zfs cache file regenerate

The /etc/zfs/zpool.cache file

Whenever a pool is imported on the system it will be added to the /etc/zfs/zpool.cache file. This file stores pool configuration information, such as the device names and pool state. If this file exists when running the zpool import command then it will be used to determine the list of pools available for import. When a pool is not listed in the cache file it will need to be detected and imported using the zpool import -d /dev/disk/by-id command.

Generating a new /etc/zfs/zpool.cache file

The /etc/zfs/zpool.cache file will be automatically updated when your pool configuration is changed. However, if for some reason it becomes stale you can force the generation of a new /etc/zfs/zpool.cache file by setting the cachefile property on the pool.

$ zpool set cachefile=/etc/zfs/zpool.cache tank

Conversely the cache file can be disabled by setting cachefile=none. This is useful for failover configurations where the pool should always be explicitly imported by the failover software.

$ zpool set cachefile=none tank

Openssh 7+ ssh-dsa error fixed

The solution is to add the following line to ~/.ssh/config on every client machine (every machine where you run the SSH client):

PubkeyAcceptedKeyTypes=+ssh-dss

If the server is using OpenSSH 7.0 or newer, you’ll also need to add this line to /etc/ssh/sshd_config on each server machine.

How Arp works in Linux

 

To understand how ARP translates IP addresses to MAC addresses, consider the following example. Assume host A has an IP address of 192.0.2.5/24 and a MAC address of fc:99:47:49:d4:a0, and wants to send a packet to host B with an IP address of 192.0.2.7. Note that the network number is the same for both hosts, so host A is able to send frames directly to host B.

The first time host A attempts to communicate with host B, the destination MAC address is not known. Host A makes an ARP request to the local network. The request is a broadcast with a message like this:

To: everybody (ff:ff:ff:ff:ff:ff). I am looking for the computer who has IP address 192.0.2.7. Signed: MAC address fc:99:47:49:d4:a0.

Host B responds with a response like this:

To: fc:99:47:49:d4:a0. I have IP address 192.0.2.7. Signed: MAC address 54:78:1a:86:00:a5.

Host A then sends Ethernet frames to host B.

You can initiate an ARP request manually using the arping command. For example, to send an ARP request to IP address 192.0.2.132:

$ arping -I eth0 192.0.2.132
ARPING 192.0.2.132 from 192.0.2.131 eth0
Unicast reply from 192.0.2.132 [54:78:1A:86:1C:0B]  0.670ms
Unicast reply from 192.0.2.132 [54:78:1A:86:1C:0B]  0.722ms
Unicast reply from 192.0.2.132 [54:78:1A:86:1C:0B]  0.723ms
Sent 3 probes (1 broadcast(s))
Received 3 response(s)

To reduce the number of ARP requests, operating systems maintain an ARP cache that contains the mappings of IP addresses to MAC address. On a Linux machine, you can view the contents of the ARP cache by using the arp command:

$ arp -n
Address                  HWtype  HWaddress           Flags Mask            Iface
192.0.2.3                ether   52:54:00:12:35:03   C                     eth0
192.0.2.2                ether   52:54:00:12:35:02   C                     eth0

The meanings of SR、LRM、LR、ER and ZR

SR、LRM、LR、ER、ZR are terms used in fiber optic communications, stand for the transmission distance of the 10G modules.

SR stands for Short Range, these transceivers support link length of 300m over multi-mode fiber and use 850nm lasers.

LRM means Long Reach Multimode, these transceivers support distance up to 220m over multi-mode fiber and use 1310nm lasers.

LR means Long Reach, these transceivers support distance up to 10km over single mode fiber and use 1310nm lasers.

ER means Extended Reach, the data rate of these transceivers support distance up to 40km over single mode fiber and use 1550nm lasers.

ZR also stands for Extended Reach which can transmit 10G data rate and 80km distance over single mode fiber and use 1550nm lasers.

Problem with SSD corruption on power failure .. flactuation

only enterprise-class SSDs can be relied upon (at all) for safe behavior on power fail.

The enterprise-class SSDs have super-capacitors that store enough power to write all data stored in the RAM within the SSD on power fail, and vendors charge three times as much as they do for consumer class SSDs.

Some vendors do certify their SSDs, but you should check them under real power-fail conditions, like pulling the plug.

It is better to use an Arduino with a relay for experimenting with cutting off power.