Class 19 CS 202 9 April 2020 On the board ------------ 1. Last time 2. Crash recovery --intro --ad-hoc --copy on write --journaling --wrap-up, summarize --------------------------------------------------------------------------- 1. Last time finished describing the implementation of directories. what's the point? one, you'll need to know the structure for lab5. two, that structure neatly separates human names from the system's name, and makes hard links very natural to implement you asked if "." and ".." are physically direntries. in standard Unix, yes; but not in lab5. described some of the design choices in FFS. all driven by actual file access patterns + the reality of disk hardware 2. Crash recovery --intro --ad-hoc --copy-on-write --journaling --There are a lot of data structures used to implement the file system: bitmap of free blocks, directories, inodes, indirect blocks, data blocks, etc. --We want these data structures to be *consistent*: we want invariants to hold --We also want to ensure that data on the disk remains consistent. --Thorny issue: *crashes* or power failures. --Making the problem worse is: (a) write-back caching and (b) non-ordered disk writes. (a) means the OS delays writing back modified disk blocks. (b) means that the modified disk blocks can go to the disk in an unspecified order. --Example: [DRAW PICTURE] INODE DATA BLOCK ADDED DATA BITMAP UPDATED crash. restart. uh-oh. --Solution: the system requires a notion of atomicity --How to think about this stuff: imagine that a crash can happen at any time. (The only thing that happens truly atomically is a write of one or a few 512-byte disk sector.) So you want to arrange for the world to look sane, regardless of where a crash happens. --> A challenge here is that metadata and data is spread across several disk blocks (and hence several sectors), so increasing size of atomic unit is not sufficient. --> Your leverage, as file system designer, is that you can arrange for some disk writes to happen *synchronously* (meaning that the system won't do anything until these disk writes complete), and you can impose some ordering on the actual writes to the disk. --So we need to arrange for higher-level operations ("add data to file") to _look_ atomic: an update either occurs or it doesn't. --Potentially useful analogy: during our concurrency unit, we had to worry about arbitrary interleavings (which we then tamed with concurrency primitives). Here, we have to worry that a crash can happen at any time (and we will tame this with abstractions like transactions). The response in both cases is a notion of atomicity. --We will mention three approaches to crash recovery in file systems: A. Ad-hoc (the book calls this "fsck") B. copy-on-write approaches C. Journaling (also known as write-ahead logging) A. Ad-hoc --Goal: metadata consistency, not data consistency (rationale: too expensive to provide data consistency; cannot live without metadata consistency.) --Approach: arrange to send file system updates to the disk in such a way that, if there is a crash, **fsck** can clean up inconsistencies --example: for file create: --write data to file --then update/write inode --then mark inode "allocated" in bitmap --then mark data blocks "allocated" in bitmap --then update directory now, cases: inode not marked allocated in bitmap --> only writes were to unallocated, unreachable blocks; the result is that the write "disappears" inode allocated, data blocks not marked allocated in bitmap --> fsck must update bitmap file created, but not yet in any directory --> fsck ultimately deletes file (after all that!) Disadvantages to this ad-hoc approach: (a) need to get ad-hoc reasoning exactly right (b) poor performance (synchronous writes of metadata) --multiple updates to same block require that they be issued separately. for example, imagine two updates to same directory block. requires first complete before doing the second (otherwise, not synchronous) --more generally, cost of crash recoverability is enormous. (a job like "untar" could be 10-20x slower) (c) slow recovery: fsck must scan entire disk --recovery gets slower as disks get bigger. if fsck takes one minute, what happens when disk gets 10 times bigger? --essentially, fsck has to scan the entire disk B. Copy-on-write approaches -- Goal: provide both metadata and data consistency, by using more space. Rationale: disks have gotten larger, space is not at a premium. -- Used by filesystems like ZFS, btrfs and APFS. [For more details read The Zettabyte File System by Jeff Bonwick, Matt Ahrens, Val Henson, Mark Maybee and Mark Shellenbaum. https://www.cs.hmc.edu/~rhodes/courses/cs134/sp19/readings/zfs.pdf] -- Approach: never modify a block, instead always make a new copy. In detail (see handout) : * The filesystem has a root block, which we refer to as the Uberblock (copying terminology from ZFS). The uberblock is the **only** block in the filesystem that is ever _modified_ (as opposed to being fully written, which the rest of the blocks are). * Start with a basic example: a modification to a file in an existing block - remember: _never modify, only copy_. so the file system allocates a new block, and writes the new version of the data to the new block - but that in turn necessitates writing a new version of the inode (to point to the new version of the block) - and that in turn _changes the inode number_, which means that parents and any directories hard-linking to the file have to change (for this to work, the inode has to store the list of hard links.) - and that in turn means that _those_ directories' inodes have to change - and so on up to the uberblock. - the change is _committed_ -- in the crucial sense that after a crash the new version will be visible -- when and only when the uberblock is modified on disk. * Note that the same thing happens when a user appends to a file, creating another block (and thus changing the inode, and so on). * And the same thing happens when creating a file (because the directory inode has to change) -- Note that to enable this picture, the uberblock is designed to fit in a sector, in order to allow **atomic updates**. -- Benefits: * Most changes can be committed in **any order**. * The only requirement is that all changes be committed before the uberblock is updated. * The ability to reorder writes in this manner has performance benefits. * On disk structure and data is **always** consistent. Do not need to use fsck, or run recovery after crash. * Most of these filesystems also make use of checksums to handle cases where data is corrupted for other reasons. * Filesystem incorporates versioning similar to Git and other version control tools you may have used. * This requires not throwing away the old versions of the blocks after writing the new ones. -- Disadvantages: * Significant write amplification: any writes require changes to several disk blocks. * Significant space overheads: the filesystem needs enough space to copy metadata blocks in order to make any changes. Consider the problem of deleting files when the disk is nearly full. * Generally necessitates the use of a garbage collection daemon in order to reclaim blocks from old versions of the file-system. C. Journaling -- Copy on write showed that crash consistency is easy to achieve when modifications **do not** modify (or destroy) the current copy. Golden rule of atomicity, per Saltzer-Kaashoek: "never modify the only copy" -- Problem is that copy-on-write carries significant write and space overheads. Want to do better without violating atomicity rule. -- Going to do so by borrowing ideas from how transaction are implemented in databases. -- Core idea: Treat file system operations as transactions. Concretely, this means that after a crash, failure recovery ensures that: * Committed file system operations are reflected in on-disk data structures. * Uncommitted file system operations are not visible after crash recovery. -- Core mechanism: Record enough information to finish applying committed operations (*redo operations*) and/or roll-back uncommitted operations (*undo operations*). This information is stored in a redo log or undo log. Discuss this in detail next. -- Redo logging * Used by Ext3 and Ext4 on Linux, going to discuss in that context. * Log is a fixed length ring buffer placed at the beginning of the disk (see handout). * Basic operations Step 1: filesystem computes what would change due to an operation. For instance, creating a new file involves changes to directory inodes, appending to a file involves changes to the file's inode and data blocks. Step 2: the file system computes where in the log it can write this transaction, and writes a transaction begin record there (TxnBegin in the handout). This record contains a transaction ID, which needs to be unique. The file system **does not** need to wait for this write to finish and can immediately proceed to the next step. Step 3: the file system writes a log detailing all the changes it computed in step 1 to the log. The file system **must** now wait for these log changes and the TxnBegin record (step 2) to finish being written to disk. Step 4: once the TxnBegin record, and all the log records from step 3 have been written, the system writes a transaction end record (TxnEnd in the handout). This record contains the same transaction ID as was written in Step 2, and the transaction is considered committed once it has been successfully written to disk. Step 5: Once the TxnEnd record has been written, the filesystem asynchronously performs the actual file system changes, this process is called **checkpointing**. While the system is free to perform checkpointing whenever it is convenient, the checkpoint rate dictates the size of the log that the system must reserve. * Crash recovery: During crash recovery, the filesystem needs to read through the logs, determine the set of **committed** operations, and then apply them. Observe that: -- The filesystem can determine whether a transaction is committed or not by comparing transaction IDs in TxnBegin and TxnEnd records. -- It is safe to apply the same redo log multiple times. Operationally, when the system is recovering from a crash, the system does the following: Step 1: The file system starts scanning from the beginning of the log. Step 2: Every time it finds a TxnBegin entry, it searches for a corresponding TxnEnd entry. Step 3: If matching TxnBegin and TxnEnd entries are found -- indicating that the transaction is committed -- the file system applies (checkpoints) the changes. Step 4: Recovery is completed once the entire log is scanned. Note, for redo logs, filesytems generally begin scanning the log from the **start of the log**. * What to log? Observe that logging can double the amount of data written to disk. To improve performance, Ext3 and 4 allow users to choose what to log. * Default is to log only metadata. The idea here is that many people are willing to accept data loss/corruption after a crash, but keeping metadata consistent is important/ This is because if metadata is inconsistent the FS may become unusable, as the data structures no longer have integrity. * Can change settings to force data to be logged, along with metadata. This incurs additional overheads, but prevents data loss on crash. -- Undo logging * Not used in isolation by any file system. * Key idea: Log contains information on how to rollback any changes made to data. Mechanically, during normal operations: Step 1: Write a TxBegin entry to the log. Step 2: For each operation, write instructions for how to undo any updates made to a block. These instructions might include the original data in the block. In-place changes to the block can be made right after these instructions have been persisted. Step 3: Wait for in-place changes (what we referred to as checkpointing) to finish for all blocks. Step 4: Write a TxnEnd entry into the block. *Note* this implies that a transaction is committed once all changes have been written to disk. During crash recovery: Step 1: Scan the log to find all uncommitted transactions, these are ones where a TxnBegin entry is present, but no TxnEnd entry is found. Step 2: For each such transaction check to see whether the undo entry is valid. This is usually done through the use of a checksum. Why do we need this? Remember a crash might occur before the undo entry has been successfully written. If that happened, then (by the procedure described above), the actual changes corresponding to this undo entry have not been written to disk, so ignoring this entry is safe. On the other hand, trying to undo using a partially complete entry might result in data corruption, so using this entry would be **unsafe**. Step 3: Apply all valid undo entries found, in order to restored the disk to a consistent location. Note, for undo logs, logs are generally scanned from the **end of the log**. * Advantage: Changes can be checkpointed to disk as soon as the undo log has been updated. This is beneficial when the amount of buffer cache is low. * Disadvantage: A transaction is not committed until all dirty blocks have been flushed to their in-place targets. [made it to here; pick up the rest next time.] -- Redo Logging vs Undo Logging This is just a recap of the advantages and disadvantages. **Redo logging** * Advantage: A transaction can commit without all in-place updates (writes to actual disk locations) being completed. Updating the journal is sufficient. Why is this useful? In-place updates might be scattered all over the disk, so the ability to delay them can help improve performance. * Disadvantage: A transaction's dirty blocks need to be kept in the buffer-cache until the transaction commits and all of the associated journal entries have been flushed to disk. This might increase memory pressure. **Undo log** * Advantage: A dirty block can be written to disk as soon as the undo-log entry has been flushed to disk. This reduces memory pressure. * Disadvantage: A transaction cannot commit until all dirty blocks have been flushed to disk. This imposes additional constraints on the disk scheduler, might result in worse performance. -- Combining Redo and Undo Logging * Done by NTFS. * Goals: - Allow dirty buffers to be flushed as soon as their associated journal entries are written. This can reduce memory pressure when necessary. - Transactions commit as soon as logging is done, so the system has greater flexibility when scheduling disk writes. * How does this work? * Basic operations Step 1: filesystem computes what would change due to an operation. For instance, creating a new file involves changes to directory inodes, appending to a file involves changes to the file's inode and data blocks. Step 2: the file system computes where in the log it can write this transaction, and writes a transaction begin record there (TxnBegin in the handout). This record contains a transaction ID, which needs to be unique. The file system **does not** need to wait for this write to finish and can immediately proceed to the next step. Step 3: the file system writes both a redo log entry and an undo log entry for each of the changes it computed in step 1. These live together. The filesystem can begin making in-place changes (checkpointing changes) the moment this undo + redo log information has been written. Step 4: Wait until the TxnBegin record, and all the log records from step 3, have been written, the system writes a transaction end record (TxnEnd in the handout). This record contains the same transaction ID as was written in Step 2, and the transaction is considered committed once it has been successfully written to disk. Step 5: Similar to the redo logging case, the filesystem asynchronously continues to checkpoint/perform in-place writes whenever it is convenient. * Recovery For crash recovery, the filesystem first goes through the log finding all committed transactions, and using the redo entry within them to apply committed changes. Next it scans through the log (backwards) finding all uncommitted transactions, and uses the undo entries associated with these to undo any in-place updates. * Why? Designed for a time when the same Operating System ran on machines with very little memory (8-32MB), and also on "big-iron" servers with lots of memory (1GB?). This was an attempt to get the best of both worlds.