Originally posted by npwx
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Btrfs Working On RAID1 Round-Robin Read Balancing
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Originally posted by pWe00Iri3e7Z9lHOX2Qx View Post
Btrfs should never have used the term "RAID", because those levels actually mean something to most people. The write or in this case read strategies are often different than what you would assume based on the RAID level. Like the write strategy for RAID 10 reordering disks meaning you are basically guaranteed to lose data when any 2nd disk fails, compared to a traditional RAID 10 where you are only screwed if the 2nd failed disk is from the same mirror. So you go from a 33% chance of losing data in a traditional four disk RAID 10 second failed disk scenario to a 100% chance with Btrfs RAID 10. And this isn't obvious from the documentation. Fun right?
Btrfs is designed to be as flexible as possible with maximizing storage / mixing disks / changing "RAID" levels. That can be great if that's the most important thing to you. But there are a slew of reasons that many people put up with the PITA that comes with out of tree modules and use ZFS.
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Originally posted by pWe00Iri3e7Z9lHOX2Qx View Post
Btrfs should never have used the term "RAID", because those levels actually mean something to most people. The write or in this case read strategies are often different than what you would assume based on the RAID level. Like the write strategy for RAID 10 reordering disks meaning you are basically guaranteed to lose data when any 2nd disk fails, compared to a traditional RAID 10 where you are only screwed if the 2nd failed disk is from the same mirror. So you go from a 33% chance of losing data in a traditional four disk RAID 10 second failed disk scenario to a 100% chance with Btrfs RAID 10. And this isn't obvious from the documentation. Fun right?
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Originally posted by Serafean View Post
I'd say in typical risk management, a 33% chance is still considered as being screwed if the situation arises, so going 100% chance doesn't seem to be a bad tradeoff for more flexibility.
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Originally posted by pWe00Iri3e7Z9lHOX2Qx View PostSo you go from a 33% chance of losing data in a traditional four disk RAID 10 second failed disk scenario to a 100% chance with Btrfs RAID 10. And this isn't obvious from the documentation. Fun right?
Anyway btrfs is much better than "classical" RAID when it comes to preventing losing data.
In a normal RAID1 (or 10), If one of the copies of the data gets corrupted, the data is lost​ (because you cannot tell which copy is the the good one).
In btrfs, being it checksummed, you can detect and fix the corrupted copy.
With a classic raid 1 you have double the chance of data corruption compared to a single device.
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