This character is also passed through without modification. An attempt to create a file that contains one of these invalid characters will return an error. DOS 8. But it maps a filename to lowercase by default when returning a filename to a QNX application, so that QNX users and programs can always see and type lowercase. You can specify how you want fs-dos.
If you use the ignore option, you can specify whether or not to truncate filename characters beyond the 8. DOS uses the concept of a volume label, which is an actual directory entry in the root of the DOS filesystem.
To distinguish between the volume label and an actual DOS file, fs-dos. An error isn't returned because a number of utilities attempt to do this and failure would result in unexpected errors.
The approach taken is "you can change anything to anything since it isn't written to disk anyway. This transparency allows processes to operate on CD-ROM files without any special knowledge or work on their part. The fs-cd. The flash filesystem drivers are standalone executables that contain both the flash filesystem code and the flash device code. There are versions of the flash filesystem driver for different embedded systems hardware as well as PCMCIA memory cards.
The naming convention for the drivers is devf- system , where system describes the embedded system. Along with the pre-built flash filesystem drivers, we provide the libraries and source code needed to build custom flash filesystem drivers for different embedded systems. For information on how to do this, see the chapter on customizing a flash filesystem driver in the Building Embedded Systems book.
The flash filesystem drivers support one or more logical flash drives. Each logical drive is called a socket , which consists of a contiguous and homogeneous region of flash memory. For example, in a system containing two different types of flash device at different addresses, where one flash device is used for the boot image and the other for the flash filesystem, each flash device would appear in a different socket. Each socket may be divided into one or more partitions.
Two types of partitions are supported: raw partitions and flash filesystem partitions. A raw partition in the socket is any partition that doesn't contain a flash filesystem. The flash filesystem driver doesn't recognize any filesystem types other than the flash filesystem. A raw partition may contain an image filesystem or some application-specific data.
The flash filesystem will make accessible through a raw mountpoint see below any partitions on the flash that aren't flash filesystem partitions. Note that the flash filesystem partitions are available as raw partitions as well. A flash filesystem partition contains the POSIX-like flash filesystem, which uses a QNX proprietary format to store the filesystem data on the flash devices.
The flash filesystem allows files and directories to be freely created and deleted. It recovers space from deleted files using a reclaim mechanism similar to garbage collection. When you start the flash filesystem driver, it will by default mount any partitions it finds in the socket. Note that you can specify the mountpoint using mkefs or flashctl e. The flash filesystem supports many advanced features, such as POSIX compatibility, multiple threads, background reclaim, fault recovery, transparent decompression, endian-awareness, wear-leveling, and error-handling.
The flash filesystem supports the standard POSIX functionality including long filenames, access privileges, random writes, truncation, and symbolic links with the following exceptions:. These design compromises allow the flash filesystem to remain small and simple, yet include most features normally found with block device filesystems. The flash filesystem stores files and directories as a linked list of extents, which are marked for deletion as they're deleted or updated.
Blocks to be reclaimed are chosen using a simple algorithm that finds the block with the most space to be reclaimed while keeping level the amount of wear of each individual block. This wear-leveling increases the MTBF mean time between failures of the flash devices, thus increasing their longevity.
The background reclaim process is performed when there isn't enough free space. The reclaim process first copies the contents of the reclaim block to an empty spare block, which then replaces the reclaim block. The reclaim block is then erased. Unlike rotating media with a mechanical head, proximity of data isn't a factor with a flash filesystem, so data can be scattered on the media without loss of performance.
The flash filesystem has been designed to minimize corruption due to accidental loss-of-power faults. Updates to extent headers and erase block headers are always executed in carefully scheduled sequences.
These sequences allow the recovery of the flash filesystem's integrity in the case of data corruption. With properly designed hardware for power faults, flash write transactions can be guaranteed to be finished when a power fault occurs as long as there's just enough power left to commit a microsec write cycle.
Note that even rename operations are guaranteed atomic -- even through loss-of-power faults -- so you can safely use rename operations to update application code with the flash filesystem. When the flash filesystem driver is started, it scans the state of every extent header on the media in order to validate its integrity and takes appropriate action, ranging from a simple block reclamation to the erasure of dangling extent links.
This process is merged with the normal mount procedure of the flash filesystem in order to achieve optimal bootstrap timings. The flash filesystem inherently supports transparent decompression of files that have been compressed using the flashcmp utility.
The utility supports two compression algorithms:. Compressed files can be manipulated with standard utilities such as cp or ftp -- they can display their compressed and uncompressed size with the ls utility if used with the proper mountpoint. These features make the management of a compressed flash filesystem seamless to a systems designer. As flash hardware wears out, its write state-machine may find that it can't write or erase a particular bit cell.
When this happens, the error status is propagated to the flash driver so it can take proper action i. Note: Running more than one pass-through filesystem or resource manager on overlapping pathname spaces might cause deadlocks. Filesystems and pathname resolution You can seamlessly locate and connect to any service or filesystem that's been registered with the process manager. When a filesystem resource manager registers a mountpoint, the process manager creates an entry in the internal mount table for that mountpoint and its corresponding server ID i.
Filesystem classes The many filesystems available can be categorized into the following classes: Image filesystem Every QNX Neutrino system image provides a simple read-only filesystem that presents the set of files built into the OS image. Power-Safe filesystem The Power-Safe filesystem is a reliable disk filesystem that can withstand power failures without losing or corrupting data. It was designed for and is intended for traditional rotating hard disk drive media. This transparency allows processes to operate on DOS files without any special knowledge or work on their part.
This transparency allows processes to operate on CD-ROM files without any special knowledge or work on their part. The drivers are standalone executables that contain both the flash filesystem code and the flash device code. The filesystem supports a fully hierarchical directory structure with POSIX semantics as shown in the table above. ETFS is a filesystem composed entirely of transactions. Every write operation, whether of user data or filesystem metadata, consists of a transaction.
A transaction either succeeds or is treated as if it never occurred. Transactions never overwrite live data. A write in the middle of a file or a directory update always writes to a new unused area. In this way, if the operation fails part way through due to a crash or power failure , the old data is still intact. Some log-based filesystems also operate under the principle that live data is never overwritten.
But ETFS takes this to the extreme by turning everything into a log of transactions. The filesystem hierarchy is built on the fly by processing the log of transactions in the device. This scan occurs at startup, but is designed such that only a small subset of the data is read and CRC-checked, resulting in faster startup times without sacrificing reliability. Transactions are position-independent in the device and may occur in any order. You could read the transactions from one device and write them in a different order to another device.
This is important because it allows bulk programming of devices containing bad blocks that may be at arbitrary locations. This design is well-suited for NAND flash memory. NAND flash is shipped with factory-marked bad blocks that may occur in any location.
ETFS is designed to survive across a power failure, even during an active flash write or block erase. The following features contribute to its reliability:. Flash memory allows a limited number of erase cycles on a flash block before the block will fail. This number can be as low as , ETFS tracks the number of erases on each block. When selecting a block to use, ETFS attempts to spread the erase cycles evenly over the device, dramatically increasing its life.
The difference can be extreme: from usage scenarios of failure within a few days without wear-leveling to over 40 years with wear-leveling.
Filesystems often consist of a large number of static files that are read but not written. These files will occupy flash blocks that have no reason to be erased. If the majority of the files in flash are static, this will cause the remaining blocks containing dynamic data to wear at a dramatically increased rate. ETFS notices these underworked static blocks and forces them into service by copying their data to an overworked block. This solves two problems: It gives the overworked block a rest, since it now contains static data, and it forces the underworked static block into the dynamic pool of blocks.
Each transaction is protected by a cyclic redundancy check CRC. This ensures quick detection of corrupted data, and forms the basis for the rollback operation of damaged or incomplete transactions at startup.
The CRC can detect multiple bit errors that may occur during a power failure. This is suitable for NAND flash memory, in which single-bit errors may occur during normal usage. An ECC error is a warning signal that the flash block the error occurred in may be getting weak, i.
ETFS will mark the weak block for a refresh operation, which copies the data to a new flash block and erases the weak block. The erase recharges the flash block. Each read operation within a NAND flash block weakens the charge maintaining the data bits. Most devices support about , reads before there's danger of losing a bit. The ECC will recover a single-bit error, but may not be able to recover multi-bit errors. ETFS solves this by tracking reads and marking blocks for refresh before the , read limit is reached.
When ETFS starts, it processes all transactions and rolls back discards the last partial or damaged transaction. The rollback code is designed to handle a power failure during a rollback operation, thus allowing the system to recover from multiple nested faults. The validity of a transaction is protected by CRC codes on each transaction.
ETFS implements a very simple directory structure on the device, allowing significant modifications with a single flash write. For example, the move of a file or directory to another directory is often a multistage operation in most filesystems.
In ETFS, a move is accomplished with a single flash write. Log-based filesystems often suffer from fragmentation, since each update or write to an existing file causes a new transaction to be created.
ETFS uses write-buffering to combine small writes into larger write transactions in an attempt to minimize fragmentation caused by lots of very small transactions.
ETFS also monitors the fragmentation level of each file and will do a background defragmenting operation on files that do become badly fragmented. Note that this background activity will always be preempted by a user data request in order to ensure immediate access to the file being defragmented. The QNX 4 filesystem fs-qnx4. The QNX 4 filesystem implements an extremely robust design, utilizing an extent-based, bitmap allocation scheme with fingerprint control structures to safeguard against data loss and to provide easy recovery.
Features include:. The Power-Safe filesystem, supported by the fs-qnx6. It was designed for and is intended for traditional rotating hard disk drive media.
Although existing disk filesystems are designed to be robust and reliable, there's still the possibility of losing data, depending on what the filesystem is doing when a catastrophic failure such as a power failure occurs:. You can get hard drives that offer atomic sector upgrades and promise you that either all of the old or new data in the sector will be readable, but these drives are rare and expensive.
To address the problems associated with existing disk filesystems, the Power-Safe filesystem never overwrites live data; it does all updates using copy-on-write COW , assembling a new view of the filesystem in unused blocks on the disk. Everything is COW: both metadata and user data are protected. To see how this works, let's consider how the data is stored.
A Power-Safe filesystem is divided into logical blocks, the size of which you can specify when you use mkqnx6fs to format the filesystem. Each inode includes 16 pointers to blocks. If the file is smaller than 16 blocks, the inode points to the data blocks directly. If the file is any bigger, those 16 blocks become pointers to more blocks, and so on. The final block pointers to the real data are all in the leaves and are all at the same level.
In some other filesystems — such as EXT2 — a file always has some direct blocks, some indirect ones, and some double indirect, so you go to different levels to get to different parts of the file. With the Power-Safe filesystem, all the user data for a file is at the same level. If you change some data, it's written in one or more unused blocks, and the original data remains unchanged. The list of indirect block pointers must be modified to refer to the newly used blocks, but again the filesystem copies the existing block of pointers and modifies the copy.
The filesystem then updates the inode — once again by modifying a copy — to refer to the new block of indirect pointers. When the operation is complete, the original data and the pointers to it remain intact, but there's a new set of blocks, indirect pointers, and inode for the modified data:.
A superblock is a global root block that contains the inodes for the system bitmap and inodes files. A Power-Safe filesystem maintains two superblocks:. The working superblock can include pointers to blocks in the stable superblock. These blocks contain data that hasn't yet been modified. The inodes and bitmap for the working superblock grow from it.
A snapshot is a consistent view of the filesystem simply a committed superblock. To take a snapshot, the filesystem:. To mount the disk at startup, the filesystem simply reads the superblocks from disk, validates their CRCs, and then chooses the one with the higher sequence number. There's no need to run chkfsys or replay a transaction log.
The time it takes to mount the filesystem is the time it takes to read a couple of blocks. The performance of the filesystem depends on how much buffer cache is available, and on the frequency of the snapshots.
Snapshots occur periodically every 10 seconds, or as specified by the snapshot option to fs-qnx6. You can also turn snapshots off if you're doing some long operation, and the intermediate states aren't useful to you. For example, suppose you're copying a very large file into a Power-Safe filesystem. The cp utility is really just a sequence of basic operations:. If the file is big enough so that copying it spans snapshots, you have on-disk views that include the file not existing, the file existing at a variety of sizes, and finally the complete file copied and its IDs and permissions set:.
Each snapshot is a valid point-in-time view of the filesystem i. If there's a power failure, the filesystem is restored to the most recent snapshot.
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