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    SubjectRe: [PATCH] btrfs: lower metadata writeback threshold on low dirty threshold
    On Thu, May 03, 2012 at 11:43:11AM +0800, Fengguang Wu wrote:
    > This helps write performance when setting the dirty threshold to tiny numbers.
    >
    > 3.4.0-rc2 3.4.0-rc2-btrfs4+
    > ------------ ------------------------
    > 96.92 -0.4% 96.54 bay/thresh=1000M/btrfs-100dd-1-3.4.0-rc2
    > 98.47 +0.0% 98.50 bay/thresh=1000M/btrfs-10dd-1-3.4.0-rc2
    > 99.38 -0.3% 99.06 bay/thresh=1000M/btrfs-1dd-1-3.4.0-rc2
    > 98.04 -0.0% 98.02 bay/thresh=100M/btrfs-100dd-1-3.4.0-rc2
    > 98.68 +0.3% 98.98 bay/thresh=100M/btrfs-10dd-1-3.4.0-rc2
    > 99.34 -0.0% 99.31 bay/thresh=100M/btrfs-1dd-1-3.4.0-rc2
    > ==> 88.98 +9.6% 97.53 bay/thresh=10M/btrfs-10dd-1-3.4.0-rc2
    > ==> 86.99 +13.1% 98.39 bay/thresh=10M/btrfs-1dd-1-3.4.0-rc2
    > ==> 2.75 +2442.4% 69.88 bay/thresh=1M/btrfs-10dd-1-3.4.0-rc2
    > ==> 3.31 +2634.1% 90.54 bay/thresh=1M/btrfs-1dd-1-3.4.0-rc2

    Well, further tests show that it behaves very unstable:

    3.4.0-rc2-btrfs4+ 3.4.0-rc2-btrfs5+
    ------------------------ ------------------------
    69.88 +16.4% 81.31 bay/thresh=1M/btrfs-10dd-1-3.4.0-rc2-btrfs4+
    71.09 +1.4% 72.05 bay/thresh=1M/btrfs-10dd-2-3.4.0-rc2-btrfs4+
    72.60 -1.7% 71.38 bay/thresh=1M/btrfs-10dd-3-3.4.0-rc2-btrfs4+
    90.54 -0.9% 89.74 bay/thresh=1M/btrfs-1dd-1-3.4.0-rc2-btrfs4+
    89.17 -90.2% ==> 8.71 bay/thresh=1M/btrfs-1dd-2-3.4.0-rc2-btrfs4+
    ==> 14.96 +495.3% 89.06 bay/thresh=1M/btrfs-1dd-3-3.4.0-rc2-btrfs4+
    408.23 +1.0% 412.26 TOTAL write_bw

    Where the -btrfs5 kernel has one more patch to remove the write plug.

    Thanks,
    Fengguang


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