Exploring Operating Systems

Day 45: File System Journaling

Introduction to File System Journaling
Journal Structure and Components
Transaction Management
Recovery Mechanisms
Checkpoint Processing
Write-Ahead Logging
Journal Space Management
Conclusion

1. Introduction to File System Journaling

File system journaling is a technique used to ensure data consistency and reliability in file systems. It works by maintaining a log (journal) of changes before they are committed to the main file system. This allows the system to recover quickly from crashes or power failures by replaying or undoing transactions recorded in the journal.

Transaction Logging: Recording changes to the file system in a log before applying them.
Atomic Operations: Ensuring that transactions are either fully completed or fully rolled back.
Crash Recovery: Using the journal to restore the file system to a consistent state after a crash.
Consistency Guarantees: Ensuring that the file system remains in a valid state at all times.

2. Journal Structure and Components

A journal is typically implemented as a circular buffer in a dedicated area of the disk. It consists of a series of journal entries, each representing a change to the file system.

Journal Structure Implementation

Below is an implementation of a basic journal structure in C:

typedef enum {
    TXN_START,
    TXN_COMMIT,
    TXN_ABORT,
    DATA_WRITE,
    METADATA_UPDATE
} JournalEntryType;

typedef struct {
    uint64_t sequence_number;
    JournalEntryType type;
    uint64_t timestamp;
    uint32_t data_length;
    void *data;
    uint32_t checksum;
} JournalEntry;

typedef struct {
    int fd;                     // Journal file descriptor
    uint64_t current_position;  // Current write position
    uint64_t size;             // Total journal size
    uint64_t wrap_point;       // Position where journal wraps
    pthread_mutex_t lock;
    bool circular;             // Whether journal is circular
} Journal;

Journal* initialize_journal(const char *path, size_t size) {
    Journal *journal = malloc(sizeof(Journal));

    journal->fd = open(path, O_RDWR | O_CREAT, 0644);
    journal->size = size;
    journal->current_position = 0;
    journal->wrap_point = size;
    journal->circular = true;

    pthread_mutex_init(&journal->lock, NULL);

    // Initialize journal header
    JournalHeader header = {
        .magic = JOURNAL_MAGIC,
        .version = JOURNAL_VERSION,
        .creation_time = time(NULL)
    };

    write(journal->fd, &header, sizeof(header));
    fsync(journal->fd);

    return journal;
}

3. Transaction Management

Transactions in a journaled file system are sequences of operations that are logged and applied atomically. Below is an implementation of transaction management:

typedef struct {
    uint64_t txn_id;
    time_t start_time;
    List *operations;
    JournalEntry *start_entry;
    bool committed;
} Transaction;

typedef struct {
    Transaction **active_txns;
    size_t txn_count;
    pthread_mutex_t txn_lock;
    Journal *journal;
} TransactionManager;

Transaction* begin_transaction(TransactionManager *tm) {
    Transaction *txn = malloc(sizeof(Transaction));
    txn->txn_id = generate_txn_id();
    txn->start_time = time(NULL);
    txn->operations = list_create();
    txn->committed = false;

    // Create start entry
    JournalEntry *start_entry = create_journal_entry(
        TXN_START,
        txn->txn_id,
        NULL,
        0
    );

    pthread_mutex_lock(&tm->journal->lock);
    write_journal_entry(tm->journal, start_entry);
    pthread_mutex_unlock(&tm->journal->lock);

    txn->start_entry = start_entry;

    // Add to active transactions
    pthread_mutex_lock(&tm->txn_lock);
    tm->active_txns[tm->txn_count++] = txn;
    pthread_mutex_unlock(&tm->txn_lock);

    return txn;
}

bool commit_transaction(TransactionManager *tm, Transaction *txn) {
    // Write all pending operations
    List *ops = txn->operations;
    ListIterator it = list_iterator_create(ops);

    pthread_mutex_lock(&tm->journal->lock);

    while (list_iterator_has_next(it)) {
        JournalEntry *entry = list_iterator_next(it);
        if (!write_journal_entry(tm->journal, entry)) {
            pthread_mutex_unlock(&tm->journal->lock);
            return false;
        }
    }

    // Write commit record
    JournalEntry *commit_entry = create_journal_entry(
        TXN_COMMIT,
        txn->txn_id,
        NULL,
        0
    );

    bool success = write_journal_entry(tm->journal, commit_entry);
    pthread_mutex_unlock(&tm->journal->lock);

    if (success) {
        txn->committed = true;
        remove_active_transaction(tm, txn);
    }

    return success;
}

4. Recovery Mechanisms

Recovery mechanisms ensure that the file system can be restored to a consistent state after a crash. Below is an implementation of crash recovery:

typedef struct {
    Journal *journal;
    List *incomplete_txns;
    List *completed_txns;
} RecoveryManager;

bool perform_recovery(RecoveryManager *rm) {
    // Scan journal from last checkpoint
    uint64_t checkpoint_position = find_last_checkpoint(rm->journal);
    set_journal_position(rm->journal, checkpoint_position);

    while (has_more_entries(rm->journal)) {
        JournalEntry *entry = read_next_entry(rm->journal);

        switch (entry->type) {
            case TXN_START:
                add_incomplete_transaction(rm, entry->txn_id);
                break;

            case TXN_COMMIT:
                move_to_completed(rm, entry->txn_id);
                break;

            case TXN_ABORT:
                remove_incomplete_transaction(rm, entry->txn_id);
                break;

            case DATA_WRITE:
            case METADATA_UPDATE:
                if (is_transaction_complete(rm, entry->txn_id)) {
                    replay_operation(entry);
                }
                break;
        }
    }

    // Rollback incomplete transactions
    return rollback_incomplete_transactions(rm);
}

bool replay_operation(JournalEntry *entry) {
    switch (entry->type) {
        case DATA_WRITE:
            return replay_data_write(entry);

        case METADATA_UPDATE:
            return replay_metadata_update(entry);

        default:
            return false;
    }
}

5. Checkpoint Processing

Checkpoints are used to reduce the amount of journal data that needs to be scanned during recovery. Below is an implementation of checkpoint processing:

typedef struct {
    uint64_t checkpoint_id;
    time_t timestamp;
    List *active_txns;
    uint64_t journal_position;
} Checkpoint;

bool create_checkpoint(Journal *journal, TransactionManager *tm) {
    pthread_mutex_lock(&tm->txn_lock);
    pthread_mutex_lock(&journal->lock);

    Checkpoint *cp = malloc(sizeof(Checkpoint));
    cp->checkpoint_id = generate_checkpoint_id();
    cp->timestamp = time(NULL);
    cp->journal_position = journal->current_position;

    // Copy active transactions
    cp->active_txns = list_create();
    for (size_t i = 0; i < tm->txn_count; i++) {
        list_append(cp->active_txns, tm->active_txns[i]);
    }

    // Write checkpoint record
    JournalEntry *cp_entry = create_checkpoint_entry(cp);
    bool success = write_journal_entry(journal, cp_entry);

    if (success) {
        fsync(journal->fd);
        update_checkpoint_pointer(journal, cp);
    }

    pthread_mutex_unlock(&journal->lock);
    pthread_mutex_unlock(&tm->txn_lock);

    return success;
}

6. Write-Ahead Logging

Write-ahead logging (WAL) ensures that changes are logged before they are applied to the file system. Below is an implementation of WAL:

typedef struct {
    void *data;
    size_t length;
    uint64_t disk_offset;
    uint32_t checksum;
} WALRecord;

bool write_ahead_log(Journal *journal, WALRecord *record) {
    // Create log entry
    JournalEntry *entry = create_journal_entry(
        DATA_WRITE,
        current_transaction_id(),
        record->data,
        record->length
    );

    // Write to journal first
    pthread_mutex_lock(&journal->lock);
    bool logged = write_journal_entry(journal, entry);
    pthread_mutex_unlock(&journal->lock);

    if (!logged) {
        return false;
    }

    // Now safe to write to actual location
    return write_to_disk(record->disk_offset,
                        record->data,
                        record->length);
}

7. Journal Space Management

Journal space management ensures that the journal does not run out of space. Below is an implementation of journal space management:

typedef struct {
    uint64_t total_space;
    uint64_t used_space;
    uint64_t reserved_space;
    List *free_segments;
} JournalSpaceManager;

bool allocate_journal_space(JournalSpaceManager *manager,
                          size_t required_space) {
    pthread_mutex_lock(&manager->lock);

    if (manager->used_space + required_space >
        manager->total_space - manager->reserved_space) {
        // Try to reclaim space
        if (!reclaim_journal_space(manager, required_space)) {
            pthread_mutex_unlock(&manager->lock);
            return false;
        }
    }

    manager->used_space += required_space;
    pthread_mutex_unlock(&manager->lock);
    return true;
}

bool reclaim_journal_space(JournalSpaceManager *manager,
                         size_t required_space) {
    // Find completed transactions that can be truncated
    List *completed_txns = find_completed_transactions(manager);
    uint64_t reclaimable_space = 0;

    ListIterator it = list_iterator_create(completed_txns);
    while (list_iterator_has_next(it)) {
        Transaction *txn = list_iterator_next(it);
        reclaimable_space += calculate_transaction_space(txn);

        if (reclaimable_space >= required_space) {
            truncate_journal(manager, txn->start_entry->position);
            return true;
        }
    }

    return false;
}

8. Conclusion

File system journaling is essential for maintaining data consistency and recovering from system crashes. Understanding the implementation details of journaling mechanisms is crucial for developing reliable file systems.

This site is open source. Improve this page.