The ZFS Pool on my server was showing degraded state. After checking the SMART status of the constituent drives and finding no problem, I discovered that there’s a bug in Solaris 10.5 where the system reports a growing number of errors and eventually fails the pool. dmesg shows an error unable to kmem_alloc enough memory for scatter/gather list, however, there is actually nothing wrong with the pool. Running zpool status shows degraded state:

root@fs:~# zpool status
  pool: rpool
 state: ONLINE
  scan: none requested
config:

        NAME        STATE     READ WRITE CKSUM      CAP            Product /Disks     IOstat mess          SN/LUN
        rpool       ONLINE       0     0     0
          c1t0d0    ONLINE       0     0     0      32.2 GB        VMware Virtual S   S:5 H:25 T:0         000000000000000

errors: No known data errors

  pool: tank
 state: DEGRADED
status: One or more devices has experienced an unrecoverable error.  An
        attempt was made to correct the error.  Applications are unaffected.
action: Determine if the device needs to be replaced, and clear the errors
        using 'zpool clear' or replace the device with 'zpool replace'.
   see: http://illumos.org/msg/ZFS-8000-9P
  scan: scrub repaired 0 in 12h15m with 0 errors on Fri Dec 21 00:08:43 2020
config:

        NAME                       STATE     READ WRITE CKSUM      CAP            Product /Disks     IOstat mess          SN/LUN
        tank                       DEGRADED     0     0     0
          raidz1-0                 DEGRADED     0     0     0
            c0t50014EE20BF0750Dd0  ONLINE       0     0     0      4 TB           WDC WD40EFRX-68W   S:0 H:0 T:0          WDWCC4E6NAXVAS
            c0t50014EE263348A3Ed0  ONLINE       0     0     0      4 TB           WDC WD40EFRX-68W   S:0 H:0 T:0          WDWCC4E0FRRRRP
            c0t50014EE2B69D2D68d0  DEGRADED     0     0    20  too many errors      4 TB           WDC WD40EFRX-68W   S:0 H:0 T:0          WDWCC4E3AN2Y99

errors: No known data errors

Running zpool clear recovers the pool:

root@fs:~# zpool clear
root@fs:~# zpool status    
  pool: rpool
 state: ONLINE
  scan: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        rpool       ONLINE       0     0     0
          c1t0d0    ONLINE       0     0     0

errors: No known data errors

  pool: tank
 state: ONLINE
status: One or more devices has experienced an unrecoverable error.  An
        attempt was made to correct the error.  Applications are unaffected.
action: Determine if the device needs to be replaced, and clear the errors
        using 'zpool clear' or replace the device with 'zpool replace'.
   see: http://illumos.org/msg/ZFS-8000-9P
  scan: none requested
config:

        NAME                       STATE     READ WRITE CKSUM
        tank                       ONLINE       0     0     0
          raidz1-0                 ONLINE       0     0     0
            c0t50014EE20BF0750Dd0  ONLINE       0     0     2
            c0t50014EE263348A3Ed0  ONLINE       0     0     0
            c0t50014EE2B69D2D68d0  ONLINE       0     0     0