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Update chunks.txt #32

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Update chunks.txt #32

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17 changes: 10 additions & 7 deletions chunks.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -72,21 +72,24 @@ block_group_offset = 4596957184 - 4593811456 = 3145728 => 3m
Btrfs actually uses the word "stripe" for 2 different types of stripe. The first
is the already introduced physical stripe. The second type stripe is the logical
stripe. Logical stripe is the granularity at which writes are mapped in btrfs,
that's generally io_width which is currently hard-coded at 64k. To calculate
that's generally io_width which is currently hard-coded at 65536 (64kb). To calculate
the logical stripe the block_group_offset is divided by logical stripe size :

logical_stripe = [block group offset ] / [logical stripe size]
logical_stripe = 3145728 / 64k = 48
logical_stripe = 3145728 / 65536 = 48
offset = [block group offset ] mod [logical stripe size]
offset = 3145728 % 65536 = 0

Aimed with those 2 variables it's possible to calculate the physical address
by:

physical_address = [physical stripe start] + [logical_stripe] * [logical stripe_size]
physical_address = 1351614464 + 48 * 64k = 1351614464 + 3145728 = 1354760192
physical_address = [physical stripe start] + [logical_stripe] * [logical stripe_size] + [offset]
physical_address_dev_1 = 2425356288 + 48 * 65536 + 0 = 2428502016
physical_address_dev_2 = 2446327808 + 48 * 65536 + 0 = 2449473536

So logical address 4596957184 corresponds to physical address 1354760192 for
that particular device. For the other chunk since the base address is different
naturally the end result is also going to be different.
So logical address 4596957184 corresponds to physical address 2428502016 for device 1, and
physical address 2449473536 for device 2 (because of RAID1 config). For the other chunk since
the base address is different naturally the end result is also going to be different.

The important thing to consider is "stripe" refers to more than one thing in
btrfs parlance, namely it could refer to a physical stripe - the physical chunk,
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