Editing NTFS (section)

=== System compression ===
{{anchor|CompactOS algorithms|CompactOS compression|CompactOS|WIMBoot}}
Since [[Windows 10]], Microsoft has introduced new file compression scheme based on the XPRESS algorithm with 4K/8K/16K block size<ref name=ms-xca>{{cite web | url=https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-xca/a8b7cb0a-92a6-4187-a23b-5e14273b96f8 | title=&#91;MS-XCA&#93;: Xpress Compression Algorithm | date=31 January 2023 }}</ref> and the [[LZX]] algorithm;<ref name=wmilib-compression>{{cite web |url=https://wimlib.net/compression.html |title=wimlib: the open source Windows Imaging (WIM) library – Compression algorithm}}</ref> both are variants of [[LZ77]] updated with [[Huffman encoding|Huffman entropy coding]] and [[range coding]], which LZNT1 lacked. These compression algorithms were taken from [[Windows Imaging Format]] (WIM file).

The new compression scheme is used by CompactOS feature, which reduces disk usage by compressing Windows system files.<ref name=compactos>{{cite web |title=Compact OS, single-instancing, and image optimization |url=https://learn.microsoft.com/en-us/windows-hardware/manufacture/desktop/compact-os |publisher=Microsoft |access-date=1 October 2019 |language=en-us}}</ref> CompactOS is not an extension of NTFS file compression and does not use the 'compressed' attribute; instead, it sets a [[NTFS reparse point#System compression|reparse point]] on each compressed file with a WOF (Windows Overlay Filter) tag,<ref name=oldnewthing-wof>{{cite web |author=Raymond Chen |url=https://devblogs.microsoft.com/oldnewthing/20190618-00/?p=102597 |title=What is WofCompressedData? Does WOF mean that Windows is a dog? |website=Microsoft DevBlogs| date=18 June 2019 }}</ref> but the actual data is stored in an alternate data stream named "WofCompressedData", which is decompressed on-the-fly by a WOF [[Installable File System|filesystem filter]] driver, and the main file is an empty [[sparse file]].<ref name=oldnewthing-wof/> This design is meant purely for read-only access, so any writes to compressed files result in an automatic decompression.<ref name=oldnewthing-wof/><ref>{{cite web |last=Biggers |first=Eric |title= NTFS-3G plugin for reading "system compressed" files |url=https://github.com/ebiggers/ntfs-3g-system-compression |website=GitHub |accessdate=1 October 2019 |date=29 April 2019}}</ref><ref>{{cite web |title=Re: [ntfs-3g-devel] Experimental support for Windows 10 "System Compressed" files |url=https://sourceforge.net/p/ntfs-3g/mailman/message/34481588/ |website=SourceForge.net |accessdate=1 October 2019}}</ref>

CompactOS compression is intended for [[OEM]]s who prepare OS images with the {{code|/compact}} flag of the [[DISM|{{code|DISM}} tool]] in [[Windows Assessment and Deployment Kit|Windows ADK]],<ref>{{cite web | url=https://learn.microsoft.com/en-us/windows-hardware/manufacture/desktop/what-is-dism | title=DISM Overview | date=15 December 2021 }}</ref> but it can also be manually turned on per file with the {{code|/exe}} flag of the {{code|compact}} command.<ref name=compact-reference>{{cite web | url=https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/compact | title=Compact | date=3 February 2023 }}</ref> CompactOS algorithm avoids [[Fragmentation (computing)|file fragmentation]] by writing compressed data in contiguously allocated chunks, unlike core NTFS compression.{{Citation needed|date=October 2020}}

CompactOS file compression is an improved version of WIMBoot feature introduced in [[Windows 8.1]]. WIMBoot reduces Windows disk usage by keeping system files in a [[Windows Imaging Format|compressed WIM image]] on a separate hidden [[disk partition]].<ref name=wimboot>{{cite web |url=https://learn.microsoft.com/en-us/previous-versions/windows/it-pro/windows-8.1-and-8/dn594399(v=win.10) |title=Windows Image File Boot (WIMBoot) Overview|date=10 March 2015 }}</ref> Similarly to CompactOS, Windows system directories only contain [[sparse file]]s marked by a reparse point with a WOF tag, and Windows Overlay Filter driver decompresses file contents on-the-fly from the WIM image. WIMBoot is less effective than CompactOS though, as new updated versions of system files need to be written to the system partition, consuming disk space.<ref name=oldnewthing-wof/>