Tag: error

iSCSI service failed after do-release-update

iSCSI service failed after do-release-update

The issue was caused by missing iscsi driver.


  • iscsid.service

Got following error

Oct 28 16:45:03 ubuntupi iscsid[6813]: can not create NETLINK_ISCSI socket [Protocol not supported]
  • iscsid.socket
     Active: failed (Result: service-start-limit-hit) since Thu 2021-10-28 16:40:07 +08; 1min 30s ago
  • depmod -a

Don't have iscsi module loaded

  • Error in dmesg
open-iscsi.service - Login to default iSCSI targets

    Process: 2333 ExecStart=/sbin/iscsiadm -m node --loginall=automatic (code=exited, status=21)

Disable CentOS 8 Load Kernel Modules ipmi_si No such device error

Disable CentOS Load Kernel Modules ipmi_si No such device error

After CentOS 8 boots up, following error appeared when starting systemd-modules-load.service in /var/log/messages

Mar  1 15:48:40 centos kernel: ipmi_si: IPMI System Interface driver
Mar  1 15:48:40 centos kernel: ipmi_si: Unable to find any System Interface(s)
Mar  1 15:48:40 centos systemd-modules-load[561]: Failed to insert 'ipmi_si': No such device


The module ipmi_si is designed for physical servers with a remote control interface - an IPMI, and the CentOS 8 is running in a VM.


Create /etc/modprobe.d/blacklist-ipmi.conf file with following lines,

blacklist ipmi_si
blacklist ipmi_devintf
blacklist ipmi_msghandler
blacklist ipmi_ssif
blacklist ipmi_watchdog
blacklist ipmi_poweroff
blacklist acpi_ipmi
blacklist ibmaem
blacklist ibmpex


Boot fails after update ipmi_si No such device

Unable to show login page for NextCloud

Unable to show login page for NextCloud

NextCloud shows Internal Server Error on first page.


In docker log, shows following message

Class 'OCP\\User' not found

and it points to following file


Warning before

There was an warning message pointing to epubreader app, shows untruested source, but after I reinstalled it, the message disappered.


Access docker

docker exec -it nextcloudpi bash

Move the epubreader app to /tmp directory.

mv /data/nextcloud/apps/epubreader/ /tmp/

Restart NextCloudPi docker container


After fixed, if try to install EPUB/CBZ/PDF ebook reader app again, following message appears.

Error: This app cannot be enabled because it makes the server unstable

If force install it, NextCloud will appear Internal Server Error again.

NextCloud menu display incorrectly

NextCloud menu display incorrectly

A strange error happens in some browsers, NextCloud menu become a big page vertically, and one password textbox there requesting password, but the password is not working.

Not specific OS or browser

In my iMac, Safari is not working. In my one Windows machine, Firefox is not working. By the way, the Firefox has sync enabled and it is working in another PC. Google Chrome and Microsoft Edge have no issue.


There is programming error, the contents were downloaded from following website directly, not the NextCloud server. Browser blocked them.



Disable "Theming App" in NextCloud Apps screen.


NextCloud page not displaying correctly

Permission error when run Fusion VM

Permission error when start Fusion VM


When trying to start a newly created virtual machine in VMware Fusion, following error occurred

Error: Could not open /dev/vmmon...

This is because Fusion has no permssion to open device file required.


  • Navigate to System Preferences > Security & Privacy on the host macOS (High Sierra, Mojave and Catalina).
  • Under the General tab towards the bottom of the window, you see error similar to following message with option to click on “Allow”:
System software from vendor "VMware, Inc." was blocked from loading.
  • Click Allow.
  • Restart Fusion, and start virtual machine again.
  • If some other permission errors occurr again, repeat above steps.


Error: Could not open /dev/vmmon: Broken pipe, while launching the Virtual Machine (80467)

ZFS Concept

ZFS Concept


ZFS pool (Zpool) is a collection of one or more virtual devices (vdevs), vdev is a group of physical disks. They have following facts.

  • The redundancy level for vdevs can be a single drive, mirror, RAID-Z1, RAID-Z2, and RAID-Z3.
  • After creating a Zpool, it may not be possible to add additional disks to the vdev except mirrors.
  • Add additional vdevs to expand the Zpool is possible.
  • The storage space allocated to the Zpool cannot be decreased.
  • The drives in vdevs that are parts of the Zpool can be exchanged.

If there is a need to change the layout of the Zpool, the data should be backed up and the Zpool destroyed.


Datasets is the space emulating a regular file system.

Datasets can be nested, which can possess different settings for snapshots, compression, deduplication and so on.


Volumes (zvols) is the space emulating a block devices.

Data Integrity

No overwritten

The copy-on-write mechanism is to keep old data on the disk.


Checksum information is written when data is written into disk, then verified when read data from disk. When checksum mismatch detected, use redundant data is used for correction.

Different checksum algorithms are used

  • Fletcher-based checksum
  • SHA-256 hash


  • Single - Zpool has a vdev consisting of a single disk, similar to RAID0.
  • Mirror – similar to RAID1.
  • RAIDZ1 – similar to RAID5 but without the write hole issue.
  • RAIDZ2 – similar to RAID6, with 2 disks redundancy.
  • RAIDZ3 – similar to RAID6, with 3 disks redundancy.

RAID write hole in a RAID5/RAID1 occurs when one of the member disks doesn't match the others and by the nature of single-redundant RAID5/RAID1 it is impossible to tell which of the disks is bad.


Checksum mismatch

ZFS is a self-healing system. If mismatched checksum is detected, ZFS tries to retrieve the data from other disks. If data correct, the system will amend the incorrect data and checksum.

Disk failure

If a disk in a Zpool fails, the pool is set to the degraded state, then data on the failed device is calculated and written to first the spare disk replaces the failed one. This is called resilvering. Once the restoration operation is complete, the status of the Zpool changes back to online. In case of when multiple disks have failed and if there are not enough redundant devices, the Zpool changes its state into unavailable.

Migrate to different system

In old system, export zpool, which unmounts Zpool’s datasets or zvols.

In new system, import zpool, which mount Zpool's datasets or zvols.



The scrubbing is consistency check operation, and try to repair corrupted data.

No defragmentation

There is no online defragmentation in ZFS, so try to keep zpools below 70% utilization instead.


On ZFS, the data changes are stored on a different location than the original location on a disk and then the metadata is updated in that place on the disk. This mechanism guarantees that the old data is safely preserved in case of power loss or system crash that in other cases would result in loss of data.


The snapshot contains information about the original version of the file system to be retained. Snapshots do not require additional disk space within the pool. Once the data rendered in a snapshot is modified, the snapshot will take the disk space since it will now be pointing to the old data.


The clone is a writeable version of a snapshot. Overwriting the blocks in the cloned volume or file system results in decrementing the reference count on the previous block. The original snapshot that the clone is depending on, can not be deleted.


Rollback command is to go back to a previous version of a dataset or a volume. Note that the rollback command cannot revert changes from other snapshots than the most recent one. If to do so, all intermediate snapshots will be automatically destroyed.


Promote command is to replace an existing volume with its clone.


ZFS Essentials – What is pooled storage?
ZFS Essentials – Copy-on-write & snapshots
ZFS Essentials – Data integrity & RAIDZ
RAID Recovery Guide