Shrinking raid array
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I'm working on shrinking a raid array and need some clarification on where the metadata goes and how large it is. I have used -G with -Z and -z to reduce the used size of each disk, but the man page says you need some extra space at the end of the disk for the metadata ( format 1.0 ), but doesn't specify exactly how much. I need to know what the exact size of the reduced partition should be.
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Quoted 7 lines · Phillip Susi
I'm working on shrinking a raid array and need some clarification on where the metadata goes and how large it is. I have used -G with -Z and -z to reduce the used size of each disk, but the man page says you need some extra space at the end of the disk for the metadata ( format 1.0 ), but doesn't specify exactly how much. I need to know what the exact size of the reduced partition should be.
I just noticed that reducing the used dev size has not moved the metadata, so it is looking like it just isn't possible to shrink the md partition with metadata format 1.0?
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Quoted 1 line · Phillip Susi
I just noticed that reducing the used dev size has not moved the metadata, so it is looking like it just isn't possible to shrink the md partition with metadata format 1.0?
From "man mdadm"
"The different sub-versions store the superblock at different locations on the device, either at the end (for 1.0), at the start (for 1.1) or 4K from the start (for 1.2)"
I don't really see how it could be shrunk with a superblock at the end, then you wouldn't be able to find it next time...
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Mikael Abrahamsson email: [email protected]
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Quoted 6 lines · Mikael Abrahamsson
Quoted 3 lines · Phillip Susi
I just noticed that reducing the used dev size has not moved the metadata, so it is looking like it just isn't possible to shrink the md partition with metadata format 1.0?
Quoted 8 lines
From "man mdadm"
"The different sub-versions store the superblock at different locations on the device, either at the end (for 1.0), at the start (for 1.1) or 4K from the start (for 1.2)"
I don't really see how it could be shrunk with a superblock at the end, then you wouldn't be able to find it next time...
You move the superblock, then shrink the partition...
But it looks like mdadm doesn't move the superblock even though it no longer is using the full partition.
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Quoted 1 line · Phillip Susi
You move the superblock, then shrink the partition...
As far as I know, mdadm has no code to shrink partitions?
Quoted 2 lines
But it looks like mdadm doesn't move the superblock even though it no longer is using the full partition.
I think that is perfectly reasonable. Otherwise the superblock wouldn't be found next time if the partition wasn't resized.
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Mikael Abrahamsson email: [email protected]
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Quoted 5 lines · Mikael Abrahamsson
Quoted 1 line · Phillip Susi
You move the superblock, then shrink the partition...
As far as I know, mdadm has no code to shrink partitions?
Of course not, you would use fdisk or parted or whatever else to do that.
Quoted 5 lines
Quoted 2 lines
But it looks like mdadm doesn't move the superblock even though it no longer is using the full partition.
I think that is perfectly reasonable. Otherwise the superblock wouldn't be found next time if the partition wasn't resized.
Yes, but the way it is now the superblock won't be found if the partition *is* resized. What is the use of reducing the amount of the partition mdadm is using if you can't then shrink the partition itself?
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Quoted 1 line · Phillip Susi
Yes, but the way it is now the superblock won't be found if the partition *is* resized. What is the use of reducing the amount of the partition mdadm is using if you can't then shrink the partition itself?
It makes sense if you for some reason want to use a smaller component drive (thinking mostly of older drives where 36 or 73GB didn't mean an exact size between manufacturers and you happened to create the array on a larger drive than you now have available.
Anyhow, I can see that what you're looking for is useful as well but I also believe it should be done through a special option and I imagine it should also output information regarding where the partition should now end (to use in fdisk) so mdadm can find the superblock next time.
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Mikael Abrahamsson email: [email protected]
Quoted 1 line · Mikael Abrahamsson
Anyhow, I can see that what you're looking for is useful as well but I also believe it should be done through a special option and I imagine it should also output information regarding where the partition should now end (to use in fdisk) so mdadm can find the superblock next time.
Actually, in addition to this, I also believe that there should be some kind of superblock at the end of the current partition that points to the location of the "new" moved superblock, so that things work even if you do not change the partition size.
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Mikael Abrahamsson email: [email protected]