[Bug 237807] ZFS: ZVOL writes fast, ZVOL reads abysmal...

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=237807

            Bug ID: 237807
           Summary: ZFS: ZVOL writes fast, ZVOL reads abysmal...
           Product: Base System
           Version: 12.0-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: nbe at renzel.net

OS: FreeBSD 12.0-STABLE #23 r346730M: Fri Apr 26 08:38:36 CEST 2019


HBA Controllers:
===========================================================================
Device Name           Chip Name        Board Name        Firmware
/dev/mps0             LSISAS2308       SAS9207-8i        14000700
/dev/mps1             LSISAS2308       SAS9207-8i        14000700
===========================================================================


ZPOOL:
===========================================================================
  pool: morsleben-grube2
 state: ONLINE
  scan: scrub repaired 0 in 0 days 07:21:42 with 0 errors on Sat May  4
10:23:33 2019
config:

        NAME        STATE     READ WRITE CKSUM
        morsleben-grube2  ONLINE       0     0     0
          da0       ONLINE       0     0     0
          da1       ONLINE       0     0     0
          da2       ONLINE       0     0     0
          da3       ONLINE       0     0     0
          da4       ONLINE       0     0     0
          da5       ONLINE       0     0     0
          da6       ONLINE       0     0     0
          da7       ONLINE       0     0     0
          da8       ONLINE       0     0     0
          da9       ONLINE       0     0     0
          da10      ONLINE       0     0     0
          da11      ONLINE       0     0     0

errors: No known data errors
===========================================================================



Writing to that ZVOL is reasonabiliy fast, but reading from that ZVOL is
abysmal at around 20MB/s.



Here's an output of "gstat -I 1000000":
===========================================================================
dT: 1.018s  w: 1.000s
 L(q)  ops/s    r/s   kBps   ms/r    w/s   kBps   ms/w   %busy Name
    0      0      0      0    0.0      0      0    0.0    0.0| ada0
    0     79     79   3791    0.3      0      0    0.0    2.2| da0
    0     77     77   3823    0.4      0      0    0.0    2.7| da1
    0     18     18   1131    0.6      0      0    0.0    0.9| da2
    0     20     20   1257   10.1      0      0    0.0   17.5| da3
    0     18     18   1131    1.5      0      0    0.0    2.2| da4
    0     20     20   1257   11.2      0      0    0.0   19.1| da5
    0     17     17   1069    0.9      0      0    0.0    1.0| da6
    0     16     16   1006    0.3      0      0    0.0    0.3| da7
    0     23     23   1446   10.3      0      0    0.0   16.9| da8
    2     22     22   1383   13.0      0      0    0.0   19.6| da9
    0     79     79   1862    0.3      0      0    0.0    1.9| da10
    0     77     77   2043    2.3      0      0    0.0   12.3| da11
    1    115    115  14709    8.5      0      0    0.0   98.0|
zvol/morsleben-grube2/test
===========================================================================



How can a ZVOL be 100% busy although its underlying drives have plenty of power
left?



TIA and BR,
Nils

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