Den 01.04.2019 11:34, skrev Kevin Wolf:
Yes, for 512 accesses on native 4k disks with O_DIRECT, the QEMU block
layer performs the necessary RMW. Of course, it still comes with a
performance penalty, so you want to avoid such setups, but they do work.
I suspect that the approximately 1/10 -th disk-speed I see (e.g 28389
vs 258719 K/sec sequential disk Output) in domU compared to dom0 must
be due to this. I have dom0 and domU drives as partitions on an md
raid6. The performance is sub-optimal to put it mildly. Here is one
example bonnie++ run on a guest:
Version 1.97 ------Sequential Output------ --Sequential Input-
--Random-
Concurrency 3 -Per Chr- --Block-- -Rewrite- -Per Chr- --Block--
--Seeks--
Machine Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP
/sec %CP
gt-credit2-on-p 16G 1062 90 28389 2 20233 2 3950 82 199633 10
1195 17
Latency 19003us 140ms 652ms 20811us 602ms 33189us
Version 1.97 ------Sequential Create------ --------Random
Create--------
gt-credit2-on-pt -Create-- --Read--- -Delete-- -Create-- --Read---
-Delete--
files /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
/sec %CP
127 34445 26 +++++ +++ 42424 27 67418 51 +++++ +++
52590 35
Latency 12979us 329us 2903us 304us 38us 668us
1.97,1.97,gt-credit2-on-pt,3,1550928114,16G,,1062,90,28389,2,20233,2,3950,82,199633,10,1195,17,127,,,,,34445,26,+++++,+++,42424,27,67418,51,+++++,+++,52590,35,19003us,140ms,652ms,20811us,602ms,33189us,12979us,329us,2903us,304us,38us,668us
Same on dom0:
Version 1.97 ------Sequential Output------ --Sequential Input-
--Random-
Concurrency 3 -Per Chr- --Block-- -Rewrite- -Per Chr- --Block--
--Seeks--
Machine Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP
/sec %CP
XEN-4.12-DOM0 16G 475 91 258719 20 151508 19 585 99
324725 22 392.3 14
Latency 16093us 1097ms 406ms 18136us 160ms 155ms
Version 1.97 ------Sequential Create------ --------Random
Create--------
XEN-4.12-DOM0 -Create-- --Read--- -Delete-- -Create--
--Read--- -Delete--
files /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
/sec %CP
8 6563 11 +++++ +++ 13522 20 10969 18 +++++ +++
9560 13
Latency 104us 68us 18179us 120us 20us 63us
1.97,1.97,XEN-4.12-DOM0,3,1547961396,16G,,475,91,258719,20,151508,19,585,99,324725,22,392.3,14,8,,,,,6563,11,+++++,+++,13522,20,10969,18,+++++,+++,9560,13,16093us,1097ms,406ms,18136us,160ms,155ms,104us,68us,18179us,120us,20us,63us
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