1 /* 2 * Copyright (C) 2011, Google Inc. and others 3 * 4 * This program and the accompanying materials are made available under the 5 * terms of the Eclipse Distribution License v. 1.0 which is available at 6 * https://www.eclipse.org/org/documents/edl-v10.php. 7 * 8 * SPDX-License-Identifier: BSD-3-Clause 9 */ 10 11 package org.eclipse.jgit.internal.storage.dfs; 12 13 import java.io.IOException; 14 import java.nio.channels.ReadableByteChannel; 15 16 /** 17 * Readable random access byte channel from a file. 18 */ 19 public interface ReadableChannel extends ReadableByteChannel { 20 /** 21 * Get the current position of the channel. 22 * 23 * @return r current offset. 24 * @throws java.io.IOException 25 * the channel's current position cannot be obtained. 26 */ 27 long position() throws IOException; 28 29 /** 30 * Seek the current position of the channel to a new offset. 31 * 32 * @param newPosition 33 * position to move the channel to. The next read will start from 34 * here. This should be a multiple of the {@link #blockSize()}. 35 * @throws java.io.IOException 36 * the position cannot be updated. This may be because the 37 * channel only supports block aligned IO and the current 38 * position is not block aligned. 39 */ 40 void position(long newPosition) throws IOException; 41 42 /** 43 * Get the total size of the channel. 44 * <p> 45 * Prior to reading from a channel the size might not yet be known. 46 * Implementors may return -1 until after the first read method call. Once a 47 * read has been completed, the underlying file size should be available. 48 * 49 * @return r total size of the channel; -1 if not yet available. 50 * @throws java.io.IOException 51 * the size cannot be determined. 52 */ 53 long size() throws IOException; 54 55 /** 56 * Get the recommended alignment for reads. 57 * <p> 58 * Starting a read at multiples of the blockSize is more efficient than 59 * starting a read at any other position. If 0 or -1 the channel does not 60 * have any specific block size recommendation. 61 * <p> 62 * Channels should not recommend large block sizes. Sizes up to 1-4 MiB may 63 * be reasonable, but sizes above that may be horribly inefficient. The 64 * {@link org.eclipse.jgit.internal.storage.dfs.DfsBlockCache} favors the 65 * alignment suggested by the channel rather than the configured size under 66 * the assumption that reads are very expensive and the channel knows what 67 * size is best to access it with. 68 * 69 * @return recommended alignment size for randomly positioned reads. Does 70 * not need to be a power of 2. 71 */ 72 int blockSize(); 73 74 /** 75 * Recommend the channel maintain a read-ahead buffer. 76 * <p> 77 * A read-ahead buffer of approximately {@code bufferSize} in bytes may be 78 * allocated and used by the channel to smooth out latency for read. 79 * <p> 80 * Callers can continue to read in smaller than {@code bufferSize} chunks. 81 * With read-ahead buffering enabled read latency may fluctuate in a pattern 82 * of one slower read followed by {@code (bufferSize / readSize) - 1} fast 83 * reads satisfied by the read-ahead buffer. When summed up overall time to 84 * read the same contiguous range should be lower than if read-ahead was not 85 * enabled, as the implementation can combine reads to increase throughput. 86 * <p> 87 * To avoid unnecessary IO callers should only enable read-ahead if the 88 * majority of the channel will be accessed in order. 89 * <p> 90 * Implementations may chose to read-ahead using asynchronous APIs or 91 * background threads, or may simply aggregate reads using a buffer. 92 * <p> 93 * This read ahead stays in effect until the channel is closed or the buffer 94 * size is set to 0. 95 * 96 * @param bufferSize 97 * requested size of the read ahead buffer, in bytes. 98 * @throws java.io.IOException 99 * if the read ahead cannot be adjusted. 100 */ 101 void setReadAheadBytes(int bufferSize) throws IOException; 102 }