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.Dd Dec 16 2024
.Dt DCDR 9
.Os Hyra
.Sh NAME
.Nm dcdr - drive cache descriptor ring
.Sh SYNOPSIS
.In dev/dcdr/cache.h

.Ft struct dcdr *
.Fn dcdr_alloc "size_t bsize" "size_t cap"
.Ft struct dcd *
.Fn dcdr_cachein "struct dcdr *dcdr" "void *block" "off_t lba"

.Ft struct dcd *
.Fn dcdr_lbc_cachein "struct dcdr *dcdr" "void *block" "off_t lba"
.Ft int
.Fn dcdr_lookup "struct dcdr *dcdr" "off_t lba" "struct dcdr_lookup *res"
.Ft int
.Fn dcdr_invldcd "struct dcdr *dcdr" "off_t lba"

.Sh DESCRIPTION
The Drive Cache Descriptor Ring (DCDR) framework is used for
caching of read/write operations for LBA-based storage media
(e.g., certain SSDs and HDDs).

The
.Ft dcd
(drive cache descriptor) structure describes
a single logical block within some storage media.

The
.Ft dcdr
structure describes a drive cache descriptor ring (DCDR). When a
logical block is cached, a drive cache descriptor (DCD)
is created and added to the DCDR.

The
.Ft dcdr_alloc
function allocates and returns a new DCDR. The size
of each logical block is denoted by
.Fa bsize .
And the maximum capacity of the DCDR is denoted by
.Fa cap .

The
.Ft dcdr_cachein
function caches a logical block into a DCDR pointed to by
.Fa dcdr .
The
.Fa block
argument points to a buffer holding the data contained within
some logical block. It is recommended for this pointer to be aligned
by the storage device's block size.
The
.Fa lba
argument is the Logical Block Address (LBA) this data exists at
within the storage device.

DCDR implements a concept called Logical Block Coalescing (LBC)
which is a method to optimize caching by combining a pair
of adjacent logical blocks into a single DCD. In other words,
two consecutive logical blocks can be represented
by a single DCD. To cache a block with LBC, the
.Ft dcdr_lbc_cachein
function can be used.

The
.Fa dcdr
argument points to the target DCDR.

The
.Fa block
argument points to the first logical block. This creates
a DCD that describes the first and next logical block.

The
.Fa lba
argument is the LBA this data exists at within the storage
device.

The
.Ft dcdr_lbc_cacheinp
function searches the DCDR for a logical block
at the LBA denoted by
.Fa lba .
The result is returned in a structure
pointed to by
.Fa res .

The
.Ft dcdr_invldcd
function invalidates a single DCD from a
DCDR pointed to by
.Fa dcdr .
The
.Fa lba
argument denotes the Logical Block Address to
be searched for.

.Sh AUTHORS
.An Ian Moffett Aq Mt ian@osmora.org