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It should be available on all Linuxes. Linux 4.1 Documentation/devices.txt says: Compulsory links These links should exist on all systems: /dev/fd /proc/self/fd symbolic File descriptors /dev/stdin fd/0 symbolic stdin file descriptor /dev/stdout fd/1 symbolic stdout file descriptor /dev/stderr fd/2 symbolic ...


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In many applications, storage devices will be intermittently busy reading data. If a system is always able to defer writes until a time when the storage device isn't busy reading data, then from an application's point of view the writes will take zero time to complete. The only situations in which writes would not be instantaneous would be when: Write ...


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As you fill the memory with apps various block/filesystem caches are getting pushed out of the same memory. These caches are crucial for fast look up of files and other stuff. When there is no space for caches the kernel will try to look up all the information directly from the filesystem which is utterly slow and hence will cause high IO (more like a ...


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iostat is part of the sysstat package, which is able to show overall iops if desired, or show them separated by reads/writes. Run iostat with the -d flag to only show the device information page, and -x for detailed information (separate read/write stats). You can specify the device you want information for by simply adding it afterwards on the command ...


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It trades a small amount of reliability for a great increase in throughput. Suppose, for example, a video compressing program. With delayed write ("write back"): spend 10ms compressing frame issue write frame to disk wait 10ms for disk to acknowledge write complete GOTO 1 Versus spend 10ms compressing frame issue write frame to disk (completes in ...


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None of the other answers mentioned delayed allocation. XFS, ext4, BTRFS, and ZFS all use it. XFS has been using it since before ext4 existed, so I'll use it as the example: XFS doesn't even decide where to put data until writeout. Delayed-allocation gives the allocator much more information to base its decisions on. When a file is first being written, ...


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All the other answers here are at a minimum mostly correct for the normal case, and I would recommend reading any of them before mine, but you mentioned dd and dd has a typical use case that May not involve write caching. Write caching is primarily implemented at the filesystem level. Raw devices do not normally do write caching (multiple device drivers such ...


2

The philosophy is unsafe-by-default. There are two reasonable and obvious strategies possible: flush writes to disk immediately or delay writing. UNIX historically chose the latter. So get safety, you need to call fsync afterwards. However, you can specify safety upfront by mounting a device with option sync, or per-file by opening them with O_SYNC. ...


13

Many good answers, but let me add one other thing... Remember that Unix is a multi-process and multi-users system, so potentially many users would be trying to do file-operations (esp. writes) at (almost) the same time. With old slow hard-disks - perhaps mounted over the network - this would not only take time (for which the programs would basically ...


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Asynchronous, buffered I/O was in use before Linux and even before Unix. Unix had it, and so have all its offshoots. Here is what Ritchie and Thompson wrote in their CACM paper The UNIX Time-Sharing System: To the user, both reading and writing of files appear to be synchronous and unbuffered. That is immediately after return from a read call the ...


7

Spinning platters are slower than RAM. We use caching of reads/writes to 'hide' this fact. The useful thing about write IO is that it doesn't require disk IO to happen immediately - unlike a read, where you can't return data to the user until the read completes on the disk. Thus writes operate under a soft time constraint - as long as our sustained ...


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It is not specific to Linux, and it is called the page cache (which Linux does quite well). See also http://linuxatemyram.com/; so if a file is written, then read again a few seconds later, very often no disk I/O is needed. The main advantage is that on many systems, there is a lot of RAM, and some of it can be used as a cache by the kernel. So some files ...


53

It simply gives an illusion of speed to programs that don't actually have to wait until a write is complete. Mount your filesystems in sync mode (which gives you your instant writes) and see how slow everything is. Sometimes files exist only temporarily... a program does some bit of work and deletes the file right after the work is done. If you delayed ...


37

What's the philosophy behind such an approach? Efficiency (better usage of the disk characteristics) and performance (allows the application to continue immediately after a write). Why isn't the data written at once? The main advantage is the OS is free to reorder and merge contiguous write operations to improve their bandwidth usage (less ...


0

Got answer for Throughput, from this program i can collect read/write throughput. #include <stdlib.h> #include <stdio.h> #include <string.h> #include <sys/sysinfo.h> int main(int argc, char **argv) { struct sysinfo si; sysinfo (&si); const double xdt = 1024; float pagein,pageout, oldvalin = 0, oldvalout = 0, res; for(;;) { ...


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There should be at least one filesystem mounted read-write. Run mount to see all mounted filesystems, and look for one mounted read-write (rw): mount | grep rw. I expect you'll find /var mounted read write, and /var/tmp and/or /tmp should be writeable.



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