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    SubjectRe: Bad SSD performance with recent kernels
    On Sun, Jan 29, 2012 at 02:13:51PM +0100, Eric Dumazet wrote:
    > Le dimanche 29 janvier 2012 à 19:16 +0800, Wu Fengguang a écrit :
    >
    >
    > > Note that as long as buffered read(2) is used, it makes almost no
    > > difference (well, at least for now) to do "dd bs=128k" or "dd bs=2MB":
    > > the 128kb readahead size will be used underneath to submit read IO.
    > >
    >
    > Hmm...
    >
    > # echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=128k count=32768
    > 32768+0 enregistrements lus
    > 32768+0 enregistrements écrits
    > 4294967296 octets (4,3 GB) copiés, 20,7718 s, 207 MB/s
    >
    >
    > # echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=2M count=2048
    > 2048+0 enregistrements lus
    > 2048+0 enregistrements écrits
    > 4294967296 octets (4,3 GB) copiés, 27,7824 s, 155 MB/s

    Interesting. Here are my test results:

    root@lkp-nex04 /home/wfg# echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=128k count=32768
    32768+0 records in
    32768+0 records out
    4294967296 bytes (4.3 GB) copied, 19.0121 s, 226 MB/s
    root@lkp-nex04 /home/wfg# echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sda of=/dev/null bs=2M count=2048
    2048+0 records in
    2048+0 records out
    4294967296 bytes (4.3 GB) copied, 19.0214 s, 226 MB/s

    Maybe the /dev/sda performance bug on your machine is sensitive to timing?

    Thanks,
    Fengguang
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