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    Subject[patch/rft] jbd2: tag journal writes as metadata I/O
    Date
    Hi,

    In running iozone for writes to small files, we noticed a pretty big
    discrepency between the performance of the deadline and cfq I/O
    schedulers. Investigation showed that I/O was being issued from 2
    different contexts: the iozone process itself, and the jbd2/sdh-8 thread
    (as expected). Because of the way cfq performs slice idling, the delays
    introduced between the metadata and data I/Os were significant. For
    example, cfq would see about 7MB/s versus deadline's 35 for the same
    workload. I also tested fs_mark with writing and fsyncing 1000 64k
    files, and a similar 5x performance difference was observed. Eric
    Sandeen suggested that I flag the journal writes as metadata, and once I
    did that, the performance difference went away completely (cfq has
    special logic to prioritize metadata I/O).

    So, I'm submitting this patch for comments and testing. I have a
    similar patch for jbd that I will submit if folks agree that this is a
    good idea.

    Cheers,
    Jeff

    Signed-off-by: Jeff Moyer <jmoyer@redhat.com>

    diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
    index 671da7f..1998265 100644
    --- a/fs/jbd2/commit.c
    +++ b/fs/jbd2/commit.c
    @@ -139,7 +139,7 @@ static int journal_submit_commit_record(journal_t *journal,
    set_buffer_ordered(bh);
    barrier_done = 1;
    }
    - ret = submit_bh(WRITE_SYNC_PLUG, bh);
    + ret = submit_bh(WRITE_SYNC_PLUG | (1<<BIO_RW_META), bh);
    if (barrier_done)
    clear_buffer_ordered(bh);

    @@ -160,7 +160,7 @@ static int journal_submit_commit_record(journal_t *journal,
    lock_buffer(bh);
    set_buffer_uptodate(bh);
    clear_buffer_dirty(bh);
    - ret = submit_bh(WRITE_SYNC_PLUG, bh);
    + ret = submit_bh(WRITE_SYNC_PLUG | (1<<BIO_RW_META), bh);
    }
    *cbh = bh;
    return ret;
    @@ -369,7 +369,7 @@ void jbd2_journal_commit_transaction(journal_t *journal)
    int tag_bytes = journal_tag_bytes(journal);
    struct buffer_head *cbh = NULL; /* For transactional checksums */
    __u32 crc32_sum = ~0;
    - int write_op = WRITE;
    + int write_op = WRITE_META;

    /*
    * First job: lock down the current transaction and wait for
    @@ -409,7 +409,7 @@ void jbd2_journal_commit_transaction(journal_t *journal)
    * instead we rely on sync_buffer() doing the unplug for us.
    */
    if (commit_transaction->t_synchronous_commit)
    - write_op = WRITE_SYNC_PLUG;
    + write_op = WRITE_SYNC_PLUG | (1<<BIO_RW_META);
    trace_jbd2_commit_locking(journal, commit_transaction);
    stats.run.rs_wait = commit_transaction->t_max_wait;
    stats.run.rs_locked = jiffies;


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