sys/fs/efs/efs.h

   1 /*      $NetBSD: efs.h,v 1.1 2007/06/29 23:30:27 rumble Exp $   */
   2
   3 /*
   4  * Copyright (c) 2006 Stephen M. Rumble <rumble@ephemeral.org>
   5  *
   6  * Permission to use, copy, modify, and distribute this software for any
   7  * purpose with or without fee is hereby granted, provided that the above
   8  * copyright notice and this permission notice appear in all copies.
   9  *
  10  * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
  11  * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
  12  * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
  13  * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
  14  * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
  15  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
  16  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  17  */
  18
  19 /*
  20  * See IRIX efs(4)
  21  */
  22
  23 #ifndef _FS_EFS_EFS_H_
  24 #define _FS_EFS_EFS_H_
  25
  26 #define EFS_DEBUG
  27
  28 /*
  29  * SGI EFS - Extent File System
  30  *
  31  * The EFS filesystem is comprised of 512-byte sectors, or "basic blocks" (bb).
  32  * These blocks are divided into cylinder groups (cg), from which extents are
  33  * allocated. An extent is a contiguous region of blocks with minimal length
  34  * of 1 and maximal length of 248.
  35  *
  36  * The filesystem is limited to 8GB by struct efs_extent's ex_bn field, which
  37  * specifies an extent's offset in terms of basic blocks. Unfortunately, it was
  38  * squished into a bitfield and given only 24bits so we are left with
  39  * 2**24 * 512 bytes. Individual files are maximally 2GB, but not due to any
  40  * limitation of on-disk structures. All sizes and offsets are stored as block,
  41  * not byte values, with the exception of sb.sb_bmsize and efs_dinode.di_size.
  42  *
  43  * An EFS filesystem begins with the superblock (struct efs_sb) at bb offset 1
  44  * (offset 0 is reserved for bootblocks and other forms of contraband). The
  45  * superblock contains various parameters including magic, checksum, filesystem
  46  * size, number of cylinder groups, size of cylinder groups, and location of the
  47  * first cylinder group. A bitmap may begin at offset bb 2. This is true of
  48  * filesystems whose magic flag is EFS_MAGIC. However, the ability to grow an
  49  * efs filesystem was added in IRIX 3.3 and a grown efs's bitmap is located
  50  * toward the end of the disk, pointed to by sb.sb_bmblock. A grown filesystem
  51  * is detected with the EFS_NEWMAGIC flag. See below for more details and
  52  * differences.
  53  *
  54  * In order to promote inode and data locality, the disk is separated into
  55  * sb.sb_ncg cylinder groups, which consist of sb.sb_cgfsize blocks each.
  56  * The cylinder groups are laid out consecutively beginning from block offset
  57  * sb.sb_firstcg. The beginning of each cylinder group is comprised of
  58  * sb.sb_cgisize inodes (struct efs_dinode). The remaining space contains
  59  * file extents, which are preferentially allocated to files whose inodes are
  60  * within the same cylinder group.
  61  *
  62  * EFS increases I/O performance by storing files in contiguous chunks called
  63  * 'extents' (struct efs_extent). Extents are variably sized from 1 to 248
  64  * blocks, but please don't ask me why 256 isn't the limit.
  65  *
  66  * Each inode (struct efs_dinode) contains space for twelve extent descriptors,
  67  * allowing for up to 1,523,712 byte files (12 * 248 * 512) to be described
  68  * without indirection. When indirection is employed, each of the twelve
  69  * descriptors may reference extents that contain up to 248 more direct
  70  * descriptors. Since each descriptor is 8 bytes we could theoretically have
  71  * in total 15,872 * 12 direct descriptors, allowing for 15,872 * 12 * 248 *
  72  * 512 = ~22GB files. However, since ei_numextents is a signed 16-bit quantity,
  73  * we're limited to only 32767 indirect extents, which leaves us with a ~3.87GB
  74  * maximum file size. (Of course, with a maximum filesystem size of 8GB, such a
  75  * restriction isn't so bad.) Note that a single full indirect extent could
  76  * reference approximately 1.877GB of data, but SGI strikes again! Earlier
  77  * versions of IRIX (4.0.5H certainly, and perhaps prior) limit indirect
  78  * extents to 32 basic blocks worth. This caps the number of extents at 12 *
  79  * 32 * 64, permitting ~2.91GB files. SGI later raised this limit to 64 blocks
  80  * worth, which exceeds the range of ei_numextents and gives a maximum
  81  * theoretical file size of ~3.87GB. However, EFS purportedly only permits
  82  * files up to 2GB in length.
  83  *
  84  * The bitmap referred to by sb_bmsize and (optionally) sb_bmblock contains
  85  * data block allocation information. I haven't looked at this at all, nor
  86  * am I aware of how inode allocation is performed.
  87  *
  88  * An EFS disk layout looks like the following:
  89  *     ____________________________________________________________________
  90  *    | unused | superblock | bitmap | pad | cyl grp | ..cyl grps... | pad |
  91  *     --------------------------------------------------------------------
  92  * bb:     0          1         2          ^-sb.sb_firstcg      sb.sb_size-^
  93  *
  94  * A cylinder group looks like the following:
  95  *     ____________________________________________________________________
  96  *    |    inodes    |           ... extents and free space ...            |
  97  *     --------------------------------------------------------------------
  98  *           0       ^-(sb.sb_cgisize *                      sb.sb_cgfsize-^
  99  *                      sizeof(struct efs_dinode))
 100  *
 101  * So far as I am aware, EFS file systems have always been big endian, existing
 102  * on mips (and perhaps earlier on m68k) machines only. While mips chips are
 103  * bi-endian, I am unaware of any sgimips machine that was used in mipsel mode.
 104  *
 105  * See efs_sb.h, efs_dir.h, and efs_dinode.h for more information regarding
 106  * directory layout and on-disk inodes, and the superblock accordingly.
 107  */
 108
 109 /*
 110  * Basic blocks are always 512 bytes.
 111  */
 112 #define EFS_BB_SHFT     9
 113 #define EFS_BB_SIZE     (1 << EFS_BB_SHFT)
 114
 115 /*
 116  * EFS basic block layout:
 117  */
 118 #define EFS_BB_UNUSED   0       /* bb 0 is unused */
 119 #define EFS_BB_SB       1       /* bb 1 is superblock */
 120 #define EFS_BB_BITMAP   2       /* bb 2 is bitmap (unless moved by growfs) */
 121 /* bitmap continues, then padding up to first aligned cylinder group */
 122
 123 /*
 124  * basic block <-> byte conversions
 125  */
 126 #define EFS_BB2BY(_x)           ((_x) << EFS_BB_SHFT)
 127 #define EFS_BY2BB(_x)           (((_x) + EFS_BB_SIZE - 1) >> EFS_BB_SHFT)
 128
 129 /*
 130  * Struct efs_extent limits us to 24 bit offsets, therefore the maximum
 131  * efs.sb_size is 2**24 blocks (8GB).
 132  *
 133  * Trivia: IRIX's mkfs_efs(1M) has claimed the maximum to be 0xfffffe for years.
 134  */
 135 #define EFS_SIZE_MAX            0x01000000
 136
 137 #ifdef _KERNEL
 138
 139 #define VFSTOEFS(mp)    ((struct efs_mount *)(mp)->mnt_data)
 140
 141 /* debug goo */
 142 #ifdef DEBUG
 143 #define EFS_DEBUG
 144 #endif
 145 #ifdef EFS_DEBUG
 146 #define EFS_DPRINTF(_x) printf _x
 147 #else
 148 #define EFS_DPRINTF(_x)
 149 #endif
 150
 151 #endif
 152
 153 #endif /* !_FS_EFS_EFS_H_ */