RAID superblock formats
From Linux Raid Wiki
Currently, the Linux RAID subsystem recognizes two distinct variant superblocks.
They are known as v0.9 and v1.x Superblock formats.
The v1.x Superblock format is as follows:
| Offset (Hex) | Offset (Dec) | Length (in bytes) |
Field Name | Usage/Meaning | Data Type | Data Value | Notes | ||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Section: Superblock/"Magic-Number" Identification area | |||||||||||||||||||||||||
| 16 Bytes, Offset 0-15 (0x00 - 0x0F) | |||||||||||||||||||||||||
| 0x00 - 0x03 | 0 - 3 | 4 | magic | "Magic Number" (Superblock ID) |
__u32 | 0xa92b4efc (little-endian) |
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| 0x04 - 0x07 | 4 - 7 | 4 | major_version | Major Version of the Superblock |
__u32 | 1 | |||||||||||||||||||
| 0x08 - 0x0B | 8 - 11 | 4 | feature_map | Feature Map | __u32 | 0 Bit-Mapped Field
|
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| 0x0C - 0x0F | 12 - 15 | 4 | pad0 | Padding Block 0 | __u32 | 0 | Always set to zero when writing |
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| Section: Per-Array Identification & Configuration area | |||||||||||||||||||||||||
| 48 Bytes, Offset 16-63 (0x10 - 0x3F) | |||||||||||||||||||||||||
| 0x10 - 0x1F | 16 - 31 | 16 | set_uuid | UUID for the Array(?) | __u8[16] | Set by user-space formatting util |
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| 0x20 - 0x3F | 32 - 63 | 32 | set_name | Name for the Array(?) | char[32] | Set and used by user-space utils |
Nt | ||||||||||||||||||
| 0x40 - 0x47 | 64 - 71 | 8 | ctime | Creation Time(?) | __u64 | low 40-bits are seconds high 24-bits are uSeconds |
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| 0x48 - 0x4B | 72 - 75 | 4 | level | RAID Level of the Array |
__u32 |
|
RAID-6 not mentioned in the mdadm source(?) |
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| 0x4C - 0x4F | 76 - 79 | 4 | layout | layout of array (RAID5 only) |
__u32 | (?) | Only used for RAID5 |
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| 0x50 - 0x57 | 80 - 87 | 8 | size | used-size of component devices | __u64 | size of component devices (in # of 512-byte sectors) |
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| 0x58 - 0x5B | 88 - 91 | 4 | chunksize | chunk-size of the array | __u32 | chunk-size of the array (in # of 512-byte sectors) |
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| 0x5C - 0x5F | 92 - 95 | 4 | raid_disks | (?)number of disks in array(?) | __u32 | # | |||||||||||||||||||
| 0x60 - 0x63 | 96 - 99 | 4 | bitmap_offset | # of sectors after superblock that bitmap starts (See note about signed value) |
__u32 | (signed) | This is only valid if feature_map[0] is set Signed value allows bitmap to appear before superblock on the disk |
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| Section: RAID-Reshape In-Process Metadata Storage/Recovery area | |||||||||||||||||||||||||
| (Only contains valid data if feature_map bit '4' is set) | |||||||||||||||||||||||||
| 64 Bytes, Offset 100-163 (0x64 - 0x7F) | |||||||||||||||||||||||||
| 0x64 - 0x67 | 100 - 103 | 4 | new_level | (Used with RESHAPE) the new RAID level being reshaped-to |
__u32 | See field named "level" above |
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| 0x68 - 0x6F | 104 - 111 | 8 | reshape_position | (Used with RESHAPE) Next address of the array to reshape |
__u64 | current position of the reshape operation | |||||||||||||||||||
| 0x70 - 0x73 | 112 - 115 | 4 | delta_disks | (Used with RESHAPE) this holds the change in # of raid disks |
__u32 | change in # of raid disks | |||||||||||||||||||
| 0x74 - 0x77 | 116 - 119 | 4 | new_layout | (Used with RESHAPE) |
__u32 | new layout for array | |||||||||||||||||||
| 0x78 - 0x7B | 120 - 123 | 4 | new_chunk | (Used with RESHAPE) |
__u32 | new chunk size (See ) |
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| 0x7C - 0x7F | 124 - 127 | 4 | pad1 | Padding Block #1 | __u8[4] | 0 | Always set to zero when writing |
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| Section: This-Member-Device Information area | |||||||||||||||||||||||||
| 64 Bytes, Offset 128-191 (0x80 - 0xbf) | |||||||||||||||||||||||||
| 0x80 - 0x87 | 128 - 135 | 8 | data_offset | the sector # upon which data starts | __u64 | sector # where data begins (Often 0) |
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| 0x88 - 0x8F | 136 - 143 | 8 | data_size | sectors in the device that are used for data |
__u64 | # of sectors that can be used for data | |||||||||||||||||||
| 0x90 - 0x97 | 144 - 151 | 8 | super_offset | # of the sector upon which this superblock starts |
__u64 | # of the sector upon which this superblock starts |
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| 0x98 - 0x9F | 152 - 159 | 8 | recovery_offset | sectors before this offset (from data_offset) have been recovered |
__u64 | sector # | |||||||||||||||||||
| 0xA0 - 0xA3 | 160 - 163 | 4 | dev_number | Fm | __u32 | Permanent identifier of this device (Not its role in RAID(?)) | |||||||||||||||||||
| 0xA4 - 0xA7 | 164 - 167 | 4 | cnt_corrected_read | Number of read-errors that were corrected by re-writing | __u32 | Dv | |||||||||||||||||||
| 0xA8 - 0xB7 | 168 - 183 | 16 | device_uuid | UUID of the member-device | __u8[16] | Set by User-Space Ignored by kernel |
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| 0xB8 | 184 | 1 | devflags | Per-Device Flags (Bit-Mapped Field) |
__u8 | Bit-Mapped Field
|
WriteMostly1 indicates that this device should only be updated on writes, not read from. (Useful with slow devices in RAID1 arrays?) | ||||||||||||||||||
| 0xB9 - 0xBF | 185 - 191 | 7 | pad2 | Padding block 2 | __u8[7] | 0 | Always set to zero when writing |
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| Section: Array-State Information area | |||||||||||||||||||||||||
| 64 Bytes, Offset 192-256 (0xC0 - 0xFF) | |||||||||||||||||||||||||
| 0xC0 - 0xC7 | 192 - 199 | 8 | utime | Fm | __u64 | low 40-bits are seconds high 24-bits are uSeconds |
Nt | ||||||||||||||||||
| 0xC8 - 0xCF | 200 - 207 | 8 | events | Event Count for the Array | __u64 | # | Updated whenever the superblock is updated. Used by mdadm in re-assembly to detect failed/out-of-sync member-devices. |
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| 0xD0 - 0xD7 | 208 - 215 | 8 | resync_offset | Offsets before this one (starting from data_offset) are 'known' to be in sync. | __u64 | Dv | |||||||||||||||||||
| 0xD8 - 0xDB | 216 - 219 | 4 | sb_csum | Checksum of this superblock up to devs[max_dev] | __u32 | # | This value will be different for each member-device's superblock. | ||||||||||||||||||
| 0xDC - 0xDF | 220 - 223 | 4 | max_dev | How many devices are part of (or related to) the array | __u32 | # | |||||||||||||||||||
| 0xE0 - 0xFF | 224 - 255 | 32 | pad3 | Padding Block 3 | __u8[32] | 0 | Always set to zero when writing |
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| Remaining sections haven't been added to the wiki yet | |||||||||||||||||||||||||
| Oh | Od | Lb | Fn | Fm | Dt | Dv | Nt | ||||||||||||||||||
