Inside Vista SP1 File Copy Improvements

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Windows Vista SP1 includes a number of enhancements over the original Vista release in the areas of application compatibility, device support, power management, security and reliability. You can see a detailed list of the changes in the Notable Changes in Windows Vista Service Pack 1 whitepaper that you can download here. One of the improvements highlighted in the document is the increased performance of file copying for multiple scenarios, including local copies on the same disk, copying files from remote non-Windows Vista systems, and copying files between SP1 systems. How were these gains achieved? The answer is a complex one and lies in the changes to the file copy engine between Windows XP and Vista and further changes in SP1. Everyone copies files, so I thought it would be worth taking a break from the “Case of…” posts and dive deep into the evolution of the copy engine to show how SP1 improves its performance.


Copying a file seems like a relatively straightforward operation: open the source file, create the destination, and then read from the source and write to the destination. In reality, however, the performance of copying files is measured along the dimensions of accurate progress indication, CPU usage, memory usage, and throughput. In general, optimizing one area causes degradation in others. Further, there is semantic information not available to copy engines that could help them make better tradeoffs. For example, if they knew that you weren’t planning on accessing the target of the copy operation they could avoid caching the file’s data in memory, but if it knew that the file was going to be immediately consumed by another application, or in the case of a file server, client systems sharing the files, it would aggressively cache the data on the destination system.


File Copy in Previous Versions of Windows

In light of all the tradeoffs and imperfect information available to it, the Windows file copy engine tries to handle all scenarios well. Prior to Windows Vista, it took the straightforward approach of opening both the source and destination files in cached mode and marching sequentially through the source file reading 64KB (60KB for network copies because of an SMB1.0 protocol limit on individual read sizes) at a time and writing out the data to the destination as it went. When a file is accessed with cached I/O, as opposed to memory-mapped I/O or I/O with the no-buffering flag, the data read or written is stored in memory, at least until the Memory Manager decides that the memory should be repurposed for other uses, including caching the data of other files.


The copy engine relied on the Windows Cache Manager to perform asynchronous read-ahead, which essentially reads the source file in the background while Explorer is busy writing data to a different disk or a remote system. It also relied on the Cache Manager’s write-behind mechanism to flush the copied file’s contents from memory back to disk in a timely manner so that the memory could be quickly repurposed if necessary, and so that data loss is minimized in the face of a disk or system failure. You can see the algorithm at work in this Process Monitor trace of a 256KB file being copied on Windows XP from one directory to another with filters applied to focus on the data reads and writes:



Explorer’s first read operation at event 0 of data that’s not present in memory causes the Cache Manager to perform a non-cached I/O, which is an I/O that reads or writes data directly to the disk without caching it in memory, to fetch the data from disk at event 1, as seen in the stack trace for event 1:



In the stack trace, Explorer’s call to ReadFile is at frame 22 in its BaseCopyStream function and the Cache Manager invokes the non-cached read indirectly by touching the memory mapping of the file and causing a page fault at frame 8.


Because Explorer opens the file with the sequential-access hint (not visible in trace), the Cache Manager’s read-ahead thread, running in the System process, starts to aggressively read the file on behalf of Explorer at events 2 and 3. You can see the read-ahead functions in the stack for event 2:



You may have noticed that the read-ahead reads are initially out of order with respect to the original non-cached read caused by the first Explorer read, which can cause disk head seeks and slow performance, but Explorer stops causing non-cached I/Os when it catches up with the data already read by the Cache Manager and its reads are satisfied from memory.  The Cache Manager generally stays 128KB ahead of Explorer during file copies.


At event 4 in the trace, Explorer issues the first write and then you see a sequence of interleaved reads and writes. At the end of the trace the Cache Manager’s write-behind thread, also running in the System process, flushes the target file’s data from memory to disk with non-cached writes.


Vista Improvements to File Copy

During Windows Vista development, the product team revisited the copy engine to improve it for several key scenarios. One of the biggest problems with the engine’s implementation is that for copies involving lots of data, the Cache Manager write-behind thread on the target system often can’t keep up with the rate at which data is written and cached in memory. That causes the data to fill up memory, possibly forcing other useful code and data out, and eventually, the target’s system’s memory to become a tunnel through which all the copied data flows at a rate limited by the disk.  


Another problem they noted was that when copying from a remote system, the file’s contents are cached twice on the local system: once as the source file is read and a second time as the target file is written. Besides causing memory pressure on the client system for files that likely won’t be accessed again, involving the Cache Manager introduces the CPU overhead that it must perform to manage its file mappings of the source and destination files.


A limitation of the relatively small and interleaved file operations is that the SMB file system driver, the driver that implements the Windows remote file sharing protocol, doesn’t have opportunities to pipeline data across high-bandwidth, high-latency networks like WLANs. Every time the local system waits for the remote system to receive data, the data flowing across the network drains and the copy pays the latency cost as the two systems wait for the each other’s acknowledgement and next block of data.


After studying various alternatives, the team decided to implement a copy engine that tended to issue large asynchronous non-cached I/Os, addressing all the problems they had identified. With non-cached I/Os, copied file data doesn’t consume memory on the local system, hence preserving memory’s existing contents. Asynchronous large file I/Os allow for the pipelining of data across high-latency network connections, and CPU usage is decreased because the Cache Manager doesn’t have to manage its memory mappings and inefficiencies of the original Vista Cache Manager for handling large I/Os contributed to the decision to use non-cached I/Os. They couldn’t make I/Os arbitrarily large, however, because the copy engine needs to read data before writing it, and performing reads and writes concurrently is desirable, especially for copies to different disks or systems. Large I/Os also pose challenges for providing accurate time estimates to the user because there are fewer points to measure progress and update the estimate. The team did note a significant downside of non-cached I/Os, though: during a copy of many small files the disk head constantly moves around the disk, first to a source file, then to destination, back to another source, and so on.


After much analysis, benchmarking and tuning, the team implemented an algorithm that uses cached I/O for files smaller than 256KB in size. For files larger than 256KB, the engine relies on an internal matrix to determine the number and size of non-cached I/Os it will have in flight at once. The number ranges from 2 for files smaller than 2MB to 8 for files larger than 8MB. The size of the I/O is the file size for files smaller than 1MB, 1MB for files up to 2MB, and 2MB for anything larger.


To copy a file 16MB file, for example, the engine issues eight 2MB asynchronous non-cached reads of the source file, waits for the I/Os to complete, issues eight 2MB asynchronous non-cached writes of the destination, waits again for the writes to complete, and then repeats the cycle. You can see that pattern in this Process Monitor trace of a 16MB file copy from a local system to a remote one:



While this algorithm is an improvement over the previous one in many ways, it does have some drawbacks. One that occurs sporadically on network file copies is out-of-order write operations, one of which is visible in this trace of the receive side of a copy:



Note how the write operation offsets jump from 327,680 to 458,752, skipping the block at offset 393,216. That skip causes a disk head seek and forces NTFS to issue an unnecessary write operation to the skipped region to zero that part of the file, which is why there are two writes to offset 393,216. You can see NTFS calling the Cache Manager’s CcZeroData function to zero the skipped block in the stack trace for the highlighted event:



A bigger problem with using non-cached I/O is that performance can suffer in publishing scenarios. If you copy a group of files to a file share that represents the contents of a Web site for example, the Web server must read the files from disk when it first accesses them. This obviously applies to servers, but most copy operations are publishing scenarios even on client systems, because the appearance of new files causes desktop search indexing, triggers antivirus and antispyware scans, and queues Explorer to generate thumbnails for display on the parent directory’s folder icon.


Perhaps the biggest drawback of the algorithm, and the one that has caused many Vista users to complain, is that for copies involving a large group of files between 256KB and tens of MB in size, the perceived performance of the copy can be significantly worse than on Windows XP. That’s because the previous algorithm’s use of cached file I/O lets Explorer finish writing destination files to memory and dismiss the copy dialog long before the Cache Manager’s write-behind thread has actually committed the data to disk; with Vista’s non-cached implementation, Explorer is forced to wait for each write operation to complete before issuing more, and ultimately for all copied data to be on disk before indicating a copy’s completion. In Vista, Explorer also waits 12 seconds before making an estimate of the copy’s duration and the estimation algorithm is sensitive to fluctuations in the copy speed, both of which exacerbate user frustration with slower copies.


SP1 Improvements

During Vista SP1’s development, the product team decided to revisit the copy engine to explore ways to improve both the real and perceived performance of copy operations for the cases that suffered in the new implementation. The biggest change they made was to go back to using cached file I/O again for all file copies, both local and remote, with one exception that I’ll describe shortly. With caching, perceived copy time and the publishing scenario both improve. However, several significant changes in both the file copy algorithm and the platform were required to address the shortcomings of cached I/O I’ve already noted.


The one case where the SP1 file copy engine doesn't use caching is for remote file copies, where it prevents the double-caching problem by leveraging support in the Windows client-side remote file system driver, Rdbss.sys. It does so by issuing a command to the driver that tells it not to cache a remote file on the local system as it is being read or written. You can see the command being issued by Explorer in the following Process Monitor capture:



Another enhancement for remote copies is the pipelined I/Os issued by the SMB2 file system driver, srv2.sys, which is new to Windows Vista and Windows Server 2008. Instead of issuing 60KB I/Os across the network like the original SMB implementation, SMB2 issues pipelined 64KB I/Os so that when it receives a large I/O from an application, it will issue multiple 64KB I/Os concurrently, allowing for the data to stream to or from the remote system with fewer latency stalls.


The copy engine also issues four initial I/Os of sizes ranging from 128KB to 1MB, depending on the size of the file being copied, which triggers the Cache Manager read-ahead thread to issue large I/Os. The platform change made in SP1 to the Cache Manager has it perform larger I/O for both read-ahead and write-behind. The larger I/Os are only possible because of work done in the original Vista I/O system to support I/Os larger than 64KB, which was the limit in previous versions of Windows. Larger I/Os also improve performance on local copies because there are fewer disk accesses and disk seeks, and it enables the Cache Manager write-behind thread to better keep up with the rate at which memory fills with copied file data. That reduces, though not necessarily eliminates, memory pressure that causes active memory contents to be discarded during a copy. Finally, for remote copies the large I/Os let the SMB2 driver use pipelining. The Cache Manager issues read I/Os that are twice the size of the I/O issued by the application, up to a maximum of 2MB on Vista and 16MB on Server 2008, and write I/Os of up to 1MB in size on Vista and up to 32MB on Server 2008.


This trace excerpt of a 16MB file copy from one SP1 system to another shows 1MB I/Os issued by Explorer and a 2MB Cache Manager read-ahead, which is distinguished by its non-cached I/O flag:



Unfortunately, the SP1 changes, while delivering consistently better performance than previous versions of Windows, can be slower than the original Vista release in a couple of specific cases. The first is when copying to or from a Server 2003 system over a slow network. The original Vista copy engine would deliver a high-speed copy, but, because of the out-of-order I/O problem I mentioned earlier, trigger pathologic behavior in the Server 2003 Cache Manager that could cause all of the server’s memory to be filled with copied file data. The SP1 copy engine changes avoid that, but because the engine issues 32KB I/Os instead of 60KB I/Os, the throughput it achieves on high-latency connections can approach half of what the original Vista release achieved.


The other case where SP1 might not perform as well as original Vista is for large file copies on the same volume. Since SP1 issues smaller I/Os, primarily to allow the rest of the system to have better access to the disk and hence better responsiveness during a copy, the number of disk head seeks between reads from the source and writes to the destination files can be higher, especially on disks that don’t avoid seeks with efficient internal queuing algorithms.


One final SP1 change worth mentioning is that Explorer makes copy duration estimates much sooner than the original Vista release and the estimation algorithm is more accurate.



File copying is not as easy as it might first appear, but the product team took feedback they got from Vista customers very seriously and spent hundreds of hours evaluating different approaches and tuning the final implementation to restore most copy scenarios to at least the performance of previous versions of Windows and drastically improve some key scenarios. The changes apply both to Explorer copies as well as to ones initiated by applications using the CopyFileEx API and you’ll see the biggest improvements over older versions of Windows when copying files on high-latency, high-bandwidth networks where the large I/Os, SMB2’s I/O pipelining, and Vista’s TCP/IP stack receive-window auto-tuning can literally deliver what would be a ten minute copy on Windows XP or Server 2003 in one minute. Pretty cool.

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  • Mark, what about the copy (command prompt), xcopy and robocopy in Vista? Which one of them use CopyFileEx API? Which command still copies using pre-Vista methods? Please answer this.

    Also, for Windows 7 maybe you can add pausing and resuming and even better would be allowing priorities, like copying in the background using low priority I/O or a mission-critical copy etc.

    As for the shell/Explorer some more things I don't like are (This is if the shell team is reading this)

    1. In views like "List view", clicking on a white space between two horizontal columns, selects the item. Unselecting an item after doing whatever you have done and before doing another thing on it is very painful.

    2. The Load/Save dialogs still show URLs!! Wow! Can I save my notepad text file to What is this? Windows Live Workspace?

    3. Autosort/Autorefresh behavior for Explorer needs getting used to, however if I paste 100+ files in a folder containing 100+ files, those files are scattered all over alphabetically! Give us an option to modify this behavior.

    4. The size and free space is not shown on the status bar without selecting the files. Clearly this is a forgotten issue? Why not give us a TweakUI?

    5. When sorting by any criteria, Vista first sorts in Descending order compared to XP which first sorted in Ascending order. Many times, (My)Computer and Recycle Bin turn up at the end of the list after sorting because of this!!

  • 1. Good article as ever. The frustrating thing about this is that many Vista beta testers including myself pointed out the sluggishness of file copying, through mutiple bug reports with lots of votes.

    2. Also, what about zip file extraction, which is also painfully slow on Vista RTM, and still slower than XP on Vista SP1. It is no surprise that the channel 9 videos often show MS Engineers with Winzip, though maybe that's for other features.

  • The Vista file copy has got to be the most frustrating and annoying part of Vista, especially over a network. How many times have I stared at the thing saying "0%... Calculating Time Remaining" and it just sits there. Horrible, horrible, horrible, how could they screw this up this badly?

  • Thank you for being honest about the memory pressure issue.  This has always been a source of great frustration for me in Windows.  Why can't the cache manager cap the amount of memory it uses to a reasonable (and preferably tweakable) value?  I think it's really disgraceful for a file copy operation to "force other useful code and data out" of memory, especially to the extent that it does.  Valuable memory contents are discarded just to cache hundreds of MB of copied file data that is almost never accessed before it leaves the cache.  Why...

  • Excellent article, as always!  This is vaguely related to file copying and performance, but one of the features I like most about Vista is Previous Versions.  I was quite surprised--and thrilled--to find out that this was accessible remotely via the admin share.  What a great admin tool--it has already saved the day several times!  

    If I could make a request, it would be for a continual file change monitoring system (rather than preset intervals), at least for user files.  Maybe this would be better implemented within the application itself, but it would be awesome if Windows took this to the next level and had some kind of continual, point-in-time restore feature for user files.

  • slow file access/copy speeds with SP1 are only slightly better and still exist! 4-16 MB/s MAX. with any hd (disk to disk, disk to all external usb 2.0 disks every tried). all patches and latest drivers installed. f.i. with all sony vaio sz 1-7 notebooks so far. read all the issues:

  • @Mark Anon: I think that's got a lot to do with the 'Attachment Security' behaviour - if the ZIP was downloaded from the web and has the attachment security enabled ('This file came from another computer' in Properties), the extract engine applies attachment security to all the contents. This typically breaks CHM files.

    WinZip 11 understands attachment security and if the ZIP is marked, will do the same, which is very slow. Clicking Unblock in Properties causes it not to apply attachment security, and extraction goes back to the expected speed.

    WinZip 11 is still faster than the built-in ZIP feature, however.

  • "In Vista, Explorer also waits 12 seconds before making an estimate of the copy’s duration"

    Question on this. Until SP1 for Vista comes out, is there a Registry modification we can make on Vista that would decrease this value? Thanks!

  • Vista sucks - plain and simple.   While file performance is one of the worst parts of Vista, network throughput is pathetic and gaming performance sucks quite a bit as well.   Sure you can buy faster hardware but then why would you throttle it with Vista - why not use a faster OS such as XP or Linux?

    Yes, I truly hate Vista but I don't hate MS and I haven't given up on MS either.  I certainly don't think Vista can be fixed any more than a house full of mold should be fixed - far better to tear it down and start again.  I really hope MS learns SOMETHING from the spectacular failure that is Vista.

    I bombarded them from numerous clients about how bad Vista performed and MS kept insisting - it must be your drivers, your hardware, your antivirus, your antispam, indexing should be turned off, aero should be turned off-  pretty much -make it even simpler than XP and you still won't get the performance of XP - I don't know if MS needs to hire just MIT people but I think they should stop hiring community college drop outs if they can't figure out with a ton of feedback that Vista is SLOW.

  • Excellent article Mark.

    I was quite interested in the copy algortitm improvements on SP1, and now I have the clue.

    That's very interesting, because some bad minds, thing the slowness during copy is a bug on Vista.

  • One of the biggest improvements I noticed with Vista's copy operations (at least in Explorer) is the fact that if it encounters an error during the process, it doesn't cancel the entire operation as it would in previous versions of Windows. You may have to skip the file, but I'm glad it doesn't kill the entire transfer. I'll take the slower performance if I get a more stable copy operation.

    It's the little improvements that nobody ever mentions that make Vista better than XP. Still, people continue to treat Vista like the plague. I'm not a big fan of Microsoft, but I have to admit that *gasp* I actually LIKE Vista. That's a first for a Microsoft OS in my case.

  • I'm running a Dutch Vista Ultimate version. I wonder if i could install SP1 when i change the language to English. As usual Mark's article is good.  Somehow i managed to fix 80% of the copy/move problems back in august with some tweaking and pre-sp1 updates.

  • While Vista SP1 may copy faster, it seems like one cannot do anything while the system is copying. IE takes ages to load, things just seem slow. I hope that is addressed and fixed later, along with the Windows rot over time.

  • vista's Windows Explorer user experience is truly the most notable improvement over other windows versions as far as an everyday work flow is concerned. so many detailed and small improvements that i cannot live without it anymore. readjusting the folder view positioning automatically, etc. brilliant.

    performance, though, needs to be improved.

  • It's possible that a XX-billion dollar company can somehow miss and not realize the simplest of truths that a single individual home or business user knows with his/her pocketbook.

    This is called missing the mark.

    I'll make this very clear.

    My statement is the single most correct, absolutely pure, direct, straightforward possible wording that can be realized.

    Due to the strangest of chaotic influences, it's possible that the next release of Windows can actually manage to *at the very least* not meet these simple executive orders from a lowly end-user.

    Here's the rules:

    Do *NOT* ship us a new OS unless:

    1) File copies and general IO ops are faster than or equal to the previous release

    2) Application loading times are faster than or equal to the previous release

    3) Frames per second in games/3d is faster than or equal to the previous release

    4) CPU usage on 2d drawing is lower than or equal to previous release

    5) UI responsiveness is faster than or equal to the previous release

    6) Boot time is lower than or equal to previous release

    7) General performance (ie, heap manager) is faster than or equal to previous release

    Before shipping windows, go through a list of these.

    If each point is not met,

    Do *NOT* ship Windows.

    I repeat.

    If fps in games is lower, do *NOT* ship Windows. There's nothing else to analyze, consider, or figure out. We do not want the product if FPS has lowered.

    How can it be any more clear?

    It doesn't get any simpler.