybe every time we actually write the segment to the disk.

Q. Abstraction layer?

• Directory
• Inode (+Itable)
• Segment (+Inode Map)
• Disk