Why is LVM reading large amounts of data when I tell it to write only?
For example, this is what iostat shows when writing to a raw partition
(when doing "dd if=/dev/zero of=/dev/sdb5", iostat output every second):
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 80.81 0.00 5171.72 0 5120
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 78.00 0.00 4992.00 0 4992
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 81.82 0.00 5236.36 0 5184
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 77.00 0.00 4928.00 0 4928
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 79.21 0.00 5069.31 0 5120
This is what happens if we go through LVM - look how much is read before
it actually writes anything:
(when doing "dd if=/dev/zero of=/dev/test/test_volume").
What's interesting, if I add i.e. bs=64k to dd, I don't see these unexpected
reads on LVM any more. Why does it happen?
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 212.87 851.49 0.00 860 0
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 956.57 3826.26 0.00 3788 0
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 1083.84 4335.35 0.00 4292 0
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 1016.00 4064.00 0.00 4064 0
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 1035.64 4142.57 0.00 4184 0
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 1034.00 4136.00 0.00 4136 0
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 965.66 3862.63 0.00 3824 0
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 1025.00 4100.00 0.00 4100 0
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 991.92 3967.68 0.00 3928 0
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 1032.00 4128.00 0.00 4128 0
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 1009.90 4039.60 0.00 4080 0
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 366.33 1330.61 4040.82 1304 3960
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 1016.16 4060.61 76.77 4020 76
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 383.17 1394.06 4055.45 1408 4096
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 741.00 2904.00 1800.00 2904 1800
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 638.61 2475.25 2273.27 2500 2296
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 421.00 1560.00 3720.00 1560 3720
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 955.56 3802.02 501.01 3764 496
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 275.76 965.66 4016.16 956 3976
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 853.00 3384.00 840.00 3384 840
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 560.40 2130.69 3223.76 2152 3256
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 500.00 1907.07 2787.88 1888 2760
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 133.00 352.00 5296.00 352 5296
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 53.00 0.00 6244.00 0 6244
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 267.00 904.00 4840.00 904 4840
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdb 52.00 0.00 6240.00 0 6240
--
Tomasz Chmielewski
http://wpkg.org
_______________________________________________
linux-lvm mailing list
linux-lvm@redhat.com
https://www.redhat.com/mailman/listinfo/linux-lvm
read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
