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    xfs: Introduce delayed logging core code · 71e330b5
    Dave Chinner 提交于
    The delayed logging code only changes in-memory structures and as
    such can be enabled and disabled with a mount option. Add the mount
    option and emit a warning that this is an experimental feature that
    should not be used in production yet.
    
    We also need infrastructure to track committed items that have not
    yet been written to the log. This is what the Committed Item List
    (CIL) is for.
    
    The log item also needs to be extended to track the current log
    vector, the associated memory buffer and it's location in the Commit
    Item List. Extend the log item and log vector structures to enable
    this tracking.
    
    To maintain the current log format for transactions with delayed
    logging, we need to introduce a checkpoint transaction and a context
    for tracking each checkpoint from initiation to transaction
    completion.  This includes adding a log ticket for tracking space
    log required/used by the context checkpoint.
    
    To track all the changes we need an io vector array per log item,
    rather than a single array for the entire transaction. Using the new
    log vector structure for this requires two passes - the first to
    allocate the log vector structures and chain them together, and the
    second to fill them out.  This log vector chain can then be passed
    to the CIL for formatting, pinning and insertion into the CIL.
    
    Formatting of the log vector chain is relatively simple - it's just
    a loop over the iovecs on each log vector, but it is made slightly
    more complex because we re-write the iovec after the copy to point
    back at the memory buffer we just copied into.
    
    This code also needs to pin log items. If the log item is not
    already tracked in this checkpoint context, then it needs to be
    pinned. Otherwise it is already pinned and we don't need to pin it
    again.
    
    The only other complexity is calculating the amount of new log space
    the formatting has consumed. This needs to be accounted to the
    transaction in progress, and the accounting is made more complex
    becase we need also to steal space from it for log metadata in the
    checkpoint transaction. Calculate all this at insert time and update
    all the tickets, counters, etc correctly.
    
    Once we've formatted all the log items in the transaction, attach
    the busy extents to the checkpoint context so the busy extents live
    until checkpoint completion and can be processed at that point in
    time. Transactions can then be freed at this point in time.
    
    Now we need to issue checkpoints - we are tracking the amount of log space
    used by the items in the CIL, so we can trigger background checkpoints when the
    space usage gets to a certain threshold. Otherwise, checkpoints need ot be
    triggered when a log synchronisation point is reached - a log force event.
    
    Because the log write code already handles chained log vectors, writing the
    transaction is trivial, too. Construct a transaction header, add it
    to the head of the chain and write it into the log, then issue a
    commit record write. Then we can release the checkpoint log ticket
    and attach the context to the log buffer so it can be called during
    Io completion to complete the checkpoint.
    
    We also need to allow for synchronising multiple in-flight
    checkpoints. This is needed for two things - the first is to ensure
    that checkpoint commit records appear in the log in the correct
    sequence order (so they are replayed in the correct order). The
    second is so that xfs_log_force_lsn() operates correctly and only
    flushes and/or waits for the specific sequence it was provided with.
    
    To do this we need a wait variable and a list tracking the
    checkpoint commits in progress. We can walk this list and wait for
    the checkpoints to change state or complete easily, an this provides
    the necessary synchronisation for correct operation in both cases.
    Signed-off-by: NDave Chinner <dchinner@redhat.com>
    Reviewed-by: NChristoph Hellwig <hch@lst.de>
    Signed-off-by: NAlex Elder <aelder@sgi.com>
    71e330b5
xfs_log_priv.h 19.4 KB