Configure Software RAID 1 in Centos
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@Lakshmana said:
@scottalanmiller Ok Sir.But my manager said that he needs to see the data to be present in the individual hard disk so only trying to see the data in individual disk.Why this cannot be done.Can you please say
a RAID is not a backup. It's not meant to be used in this manner,
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@Lakshmana said:
Why this cannot be done.
Because you are breaking the RAID system. RAID is never meant to be torn apart and used in parts. The array is a singular storage system. When you break it apart not everything is there, and worse, the system now has failed components and needs to repair. Even if you found what you needed, now things are broken and files all have to be copied again.
It's like Schrodinger's cat. If you open the box, you ruin everything.
You need to use the RAID tools and let RAID do its job. If he doesn't trust the RAID, why are you using it?
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@thecreativeone91 Whether that method is not possible with the one hard disk Sir.You said that the data loss may be occured but I tried the process by booting sda hard disk alone in rescue mode and seen the data with out any loss in the shell mode.
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@Lakshmana said:
@thecreativeone91 Whether that method is not possible with the one hard disk Sir.You said that the data loss may be occured but I tried the process by booting sda hard disk alone in rescue mode and seen the data with out any loss in the shell mode.
The data loss is not at this time, it is when you go to use the system again. What is the purpose of a storage system that you "break" to verify? That doesn't work. How will you use this once it is in production?
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What was the point of this? if the raid failed you just put a new one in and rebuild it. The RAID Controller/Software will report when a raid is degraded, there is no need to "check it" manually.
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@thecreativeone91 said:
What was the point of this? if the raid failed you just put a new one in and rebuild it. The RAID Controller/Software will report when a raid is degraded, there is no need to "check it" manually.
"Checking it" is what will break things, too. It's just pointless, it is actually reckless and self defeating. If you want to ensure that the data is good, you can't do it this way.
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@scottalanmiller said:
you can't do it this way.
Ok Sir.I will explain this to my Manager and after that I acknowledge you sir
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@scottalanmiller
Sir Please refer this image one person said me that RAID 1 member can be boooted by installing bootloader.Whether it is possible Sir.I don't know about that one small doubt came after this image seen -
Yes, in case of a broken RAID array you can install a bootloader, of course. But that doesn't change that what you are doing should not be done. This is for cases where the other drive is broken, not when a manager asks you to damage the system.
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@scottalanmiller Ok
I understood.I went to Manager said that it cannot be done and the RAID configured device will lost the data and this is impossible.But he ask the explanation behind this and said to explain briefly -
I'm a bit lost - is @Lakshmana using hardware RAID or software RAID inside CentOS?
If it's hardware RAID, and he pulls a drive, won't the system always work? because both drives are suppose to be identical?
If it's software RAID, then I understand why he has failures.
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@Dashrender Its Sofware RAID1
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@Lakshmana said:
@scottalanmiller Ok
I understood.I went to Manager said that it cannot be done and the RAID configured device will lost the data and this is impossible.But he ask the explanation behind this and said to explain brieflyIt's not that it is impossible, of course it can be done, the problem is is that you break the RAID device so the system is broken. So you are "checking" data that is no longer valid. You can verify that the RAID was working, but you've broken that RAID device and are starting over. So there was no value in checking it because now you have to rebuild it and.... check again? The act of checking it breaks the array.
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@Dashrender said:
I'm a bit lost - is @Lakshmana using hardware RAID or software RAID inside CentOS?
Linux MD RAID.
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No wonder pulling a drive causes this to fail.
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@Dashrender said:
If it's hardware RAID, and he pulls a drive, won't the system always work? because both drives are suppose to be identical?
It's not hardware but hot swap that enables this. You can easily do this with MD RAID too, if done correctly. The problem is, the moment that you do this, the drive that you pull is out of date and no longer a valid member of the array. So anything that you "check" is now ruined AND you are running without RAID until you replace the drive and it rebuilds. So the thing that you check gets blown away the instant that you check it.
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@scottalanmiller said:
@Dashrender said:
If it's hardware RAID, and he pulls a drive, won't the system always work? because both drives are suppose to be identical?
It's not hardware but hot swap that enables this. You can easily do this with MD RAID too, if done correctly. The problem is, the moment that you do this, the drive that you pull is out of date and no longer a valid member of the array. So anything that you "check" is now ruined AND you are running without RAID until you replace the drive and it rebuilds. So the thing that you check gets blown away the instant that you check it.
I completely understand that - and maybe @Lakshmana does as well. So the act of testing this only proves the system will continue to function while the system is running, but probably won't survive if you reboot the system or power it down and back up. And once you put reattach a drive, even if it's the original one you pulled for the test, it has to be completely rebuilt. But pulling the drive does prove if the system is working or not.
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@Dashrender said:
@scottalanmiller said:
@Dashrender said:
If it's hardware RAID, and he pulls a drive, won't the system always work? because both drives are suppose to be identical?
It's not hardware but hot swap that enables this. You can easily do this with MD RAID too, if done correctly. The problem is, the moment that you do this, the drive that you pull is out of date and no longer a valid member of the array. So anything that you "check" is now ruined AND you are running without RAID until you replace the drive and it rebuilds. So the thing that you check gets blown away the instant that you check it.
I completely understand that - and maybe @Lakshmana does as well. So the act of testing this only proves the system will continue to function while the system is running, but probably won't survive if you reboot the system or power it down and back up. And once you put reattach a drive, even if it's the original one you pulled for the test, it has to be completely rebuilt. But pulling the drive does prove if the system is working or not.
No, it proves that the system was working. It also proves that the is not working anymore. The act of pulling the drive breaks the array. So yes, you can verify that you used to have things right. But it puts you into a degraded state and you now have to get the system to repair itself. Presumably, you'd want to check that too.... and the cycle of never having a working array begins. It is literally Schrodinger's cat. The act of observation changes the system.
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@DashrenderWhen I reboot with two hard disk(sda,sdb) there is no issue.When I connect a new hard dis(sdc) after removing sdb I check the hard disk separately the ssue came as kernel panic
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@Lakshmana said:
@DashrenderWhen I reboot with two hard disk(sda,sdb) there is no issue.When I connect a new hard dis(sdc) after removing sdb I check the hard disk separately the ssue came as kernel panic
That's likely because the one that was removed was the one with the bootloader on it. So replacing the bootloader is needed.
In theory, you can also keep the bootloader on another device, like a USB stick.
