dcachecbio.c

FAT文件系统原代码

/* dcacheCbio.c - Disk Cache Driver */

/* Copyright 1999-2002 Wind River Systems, Inc. */

/*
modification history
--------------------
01z,03mar02,jkf  SPR#32277, adding dcacheDevEnable and Disable(), orig by chn
01y,21dec01,chn  SPRs 30130, 22463, 21975 (partial). Disabled defaulting
                 tuneable parameters after they are explicitly set by user.
01x,12dec01,jkf  fixing diab build warnings.
01w,09dec01,jkf  SPR#71637, fix for SPR#68387 caused ready changed bugs.
01v,09nov01,jkf  SPR#71633, dont set errno when DevCreate is called w/BLK_DEV
                 SPR#65431, fixing typo in man page
01u,29aug01,jkf  SPR#69031, common code for both AE & 5.x.
01t,01aug01,jyo  Fixed SPR#68387: readyChanged bit is not correctly checked to
                 verify the change in the media, SPR#69411: Change in media's
                 readyChanged bit is not being propogated appropriately to the
                 layers above.
01s,13jun01,jyo  SPR#67729: Updating blkSubDev, cbioSubDev and isDriver in
                 dcacheDevCreate().
01r,19apr00,dat  doc fixup
01q,09mar00,jkf  removed taskUndelay & taskPrioritySet from CBIO_FLUSH
                 now forcing ioctl flushes inline.
01p,29feb00,jkf  T3 changes
01o,31aug99,jkf  changes for new CBIO API.
01n,31jul99,jkf  T2 merge, tidiness & spelling.
01i,17nov98,lrn  zero-fill blocks allocated with CBIO_CACHE_NEWBLK
01h,29oct98,lrn  pass along 3rd arg on CBIO_RESET, removed old mod history
01g,20oct98,lrn  fixed SPR#22553, SPR#22731
01f,14sep98,lrn  refined error handling, made updTask unconditional
01e,08sep98,lrn  add hash for speed (SPR#21972), size change (SPR#21975)
01d,06sep98,lrn  modify to work on top of CBIO (SPR#21974), and use
                 wrapper for block devices b.c.
01c,30jul98,wlf  partial doc cleanup
01b,01jul98,lrn  written.
01a,28jan98,lrn  written, preliminary
*/

/*
DESCRIPTION

This module implements a disk cache mechanism via the CBIO API.
This is intended for use by the VxWorks DOS file system, to store
frequently used disk blocks in memory.  The disk cache is unaware 
of the particular file system format on the disk, and handles the
disk as a collection of blocks of a fixed size, typically the sector
size of 512 bytes.  

The disk cache may be used with SCSI, IDE, ATA, Floppy or any other
type of disk controllers.  The underlying device driver may be either
comply with the CBIO API or with the older block device API.  

This library interfaces to device drivers implementing the block 
device API via the basic CBIO BLK_DEV wrapper provided by cbioLib.

Because the disk cache complies with the CBIO programming interface on
both its upper and lower layers, it is both an optional and a stackable
module.   It can be used or omitted depending on resources available and 
performance required.

The disk cache module implements the CBIO API, which is used by the file
system module to access the disk blocks, or to access bytes within a 
particular disk block.  This allows the file system to use the disk cache
to store file data as well as Directory and File Allocation Table blocks, 
on a Most Recently Used basis, thus keeping a controllable subset of these
disk structures in memory.  This results in minimized memory requirements 
for the file system, while avoiding any significant performance degradation.

The size of the disk cache, and thus the memory consumption of the disk
subsystem, is configured at the time of initialization (see 
dcacheDevCreate()), allowing the user to trade-off memory consumption
versus performance.  Additional performance tuning capabilities are
available through dcacheDevTune().

Briefly, here are the main techniques deployed by the disk cache:
.IP
Least Recently Used block re-use policy
.IP
Read-ahead
.IP
Write-behind with sorting and grouping
.IP
Hidden writes
.IP
Disk cache bypass for large requests
.IP
Background disk updating (flushing changes to disk) with an adjustable
update period (ioctl flushes occur without delay.)
.LP

Some of these techniques are discussed in more detail below; others 
are described in varrious professional and academic publications.

DISK CACHE ALGORITHM
The disk cache is composed internally of a number cache blocks, of
the same size as the disk physical block (sector). These cache blocks
are maintained in a list in "Most Recently Used" order, that is, blocks
which are used are moved to the top of this list. When a block needs to
be relinquished, and made available to contain a new disk block, the
Least Recently Used block will be used for this purpose.

In addition to the regular cache blocks, some of the memory allocated
for cache is set aside for a "big buffer", which may range from 1/4 of
the overall cache size up to 64KB.  This buffer is used for:

.IP
Combining cache blocks with adjacent disk block numbers, in order to
write them to disk in groups, and save on latency and overhead 
.IP
Reading ahead a group of blocks, and then converting them to normal
cache blocks.
.LP

Because there is significant overhead involved in accessing the disk
drive, read-ahead improves performance significantly by reading groups
of blocks at once.

TUNABLE PARAMETERS
There are certain operational parameters that control the disk cache
operation which are tunable. A number of
.I preset
parameter sets is provided, dependent on the size of the cache. These
should suffice for most purposes, but under certain types of workload,
it may be desirable to tune these parameters to better suite the
particular workload patterns.

See dcacheDevTune() for description of the tunable parameters. It is
recommended to call dcacheShow() after calling dcacheTune() in order 
to verify that the parameters where set as requested, and to inspect 
the cache statistics which may change dramatically. Note that the hit 
ratio is a principal indicator of cache efficiency, and should be inspected
during such tuning.

BACKGROUND UPDATING
A dedicated task will be created to take care of updating the disk with
blocks that have been modified in cache. The time period between updates
is controlled with the tunable parameter syncInterval. Its priority
should be set above the priority of any CPU-bound tasks so as to assure
it can wake up frequently enough to keep the disk synchronized with the
cache.   There is only one such task for all cache devices configured.  
The task name is tDcacheUpd

The updating task also has the responsibility to invalidate disk cache
blocks for removable devices which have not been used for 2 seconds or more.

There are a few global variables which control the parameters of this
task, namely:

.IP <dcacheUpdTaskPriority>
controls the default priority of the update task, and is set by default to 250.

.IP <dcacheUpdTaskStack>
is used to set the update task stack size.

.IP <dcacheUpdTaskOptions>
controls the task options for the update task.
.LP
All the above global parameters must be set prior to calling
dcacheDevCreate() for the first time, with the exception of
dcacheUpdTaskPriority, which may be modified in run-time, and takes
effect almost immediately. It should be noted that this priority is not
entirely fixed, at times when critical disk operations are performed,
and FIOFLUSH ioctl is called, the caller task will temporarily
.I loan
its priority to the update task, to insure the completion of the flushing
operation.

REMOVABLE DEVICES
For removable devices, disk cache provides these additional features:

.IP "disk updating"
is performed such that modified blocks will be written to disk within
one second, so as to minimize the risk of losing data in case of a
failure or disk removal.
.IP "error handling"
includes a test for disk removal, so that if a disk is removed from the
drive while an I/O operation is in progress, the disk removal event will
be set immediately.
.IP "disk signature"
which is a checksum of the disk's boot block, is maintained by the cache
control structure, and it will be verified against the disk if it was
idle for 2 seconds or more. Hence if during that idle time a disk was
replaced, the change will be detected on the next disk access, and the
condition will be flagged to the file system.

.IP NOTE
It is very important that removable disks should all have a unique
volume label, or volume serial number, which are stored in the disk's
boot sector during formatting. Changing disks which have an identical
boot sector may result in failure to detect the change, resulting in
unpredictable behavior, possible file system corruption.
.LP


CACHE IMPLEMENTATION

Most Recently Used (MRU) disk blocks are stored in a collection of memory
buffers called the disk cache.  The purpose of the disk cache is to reduce 
the number of disk accesses and to accelerate disk read and write operations, 
by means of the following techniques:

.IP
Most Recently Used blocks are stored in RAM, which results in the most
frequently accessed data being retrieved from memory rather than from disk.
.IP
Reading data from disk is performed in large units, relying on the read-ahead
feature, one of the disk cache