docs: man: Add dcdr(9)

Signed-off-by: Ian Moffett <ian@osmora.org>

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+.\" Copyright (c) 2023-2024 Ian Marco Moffett and the Osmora Team.
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions are met:
+.\"
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+.\"    this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
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+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+.\" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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+.\" LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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+.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+.\" POSSIBILITY OF SUCH DAMAGE.
+.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