[linux-lvm] tps for block device vs lvm device

[linux-lvm] tps for block device vs lvm device

[unlisted]

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Why would I see 50x the amount of transactions on the lvm volume vs the
actual underlying block device ?

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.00    0.00   11.41   57.07    0.00   31.51
Device:            tps    MB_read/s    MB_wrtn/s    MB_read    MB_wrtn
sda             211.33         0.00        56.12          0        113
sdb             268.47        63.05         0.00        128          0
dm-0          14365.52         0.00        56.11          0        113
dm-1              0.00         0.00         0.00          0          0
sdc               0.00         0.00         0.00          0          0
211 tps versus 14365 tps.  The box is idle and the following command was
running:

dd if=/dev/sdb of=/home/xxxxxxx/bigrandomfile.txt bs=4k count=10000000

Sam

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Re: [linux-lvm] tps for block device vs lvm device

[malahal]

I believe, that is due to bio vs request. LVM/DM block devices receive
and use bio requests where as sdX uses requests (which are coalesced
from these bio's using elevator algorithms). So the actual disk
transactions would be fewer.

--Malahal.

unlisted [unlisted@gmail.com] wrote:
>    Why would I see 50x the amount of transactions on the lvm volume vs the
>    actual underlying block device ?
> 
>    avg-cpu:  %user   %nice %system %iowait  %steal   %idle
>               0.00    0.00   11.41   57.07    0.00   31.51
>    Device:            tps    MB_read/s    MB_wrtn/s    MB_read    MB_wrtn
>    sda             211.33         0.00        56.12          0        113
>    sdb             268.47        63.05         0.00        128          0
>    dm-0          14365.52         0.00        56.11          0        113
>    dm-1              0.00         0.00         0.00          0          0
>    sdc               0.00         0.00         0.00          0          0
>    211 tps versus 14365 tps.  The box is idle and the following command was
>    running:
> 
>    dd if=/dev/sdb of=/home/xxxxxxx/bigrandomfile.txt bs=4k count=10000000
>    Sam

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