linux/drivers/md/dm-vdo/indexer/geometry.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * Copyright 2023 Red Hat
 */

#include "geometry.h"

#include <linux/compiler.h>
#include <linux/log2.h>

#include "errors.h"
#include "logger.h"
#include "memory-alloc.h"
#include "permassert.h"

#include "delta-index.h"
#include "indexer.h"

/*
 * An index volume is divided into a fixed number of fixed-size chapters, each consisting of a
 * fixed number of fixed-size pages. The volume layout is defined by two constants and four
 * parameters. The constants are that index records are 32 bytes long (16-byte block name plus
 * 16-byte metadata) and that open chapter index hash slots are one byte long. The four parameters
 * are the number of bytes in a page, the number of record pages in a chapter, the number of
 * chapters in a volume, and the number of chapters that are sparse. From these parameters, we can
 * derive the rest of the layout and other index properties.
 *
 * The index volume is sized by its maximum memory footprint. For a dense index, the persistent
 * storage is about 10 times the size of the memory footprint. For a sparse index, the persistent
 * storage is about 100 times the size of the memory footprint.
 *
 * For a small index with a memory footprint less than 1GB, there are three possible memory
 * configurations: 0.25GB, 0.5GB and 0.75GB. The default geometry for each is 1024 index records
 * per 32 KB page, 1024 chapters per volume, and either 64, 128, or 192 record pages per chapter
 * (resulting in 6, 13, or 20 index pages per chapter) depending on the memory configuration. For
 * the VDO default of a 0.25 GB index, this yields a deduplication window of 256 GB using about 2.5
 * GB for the persistent storage and 256 MB of RAM.
 *
 * For a larger index with a memory footprint that is a multiple of 1 GB, the geometry is 1024
 * index records per 32 KB page, 256 record pages per chapter, 26 index pages per chapter, and 1024
 * chapters for every GB of memory footprint. For a 1 GB volume, this yields a deduplication window
 * of 1 TB using about 9GB of persistent storage and 1 GB of RAM.
 *
 * The above numbers hold for volumes which have no sparse chapters. A sparse volume has 10 times
 * as many chapters as the corresponding non-sparse volume, which provides 10 times the
 * deduplication window while using 10 times as much persistent storage as the equivalent
 * non-sparse volume with the same memory footprint.
 *
 * If the volume has been converted from a non-lvm format to an lvm volume, the number of chapters
 * per volume will have been reduced by one by eliminating physical chapter 0, and the virtual
 * chapter that formerly mapped to physical chapter 0 may be remapped to another physical chapter.
 * This remapping is expressed by storing which virtual chapter was remapped, and which physical
 * chapter it was moved to.
 */

int uds_make_index_geometry(size_t bytes_per_page, u32 record_pages_per_chapter,
			    u32 chapters_per_volume, u32 sparse_chapters_per_volume,
			    u64 remapped_virtual, u64 remapped_physical,
			    struct index_geometry **geometry_ptr)
{ … }

int uds_copy_index_geometry(struct index_geometry *source,
			    struct index_geometry **geometry_ptr)
{ … }

void uds_free_index_geometry(struct index_geometry *geometry)
{ … }

u32 __must_check uds_map_to_physical_chapter(const struct index_geometry *geometry,
					     u64 virtual_chapter)
{ … }

/* Check whether any sparse chapters are in use. */
bool uds_has_sparse_chapters(const struct index_geometry *geometry,
			     u64 oldest_virtual_chapter, u64 newest_virtual_chapter)
{ … }

bool uds_is_chapter_sparse(const struct index_geometry *geometry,
			   u64 oldest_virtual_chapter, u64 newest_virtual_chapter,
			   u64 virtual_chapter_number)
{ … }

/* Calculate how many chapters to expire after opening the newest chapter. */
u32 uds_chapters_to_expire(const struct index_geometry *geometry, u64 newest_chapter)
{ … }