The Buffer Cache

1. The Buffer Cache Jeff Chase Duke University

2. The kernel syscall trap/return fault/return system call layer: file API fault entry: VM page faults memory management: block/page cache policy I/O completions timer ticks interrupt/return

3. DeFiler interfaces: overview create, destroy, read, write a dfile list dfiles DFS read(), write() startFetch(), startPush() waitValid(), waitClean() DBuffer dbuf = getBlock(blockID) releaseBlock(dbuf) DBufferCache DBuffer ioComplete() startRequest(dbuf, r/w) VirtualDisk

4. Memory Allocation Howshould an OS allocate its memory resources among contending demands? • Virtual address spaces: fork, exec, sbrk, page fault. • The kernel controls how many machine memory frames back the pages of each virtual address space. • The kernel can take memory away from a VAS at any time. • The kernel always gets control if a VAS (or rather a thread running within a VAS) asks for more. • The kernel controls how much machine memory to use as a cache for data blocks whose home is on slow storage. • Policy choices: which pages or blocks to keep in memory? And which ones to evict from memory to make room for others?

5. Memory/storage hierarchy small and fast (ns) registers caches L1/L2 • In general, each layer is a cache over the layer below. • inclusion property • Technology trends  rapid change • The triangle is expanding vertically  bigger gaps, more levels Terms to know cache index/directory cache line/entry, associativity cache hit/miss, hit ratio spatial locality of reference temporal locality of reference eviction / replacement write-through / writeback dirty/clean off-core L3 off-chip main memory (RAM) big and slow (ms) off-module disk, other storage, network RAM

7. Memory as a cache Processes access external storage objects through file APIs and VM abstraction. The OS kernel manages caching of pages/blocks in main memory. virtual address spaces data data files and filesystems, databases, other storage objects disk and other storage network RAM memory (frames) backing storage volumes (pages and blocks) page/block read/write accesses

8. The Buffer Cache Proc Memory File cache Ritchie and Thompson The UNIX Time-Sharing System, 1974

9. Editing Ritchie/Thompson The system maintains a buffer cache (block cache, file cache) to reduce the number of I/O operations. Suppose a process makes a system call to access a single byte of a file. UNIX determines the affected disk block, and finds the block if it is resident in the cache. If it is not resident, UNIX allocates a cache buffer and reads the block into the buffer from the disk. Then, if the op is a write, it replaces the affected byte in the buffer. A buffer with modified data is marked dirty: an entry is made in a list of blocks to be written. The write call may then return. The actual write may not be completed until a later time. If the op is a read, it picks the requested byte out of the buffer and returns it, leaving the block in the cache. Proc Memory File cache

10. The DeFiler buffer cache File abstraction implemented in upper DFS layer. All knowledge of how files are laid out on disk is at this layer. Access underlying disk volume through buffer cache API. Obtain buffers (dbufs), write/read to/from buffers, orchestrate I/O. read(), write() startFetch(), startPush() waitValid(), waitClean() DBuffer dbuf = getBlock(blockID) releaseBlock(dbuf) DBufferCache DBuffer Device I/O interface Asynchronous I/O to/from buffers block read and write Blocks numbered by blockIDs

11. Page/block cache internals HASH(blockID) Each frame/buffer of memory is described by a meta-object (header). Resident pages or blocks are accessible through through a global hash table. An ordered list of eviction candidates winds through the hash chains. Some frames/buffers are free (no valid data). These are on a free list.

12. DBufferCache internals HASH(blockID) Any given block (blockID) is either resident or not. If resident, then it has exactly one copy (dbuf) in the cache. If it is resident then getBlock finds the dbuf (cache hit). This requires some kind of cache index, e.g., a hash table. DBuffer dbuf = getBlock(blockID) DBufferCache DBuffer I/O cache buffers Each is byte[blocksize] Buffer headers DBuffer dbuf There is a one-to-one correspondence of dbufs to buffers.

13. DBufferCache internals HASH(blockID) If the requested block is not resident, then getBlock allocates a dbuf for the block and places the correct block contents in its buffer (cache miss). If there are no free dbufs in the cache, then we must evict some other block from the cache and reuse its dbuf. DBuffer dbuf = getBlock(blockID) DBufferCache DBuffer I/O cache buffers Each is byte[blocksize] Buffer headers DBuffer dbuf There is a one-to-one correspondence of dbufs to buffers.

14. Page/block cache internals HASH(blockID) List(s) of free buffers (bufs) or eviction candidates. These dbufs might be listed in the cache directory if they contain useful data, or not, if they are truly free. To replace a dbuf Remove from free/eviction list. Remove from cache directory. Change dbuf blockID and status. Enter in directory w/ new blockID. Re-register on eviction list. Beware of concurrent accesses. cache directory

15. Dbuffer (dbuf) states DFS read(…) write(...) startFetch(), startPush() waitValid(), waitClean() A DBuffer dbuf returned by getBlock is always associated with exactly one block in the disk volume. But it might or might not be “in sync” with the underlying disk contents. DBuffer A dbuf is valid iff it has the “correct” copy of the data. A dbuf is dirty iff it is valid and has an update (a write) that has not yet been written to disk. A valid dbuf is clean if it is not dirty. Your DeFiler should return only valid data to a client. That may require you to zero the dbuf or fetch data from the disk. Your DeFiler should ensure that all dirty data is eventually pushed to disk.

16. Asynchronous I/O on dbufs Start I/O on a dbuf by posting it to a producer/consumer queue for service by a device thread. startFetch(), startPush() waitValid(), waitClean() startFetch(), startPush() Client threads may wait on the dbuf for asynchronous I/O to complete. DBuffer waitValid(), waitClean() ioComplete() startRequest(dbuf, r/w) Thread upcalls dbuf ioComplete when I/O operation is done. Device I/O interface Async I/O on dbufs device threads VirtualDisk

17. More dbuf states Do not evict a dbuf that is in active use (busy)! A dbuf is pinned if I/O is in progress, i.e., a disk request has started but not yet completed. A dbuf is held if DFS obtained a reference to the dbuf from getBlock but has not yet released the dbuf. DFS dbuf = getBlock(blockID) releaseBlock(dbuf) DBufferCache DBuffer startRequest(dbuf, r/w); ioComplete() VirtualDisk

18. File system layer (DFS) create, destroy, read, write a dfile list dfiles Allocate blocks to files and file metadata. Allocate DFileIDs to files. Track which blockIDs and DFileIDs are free and which are in use. Maintain a block map “inode” for each file, as metadata stored on disk. “inode” read(), write() startFetch(), startPush() waitValid(), waitClean() DBuffer dbuf = getBlock(blockID) releaseBlock(dbuf) sync() DBufferCache DBuffer

19. A Filesystem On Disk sector 0 sector 1 allocation bitmap file directory file 11100010 00101101 10111101 wind: 18 0 0 snow: 62 rain: 32 hail: 48 10011010 00110001 00010101 00101110 00011001 01000100 once upo n a time /n in a l and far far away , lived th Data

20. A Filesystem On Disk sector 0 sector 1 allocation bitmap file directory file 11100010 00101101 10111101 wind: 18 0 0 snow: 62 rain: 32 hail: 48 10011010 00110001 00010101 00101110 00011001 01000100 once upo n a time /n in a l and far far away , lived th Metadata

21. Managing files create, destroy, read, write a dfile list dfiles Each file has a size: it is the first byte offset in the file that has never been written. Never return data past a file’s size. Fetch blocks for data and metadata (or zero new ones fresh), read and write in place, and push dirty blocks back to the disk. Serialize DFS read/write on each inode. “inode” read(), write() startFetch(), startPush() waitValid(), waitClean() DBuffer dbuf = getBlock(blockID) releaseBlock(dbuf) sync() DBufferCache DBuffer

22. Representing a File On Disk file attributes e.g., size once upo n a time /nin a l logical block 0 block map Index by logical block number maps to a blockID and far far away ,/nlived t logical block 1 blockID access blocks through the block cache with getBlock, startFetch, waitValid, read, releaseBlock. he wise and sage wizard. logical block 2 “inode”

23. Filesystem layout on disk inode 0 bitmap file inode 1 root directory inode 1 root directory fixed locations on disk 11100010 00101101 10111101 11100010 00101101 10111101 wind: 18 0 0 snow: 62 rain: 32 hail: 48 10011010 00110001 00010101 allocation bitmap file blocks 00101110 00011001 01000100 once upo n a time /n in a l file blocks and far far away , lived th inode This is a toy example (Nachos).

24. Filesystem layout on disk inode 0 bitmap file inode 1 root directory 11100010 00101101 10111101 0 rain: 32 hail: 48 X X DeFiler must be able to find all valid inodes on disk. X X once upo n a time /n in a l Your DeFiler volume is small. You can keep the free block/inode maps in memory. You don’t need metadata structures on disk for that. But you have to scan the disk to rebuild the in-memory structures on initialization. file blocks DeFiler has no directories. You just need to keep track of which DFileIDs are currently valid, and return a list. and far far away , lived th inode

25. Disk layout: the easy way DeFiler must be able to find all valid inodes on disk. Given a list of valid inodes, you can determine which inodes and blocks are free and which are in use. once upo n a time /n in a l file blocks and far far away , lived th inode