Quick Reference The goal of this chapter is to write a complete char device driver. We'll develop a character driver because this class is suitable for most simple hardware devices. Char drivers are also easier to understand than, for example, block drivers or network drivers. Our ultimate aim is to write a modularized char driver, but we won't talk about modularization issues in this chapter.
Mount time linearly depends on the flash size True, the dependency is linear, because JFFS2 has to scan whole flash media when mounting. UBIFS mount time does not depend on the flash size.
Memory consumption linearly depends on the flash size True, the dependency is linear. But it is not difficult to implement the LPT shrinker and get rid of the dependency.
It is not implemented only because the memory consumption is too small to make the coding work worth it. UBI memory consumption linearly depends on flash size.
Mount time linearly depends on the file system contents True, the more data are stored on the file system, the longer it takes to mount it, because JFFS2 has to do more scanning work.
False, give read write access file linux time does not depend on the file system contents. At the worst case if there was an unclean rebootUBIFS has to scan and replay the journal which has fixed and configurable size.
Full file system checking is required after each mount True. For example, this may be observed by running top just after JFFS2 has been mounted. This slows down overall system boot-up time. Fundamentally, this is needed because JFFS2 does not store space accounting information i.
Memory consumption linearly depends on file system contents True. UBIFS memory consumption does not depend on how much data are stored on the flash media. File access time linearly depends on its size True. The fragment tree is an in-memory RB-tree which is indexed by file offset and refers on-flash nodes corresponding to this offset.
The fragment tree is not stored on the flash media. Instead, it is built on-the-flight when the file is opened for the first time. To build the fragment tree, JFFS2 has to read each data node corresponding to this inode from the flash. This means, the larger is the file, the longer it takes to open it for the first time.
And the larger is the file the more memory it takes when it is opened. Depending on the system, JFFS2 becomes nearly unusable starting from certain file size. Whenever a piece of data has to be read from the file system, the B-tree is looked-up and the corresponding flash address to read is found.
There is a TNC cache which caches the B-tree nodes when the B-tree is looked-up, and the cache is shrinkable, which means it can be shrunk when the kernel needs more memory.
Since JFFS2 is fully synchronous, it writes data to the flash media as soon as the data arrives. If one changes few bytes in the middle of a file, JFFS2 writes a data node which contains those bytes to the flash. If there are many random small writes all over the place, the file system becomes fragmented.
But this "defragmentation" is happening during garbage collection and at random time, because JFFS2 wear-leveling algorithm is based on random eraseblock selection. So if there were a lot of small writes, JFFS2 becomes slower some time later - the performance just goes down out of the blue which makes the system less predictable.
This does not hurt the performance much because of the write-back support: And write-back usually happens in background. Write-back support UBIFS supports write-back, which means that file changes do not go to the flash media straight away, but they are cached and go to the flash later, when it is absolutely necessary.
Write-back caching is a standard technique which is used by most file systems like ext3 or XFS. This buffer contains last written data and is flushed once it is full.
However, because the amount of cached data are very small, JFFS2 is very close to a synchronous file system. Write-back support requires the application programmers to take extra care about synchronizing important files in time. Otherwise the files may corrupt or disappear in case of power-cuts, which happens very often in many embedded devices.
Let's take a glimpse at Linux manual pages:On computer filesystems, different files and directories have permissions that specify who and what can read, write, modify and access them.
This is important because WordPress may need access to write to files in your wp-content directory to enable certain functions.. Permission Modes. Jun 25, · Linux can establish different types of groups for file access. In a one home computer environment anyone who uses the computer can read this file but cannot write to (modify) it.
This is a completely normal situation. Is it possible in linux to give a user read/write access to all files and directories like root?
EDIT I have a special case. I need to create a folder structure on . How can I (programmatically) give write permission on a file to a particular user in Linux? Like, for example, its owner? Everyone has read access to this file. Give read-only access to specific folders?
read: permitted to view files and sub-directories in that directory; jack and jack's group will have read+write access to /home/jack and all it's sub-directories.
The rest will have only read access. I'm running a server, and I need to give read/write access to a particular directory to a single user. I've tried the following: sudo adduser abcd sudo groupadd abcdefg chown timberdesignmag.comg /var/www/.