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Merge pull request #51933 from aclamk/wip-aclamk-bs-onode-doc

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os/bluestore: Some in-code documentation for Onode / blob
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Adam Kupczyk 2023-07-25 14:06:02 +02:00 committed by GitHub
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src/os/bluestore/BlueStore.cc
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@ -3642,6 +3642,54 @@ BlueStore::BlobRef BlueStore::ExtentMap::split_blob(
}
// Onode
//
// Mapping blobs over Onode's logical offsets.
//
// Blob is always continous. Blobs may overlap.
// Non-mapped regions are "0" when read.
// 1 2 3
// 0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef
// <blob.a.blob.a><blob.b.blo> <blob.c.blob.c.blob.c.blob>
// <blob.d.blob.d.b> <blob.e.blob.e>
// blob.a starts at 0x0 length 0xe
// blob.b starts at 0xf length 0xb
// blob.c starts at 0x23 length 0x1b
// blob.d starts at 0x06 length 0x12
// blob.e starts at 0x2d length 0xf
//
// Blobs can have non-encoded parts:
// 1 2 3
// 0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef
// aaaaaa......aaabbbbb...bbbb ccccccccccccccc..........cc
// dddddd........ddd .....eeeeeeeeee
// "." - non-encoded parts of blob (holes)
//
// Mapping logical to blob:
// extent_map maps {Onode's logical offset, length}=>{Blob, in-blob offset}
// {0x0, 0x6}=>{blob.a, 0x0}
// {0x6, 0x6}=>{blob.d, 0x0}
// {0xc, 0x3}=>{blob.a, 0xc}
// {0xf, 0x5}=>{blob.b, 0x0}
// {0x14, 0x3}=>{blob.d, 0xe}
// {0x17, 0x4}=>{blob.b, 0x8}
// a hole here
// {0x23, 0xe}=>{blob.c, 0x0}
// and so on...
//
// Compressed blobs do not have non-encoded parts.
// Same example as above but all blobs are compressed:
// 1 2 3
// 0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef
// aaaaaaAAAAAAaaabbbbbBBBbbbb cccccccccccccccCCCCCCCCCCcc
// ddddddDDDDDDDDddd EEEEEeeeeeeeeee
// A-E: parts of blobs that are never used.
// This can happen when a compressed blob is overwritten partially.
// The target ranges are no longer used, but are left there because they are necessary
// for successful decompression.
//
// In compressed blobs PExtentVector and csum refer to actually occupied disk space.
// Blob's logical length is larger then occupied disk space.
// Mapping from extent_map always uses offsets of decompressed data.
#undef dout_prefix
#define dout_prefix *_dout << "bluestore.onode(" << this << ")." << __func__ << " "