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    Alibaba Cloud Linux:JBD2 optimization interface

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    Alibaba Cloud Linux:JBD2 optimization interface

    Last Updated:Feb 28, 2026

    Alibaba Cloud Linux provides two kernel interfaces, force_copy and stats, to optimize JBD2 performance on ext4 file systems. Use force_copy to reduce journaling latency caused by the BH_Shadow state, and stats to diagnose whether JBD2 is the source of file system quality of service (QoS) issues.

    How it works

    ext4 is one of the most common journaling file systems. JBD2 is the kernel thread responsible for updating the ext4 journal and acts as a global resource for the file system. When JBD2 attempts to access a cache page, the page may be in the BH_Shadow state, which forces JBD2 to wait for the cache page to write back to disk. This wait increases latency and degrades system performance.

    The force_copy interface copies the cache page instead of waiting for the BH_Shadow state to clear, reducing the time JBD2 spends blocked on disk writeback.

    Supported versions

    Distribution

    Minimum kernel version

    Alibaba Cloud Linux 2

    4.19.81-17.al7

    Alibaba Cloud Linux 3

    All versions

    Interface reference

    force_copy

    Property

    Value

    Path

    /proc/fs/jbd2/<device>-8/force_copy

    Default

    Disabled (0)

    Valid values

    1 (enable), 0 (disable)

    Function

    Forces a cache page copy to avoid BH_Shadow wait time

    Replace <device> with the block storage device name, such as nvme0n1.

    Important

    Enabling force_copy increases memory consumption. Evaluate your available memory before enabling this interface on memory-constrained instances.

    stats

    Property

    Value

    Path

    /proc/fs/jbd2/<device>-8/stats

    Function

    Outputs JBD2 transaction statistics for diagnosing file system QoS issues

    Replace <device> with the block storage device name, such as nvme0n1.

    Enable force_copy

    Run the following command to enable force_copy. Replace nvme0n1 with your block storage device name.

    echo 1 > /proc/fs/jbd2/nvme0n1-8/force_copy

    To disable force_copy:

    echo 0 > /proc/fs/jbd2/nvme0n1-8/force_copy

    Read stats output

    Run the following command to query JBD2 transaction statistics:

    cat /proc/fs/jbd2/nvme0n1-8/stats

    Sample output:

    337 336 65536 0 14837 1701504 16 0 20058 5 33082732 605 942 1000 1000

    The output contains 15 space-separated fields. The following table describes each field.

    Field

    Position

    Description

    Transaction ID

    1st

    The ID of the current transaction

    Transactions requested

    2nd

    The number of transactions requested

    Max cached transactions

    3rd

    The maximum number of cached transactions

    Wait time

    4th

    The time the transaction spent waiting to be processed

    Request latency

    5th

    The latency of the transaction request

    Run time

    6th

    The time the transaction ran

    Lock time

    7th

    The time the transaction was locked

    Flush time

    8th

    The time it took to flush the transaction

    Log time

    9th

    The time it took to log the transaction

    Average commit time

    10th

    The average time it took to commit the transaction

    Handles

    11th

    The number of handles in the transaction

    Blocks

    12th

    The number of blocks in the transaction

    Logged blocks

    13th

    The number of blocks recorded for the transaction

    Time constant (Hz)

    14th

    The kernel time constant (CONFIG_HZ). Unit: hertz

    Time constant (ms)

    15th

    The kernel time constant. Unit: milliseconds

    Use fields 14 and 15 to convert time-based fields from ticks to the unit of your choice. Compare stats output before and after enabling force_copy to measure the latency improvement on your workload.