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| -rw-r--r-- | fs/btrfs/raid56.h | 70 |
1 files changed, 70 insertions, 0 deletions
diff --git a/fs/btrfs/raid56.h b/fs/btrfs/raid56.h index 42a45716fb03..3ed23303d7fa 100644 --- a/fs/btrfs/raid56.h +++ b/fs/btrfs/raid56.h @@ -24,6 +24,76 @@ enum btrfs_rbio_ops { BTRFS_RBIO_PARITY_SCRUB, }; +/* + * Overview of btrfs_raid_bio. + * + * One btrfs_raid_bio represents a full stripe of RAID56, including both data + * and P/Q stripes. For now, each data and P/Q stripe is of a fixed length (64K). + * + * One btrfs_raid_bio can have one or more bios from higher layer, covering + * part or all of the data stripes. + * + * [PAGES FROM HIGHER LAYER BIOS] + * Higher layer bios are in the btrfs_raid_bio::bio_list. + * + * Pages from the bio_list are represented like the following: + * + * bio_list: |<- Bio 1 ->| |<- Bio 2 ->| ... + * bio_paddrs: [0] [1] [2] [3] [4] [5] ... + * + * If there is a bio covering a sector (one btrfs fs block), the corresponding + * pointer in btrfs_raid_bio::bio_paddrs[] will point to the physical address + * (with the offset inside the page) of the corresponding bio. + * + * If there is no bio covering a sector, then btrfs_raid_bio::bio_paddrs[i] will + * be INVALID_PADDR. + * + * The length of each entry in bio_paddrs[] is sectorsize. + * + * [PAGES FOR INTERNAL USAGES] + * Pages not covered by any bio or belonging to P/Q stripes are stored in + * btrfs_raid_bio::stripe_pages[] and stripe_paddrs[], like the following: + * + * stripe_pages: |<- Page 0 ->|<- Page 1 ->| ... + * stripe_paddrs: [0] [1] [2] [3] [4] ... + * + * stripe_pages[] array stores all the pages covering the full stripe, including + * data and P/Q pages. + * stripe_pages[0] is the first page of the first data stripe. + * stripe_pages[BTRFS_STRIPE_LEN / PAGE_SIZE] is the first page of the second + * data stripe. + * + * Some pointers inside stripe_pages[] can be NULL, e.g. for a full stripe write + * (the bio covers all data stripes) there is no need to allocate pages for + * data stripes (can grab from bio_paddrs[]). + * + * If the corresponding page of stripe_paddrs[i] is not allocated, the value of + * stripe_paddrs[i] will be INVALID_PADDR. + * + * The length of each entry in stripe_paddrs[] is sectorsize. + * + * [LOCATING A SECTOR] + * To locate a sector for IO, we need the following info: + * + * - stripe_nr + * Starts from 0 (representing the first data stripe), ends at + * @nr_data (RAID5, P stripe) or @nr_data + 1 (RAID6, Q stripe). + * + * - sector_nr + * Starts from 0 (representing the first sector of the stripe), ends + * at BTRFS_STRIPE_LEN / sectorsize - 1. + * + * All existing bitmaps are based on sector numbers. + * + * - from which array + * Whether grabbing from stripe_paddrs[] (aka, internal pages) or from the + * bio_paddrs[] (aka, from the higher layer bios). + * + * For IO, a physical address is returned, so that we can extract the page and + * the offset inside the page for IO. + * A special value INVALID_PADDR represents when the physical address is invalid, + * normally meaning there is no page allocated for the specified sector. + */ struct btrfs_raid_bio { struct btrfs_io_context *bioc; |