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COW file size: better explain the exception size

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The exception size was hardcoded and its explanation hidden in one of
the comments.
Move it to a separate constant e better explain why that is 64 * 2 / 8.

Bug: 176972301
Test: m
Signed-off-by: Alessio Balsini <balsini@google.com>
Change-Id: Ifcb527540882222916ada07dacf3f76f87609539
This commit is contained in:
Alessio Balsini 2021-02-16 11:00:52 +00:00
parent aab166132a
commit 2f9a4ff57d
1 changed files with 14 additions and 10 deletions
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24
fs_mgr/libsnapshot/dm_snapshot_internals.h
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@ -28,7 +28,7 @@ class DmSnapCowSizeCalculator {
DmSnapCowSizeCalculator(unsigned int sector_bytes, unsigned int chunk_sectors)
: sector_bytes_(sector_bytes),
chunk_sectors_(chunk_sectors),
exceptions_per_chunk(chunk_sectors_ * sector_bytes_ / (64 * 2 / 8)) {}
exceptions_per_chunk(chunk_sectors_ * sector_bytes_ / exception_size_bytes) {}
void WriteByte(uint64_t address) { WriteSector(address / sector_bytes_); }
void WriteSector(uint64_t sector) { WriteChunk(sector / chunk_sectors_); }
@ -124,18 +124,22 @@ class DmSnapCowSizeCalculator {
const uint64_t chunk_sectors_;
/*
* The COW device stores tables to map the modified chunks. Each table
* has the size of exactly 1 chunk.
* Each row of the table (also called exception in the kernel) contains two
* 64 bit indices to identify the corresponding chunk, and this 128 bit row
* size is a constant.
* The number of exceptions that each table can contain determines the
* number of data chunks that separate two consecutive tables. This value
* is then fundamental to compute the space overhead introduced by the
* tables in COW devices.
* The COW device stores tables to map the modified chunks. Each table has
* the size of exactly 1 chunk.
* Each entry of the table is called exception and the number of exceptions
* that each table can contain determines the number of data chunks that
* separate two consecutive tables. This value is then fundamental to
* compute the space overhead introduced by the tables in COW devices.
*/
const uint64_t exceptions_per_chunk;
/*
* Each row of the table (called exception in the kernel) contains two
* 64 bit indices to identify the corresponding chunk, and this 128 bit
* pair is constant in size.
*/
static constexpr unsigned int exception_size_bytes = 64 * 2 / 8;
/*
* Validity check for the container.
* It may happen that the caller attempts the write of an invalid chunk