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In retrospect, if you're looking at file systems as a means to prevent write holes with RAID 5/6 then you're going about it wrong. Write holes happen with every RAID level. They happen with RAID 5 and 6. They happen with RAID 1 and RAID 10. Do not believe anyone who says that write holes are unique to RAID 5/6 and their derivatives. They are mistaken. Any two or more storage devices in a RAID set that are not atomically locked together can suffer write holes. They can even happen with ZFS. This is not a RAID issue. RAID is about making the hardware tolerant to faults. RAID does not care about the integrity of your data. Write holes happen when power to the storage devices is lost during write operations. UPS and redundant power are the primary ways of preventing write holes. If the server doesn't lose power, or it has time to perform a graceful shutdown when mains fail, then no holes appear in the data it holds. Battery-backed cache is the second line of defense against write holes. The battery prevents cache loss if redundant and backup power fail. Non-volatile cache (SSD) is becoming a popular alternative to battery-backed cache, although flash has its own set of power-related problems. The last line of defense against corruption is a good backup history. ZFS and Btrfs will detect and if possible correct single-bit errors. They may be able to prevent write holes if they can reliably control every piece of I/O cache in the data stream. This includes the write acceleration cache found on most modern disks' on-board controllers. Not all of these reliably honor cache flush instructions from the host and because of this they cannot be relied upon to maintain data integrity under power fault conditions. -- Rich P.
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