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Having multiple simultaneous writers (no reader) to a single file. Is it possible to accomplish in a performant way in .NET?

I'm developing a multiple segment file downloader. To accomplish this task I'm currently creating as many temporary files on disk as segments I have (they are fixed in number during the file downloading). In the end I just create a new file f and copy all the segments' contents onto f.
I was wondering if there's not a better way to accomplish this. My idealization is of initially creating f in its full-size and then have the different threads write directly onto their portion. There need not to be any kind of interaction between them. We can assume any of them will start at its own starting point in the file and then only fill information sequentially in the file until its task is over.
I've heard about Memory-Mapped files (http://msdn.microsoft.com/en-us/library/dd997372(v=vs.110).aspx) and I'm wondering if they are the solution to my problem or not.
Thanks

Using the memory mapped API is absolute doable and it will probably perform quite well - of cause some testing would be recommended.
If you want to look for a possible alternative implementation, I have the following suggestion.
Create a static stack data structure, where the download threads can push each file segment as soon as it's downloaded.
Have a separate thread listen for push notifications on the stack. Pop the stack file segments and save each segment into the target file in a single threaded way.
By following the above pattern, you have separated the download of file segments and the saving into a regular file, by putting a stack container in between.
Depending on the implementation of the stack handling, you will be able to implement this with very little thread locking, which will maximise performance.
The pros of this is that you have 100% control on what is going on and a solution that might be more portable (if that ever should be a concern).
The stack decoupling pattern you do, can also be implemented pretty generic and might even be reused in the future.
The implementation of this is not that complex and probably on par with the implementation needed to be done around the memory mapped api.
Have fun...
/Anders

The answers posted so far are, of course addressing your question but you should also consider the fact that multi-threaded I/O writes will most likely NOT give you gains in performance.
The reason for multi-threading downloads is obvious and has dramatic results. When you try to combine the files though, remember that you are having multiple threads manipulate a mechanical head on conventional hard drives. In case of SSD's you may gain better performance.
If you use a single thread, you are by far exceeding the HDD's write capacity in a SEQUENTIAL way. That IS by definition the fastest way to write to conventions disks.
If you believe otherwise, I would be interested to know why. I would rather concentrate on tweaking the write performance of a single thread by playing around with buffer sizes, etc.

Yes, it is possible but the only precaution you need to have is to control that no two threads are writing at the same location of file, otherwise file content will be incorrect.
FileStream writeStream = new FileStream(destinationPath, FileMode.OpenOrCreate, FileAccess.Write, FileShare.Write);
writeStream.Position = startPositionOfSegments; //REMEMBER This piece of calculation is important
// A simple function to write the bytes ... just read from your source and then write
writeStream.Write(ReadBytes, 0 , bytesReadFromInputStream);
After each Write we used writeStream.Flush(); so that buffered data gets written to file but you can change according to your requirement.
Since you have code already working which downloads the file segments in parallel. The only change you need to make is just open the file stream as posted above, and instead of creating many segments file locally just open stream for a single file.
The startPositionOfSegments is very important and calculate it perfectly so that no two segments overwrite the desired downloaded bytes to same location on file otherwise it will provide incorrect result.
The above procedure works perfectly fine at our end, but this can be problem if your segment size are too small (We too faced it but after increasing size of segments it got fixed). If you face any exception then you can also synchronize only the Write part.

Efficient log backup program in C#

I am writing a log backup program in C#. The main objective is to take logs from multiple servers, copy and compress the files and then move them to a central data storage server. I will have to move about 270Gb of data every 24 hours. I have a dedicated server to run this job and a LAN of 1Gbps. Currently I am reading lines from a (text)file, copying them into a buffer stream and writing them to the destination.
My last test copied about 2.5Gb of data in 28 minutes. This will not do. I will probably thread the program for efficiency, but I am looking for a better method to copy the files.
I was also playing with the idea of compressing everything first and then using a stream buffer a bit to copy. Really, I am just looking for a little advice from someone with more experience than me.
Any help is appreciated, thanks.

You first need to profile as Umair said so that you can figure out how much of the 28 minutes is spent compressing vs. transmitting. Also measure the compression rate (bytes/sec) with different compression libraries, and compare your transfer rate against other programs such as Filezilla to see if you're close to your system's maximum bandwidth.
One good library to consider is DotNetZip, which allows you to zip to a stream, which can be handy for large files.
Once you get it fine-tuned for one thread, experiment with several threads and watch your processor utilization to see where the sweet spot is.

One of the solutions can be is what you mantioned: compress files in one Zip file and after transfer them via network. This will bemuch faster as you are transfering one file and often on of principal bottleneck during file transfers is Destination security checks.
So if you use one zip file, there should be one check.
In short:
Compress
Transfer
Decompress (if you need)
This already have to bring you big benefits in terms of performance.

Compress the logs at source and use TransmitFile (that's a native API - not sure if there's a framework equivalent, or how easy it is to P/Invoke this) to send them to the destination. (Possibly HttpResponse.TransmitFile does the same in .Net?)
In any event, do not read your files linewise - read the files in blocks (loop doing FileStream.Read for 4K - say - bytes until read count == 0) and send that direct to the network pipe.

Trying profiling your program... bottleneck is often where you least expect it to be. As some clever guy said "Premature optimisation is the root of all evil".
Once in a similar scenario at work, I was given the task to optimise the process. And after profiling the bottleneck was found to be a call to sleep function (which was used for synchronisation between thread!!!! ).

When to use memory-mapped files?

I have an application that receives chunks of data over the network, and writes these to disk.
Once all chunks have been received, they can be decoded/recombined into the single file they actually represent.
I'm wondering if it's useful to use memory-mapped files or not - first for writing the single chunks to disk, second for the single file into which all of them are decoded.
My own feeling is that it might be useful for the second case only, anyone got some ideas on this?
Edit:
It's a C# app, and I'm only planning an x64 version.
(So running into the 'largest contigious free space' problem shouldn't be relevant)

Memory-mapped files are beneficial for scenarios where a relatively small portion (view) of a considerably larger file needs to be accessed repeatedly.
In this scenario, the operating system can help optimize the overall memory usage and paging behavior of the application by paging in and out only the most recently used portions of the mapped file.
In addition, memory-mapped files can expose interesting features such as copy-on-write or serve as the basis of shared-memory.
For your scenario, memory-mapped files can help you assemble the file if the chunks arrive out of order. However, you would still need to know the final file size in advance.
Also, you should be accessing the files only once, for writing a chunk. Thus, a performance advantage over explicitly implemented asynchronous I/O is unlikely, but it may be easier and quicker to implement your file writer correctly.
In .NET 4, Microsoft added support for memory-mapped files and there are some comprehensive articles with sample code, e.g. http://blogs.msdn.com/salvapatuel/archive/2009/06/08/working-with-memory-mapped-files-in-net-4.aspx.

Memory-mapped files are primarily used for Inter-Process Communication or I/O performance improvement.
In your case, are you trying to get better I/O performance?
Hate to point out the obivious, but Wikipedia gives a good rundown of the situation...
http://en.wikipedia.org/wiki/Memory-mapped_file
Specifically...
The memory mapped approach has its cost in minor page faults - when a block of data is loaded in page cache, but not yet mapped in to the process's virtual memory space. Depending on the circumstances, memory mapped file I/O can actually be substantially slower than standard file I/O.
It sounds like you're about to prematurely optimize for speed. Why not a regular file approach, and then refactor for MM files later if needed?

I'd say both cases are relevant. Simply write the single chunks to their proper place in the memory mapped file, out of order, as they come in. This of course is only useful if you know where each chunk should go, like in a bittorrent downloader. If you have to perform some extra analysis to know where the chunk should go, the benefit of a memory mapped file might not be as large.

Logic in Disk Defragmantation & Disk Check

What is the logic behind disk defragmentation and Disk Check in Windows? Can I do it using C# coding?

For completeness sake, here's a C# API wrapper for defragmentation:
http://blogs.msdn.com/jeffrey_wall/archive/2004/09/13/229137.aspx
Defragmentation with these APIs is (supposed to be) very safe nowadays. You shouldn't be able to corrupt the file system even if you wanted to.
Commercial defragmentation programs use the same APIs.

Look at Defragmenting Files at msdn for possible API helpers.
You should carefully think about using C# for this task, as it may introduce some undesired overhead for marshaling into native Win32.

If you don't know the logic for defragmentation, and if you didn't write the file system yourself so you can't authoritatively check it for errors, why not just start new processes running 'defrag' and 'chkdsk'?

Mark Russinovich wrote an article Inside Windows NT Disk Defragmentation a while ago which gives in-depth details. If you really want to do this I would really advise you to use the built-in facilities for defragmenting. More so, on recent OSes I have never seen a need as a user to even care about defragmenting; it will be done automatically on a schedule and the NTFS folks at MS are definitely smarter at that stuff than you (sorry, but they do this for some time now, you don't).

Despite its importance, the file system is no more than a data structure that maps file names into lists of disk blocks. And keeps track of meta-information such as the actual length of the file and special files that keep lists of files (e.g., directories). A disk checker verifies that the data structure is consistent. That is, every disk block must either be free for allocation to a file or belong to a single file. It can also check for certain cases where a set of disk blocks appears to be a file that should be in a directory but is not for some reason.
Defragmentation is about looking at the lists of disk blocks assigned to each file. Files will generally load faster if they use a contiguous set of blocks rather than ones scattered all over the disk. And generally the entire file system will perform best if all the disk blocks in use confine themselves to a single congtiguous range of the disk. Thus the trick is moving disk blocks around safely to achieve this end while not destroying the file system.
The major difficulty here is running these application while a disk is in use. It is possible but one has to be very, very, very careful not to make some kind of obvious or extremely subtle error and destroy most or all of the files. It is easier to work on a file system offline.
The other difficulty is dealing with the complexities of the file system. For example, you'd be much better off building something that supports FAT32 rather than NTFS because the former is a much, much simpler file system.
As long as you have low-level block access and some sensible way for dealing with concurrency problems (best handled by working on the file system when it is not in use) you can do this in C#, perl or any language you like.
BUT BE VERY CAREFUL. Early versions of the program will destroy entire file systems. Later versions will do so but only under obscure circumstances. And users get extremely angry and litigious if you destroy their data.

What's the best way to read and parse a large text file over the network?

I have a problem which requires me to parse several log files from a remote machine.
There are a few complications:
1) The file may be in use
2) The files can be quite large (100mb+)
3) Each entry may be multi-line
To solve the in-use issue, I need to copy it first. I'm currently copying it directly from the remote machine to the local machine, and parsing it there. That leads to issue 2. Since the files are quite large copying it locally can take quite a while.
To enhance parsing time, I'd like to make the parser multi-threaded, but that makes dealing with multi-lined entries a bit trickier.
The two main issues are:
1) How do i speed up the file transfer (Compression?, Is transferring locally even neccessary?, Can I read an in use file some other way?)
2) How do i deal with multi-line entries when splitting up the lines among threads?
UPDATE: The reason I didnt do the obvious parse on the server reason is that I want to have as little cpu impact as possible. I don't want to affect the performance of the system im testing.

If you are reading a sequential file you want to read it in line by line over the network. You need a transfer method capable of streaming. You'll need to review your IO streaming technology to figure this out.
Large IO operations like this won't benefit much by multithreading since you can probably process the items as fast as you can read them over the network.
Your other great option is to put the log parser on the server, and download the results.

The better option, from the perspective of performance, is going to be to perform your parsing at the remote server. Apart from exceptional circumstances the speed of your network is always going to be the bottleneck, so limiting the amount of data that you send over your network is going to greatly improve performance.
This is one of the reasons that so many databases use stored procedures that are run at the server end.
Improvements in parsing speed (if any) through the use of multithreading are going to be swamped by the comparative speed of your network transfer.
If you're committed to transferring your files before parsing them, an option that you could consider is the use of on-the-fly compression while doing your file transfer.
There are, for example, sftp servers available that will perform compression on the fly.
At the local end you could use something like libcurl to do the client side of the transfer, which also supports on-the-fly decompression.

The easiest way considering you are already copying the file would be to compress it before copying, and decompress once copying is complete. You will get huge gains compressing text files because zip algorithms generally work very well on them. Also your existing parsing logic could be kept intact rather than having to hook it up to a remote network text reader.
The disadvantage of this method is that you won't be able to get line by line updates very efficiently, which are a good thing to have for a log parser.

I guess it depends on how "remote" it is. 100MB on a 100Mb LAN would be about 8 secs...up it to gigabit, and you'd have it in around 1 second. $50 * 2 for the cards, and $100 for a switch would be a very cheap upgrade you could do.
But, assuming it's further away than that, you should be able to open it with just read mode (as you're reading it when you're copying it). SMB/CIFS supports file block reading, so you should be streaming the file at that point (of course, you didn't actually say how you were accessing the file - I'm just assuming SMB).
Multithreading won't help, as you'll be disk or network bound anyway.

Use compression for transfer.
If your parsing is really slowing you down, and you have multiple processors, you can break the parsing job up, you just have to do it in a smart way -- have a deterministic algorithm for which workers are responsible for dealing with incomplete records. Assuming you can determine that a line is part of a middle of a record, for example, you could break the file into N/M segments, each responsible for M lines; when one of the jobs determines that its record is not finished, it just has to read on until it reaches the end of the record. When one of the jobs determines that it's reading a record for which it doesn't have a beginning, it should skip the record.

If you can copy the file, you can read it. So there's no need to copy it in the first place.
EDIT: use the FileStream class to have more control over the access and sharing modes.
new FileStream("logfile", FileMode.Open, FileAccess.Read, FileShare.ReadWrite)
should do the trick.

I've used SharpZipLib to compress large files before transferring them over the Internet. So that's one option.
Another idea for 1) would be to create an assembly that runs on the remote machine and does the parsing there. You could access the assembly from the local machine using .NET remoting. The remote assembly would need to be a Windows service or be hosted in IIS. That would allow you to keep your copies of the log files on the same machine, and in theory it would take less time to process them.

i think using compression (deflate/gzip) would help

The given answer do not satisfy me and maybe my answer will help others to not think it is super complicated or multithreading wouldn't benefit in such a scenario. Maybe it will not make the transfer faster but depending on the complexity of your parsing it may make the parsing/or analysis of the parsed data faster.
It really depends upon the details of your parsing. What kind of information do you need to get from the log files? Are these information like statistics or are they dependent on multiple log message?
You have several options:
parse multiple files at the same would be the easiest I guess, you have the file as context and can create one thread per file
another option as mentioned before is use compression for the network communication
you could also use a helper that splits the log file into lines that belong together as a first step and then with multiple threads process these blocks of lines; the parsing of this depend lines should be quite easy and fast.
Very important in such a scenario is to measure were your actual bottleneck is. If your bottleneck is the network you wont benefit of optimizing the parser too much. If your parser creates a lot of objects of the same kind you could use the ObjectPool pattern and create objects with multiple threads. Try to process the input without allocating too much new strings. Often parsers are written by using a lot of string.Split and so forth, that is not really as fast as it could be. You could navigate the Stream by checking the coming values without reading the complete string and splitting it again but directly fill the objects you will need after parsing is done.
Optimization is almost always possible, the question is how much you get out for how much input and how critical your scenario is.