Editing NTFS (section)

=== File compression ===
Compression is enabled on a per-folder or per-file basis by setting the 'compressed' attribute. When compression is enabled on a folder, any files moved or saved to that folder will be automatically [[data compression|compressed]] using LZNT1 algorithm (a variant of [[LZ77]]).<ref>{{cite web |url=http://msdn.microsoft.com/library/default.asp?url=/library/en-us/fileio/fs/file_compression_and_decompression.asp |title=File Compression and Decompression |publisher=MSDN Platform SDK: File Systems |access-date=2005-08-18}}</ref> The compression algorithm is designed to support cluster sizes of up to 4&nbsp;KB; when the cluster size is greater than 4&nbsp;KB on an NTFS volume, NTFS compression is not available.<ref>{{cite web |url= http://support.microsoft.com/kb/314878 |title= The Default Cluster Size for the NTFS and FAT File Systems |publisher= Microsoft |date= January 31, 2002 |access-date= 2012-01-10}}</ref> Data is compressed in 16-cluster chunks (up to 64&nbsp;KB in size); if the compression reduces 64{{nbsp}}KB of data to 60{{nbsp}}KB or less, NTFS treats the unneeded 4{{nbsp}}KB pages like empty [[sparse file]] clusters—they are not written. This allows for reasonable random-access times as the OS merely has to follow the chain of fragments.

Compression works best with files that have repetitive content, are seldom written, are usually accessed sequentially, and are not themselves compressed. Single-user systems with limited hard disk space can benefit from NTFS compression for small files, from 4{{nbsp}}KB to 64{{nbsp}}KB or more, depending on compressibility. Files smaller than approximately 900&nbsp;bytes are stored within the directory entry of the [[#Master File Table|MFT]].<ref name="How NTFS Works" />

For operating systems besides Windows, all versions of the NTFS-3G driver support reading compressed files according to its developers, but support for appending data to compressed files, which means adding new data resulting in increasing the size of a file, was added in November 2009. Overwriting existing compressed data is supported since August 2010.<ref>{{cite web |author1=Jean-Pierre André |title=Data Compression |url=http://www.tuxera.com/community/ntfs-3g-advanced/data-compression/ |website=Tuxera |access-date=25 April 2025 |archive-url=https://archive.today/20130630121815/http://www.tuxera.com/community/ntfs-3g-advanced/data-compression/ |archive-date=2013-06-30 |url-status=dead}}</ref>

==== Advantages ====
Users of fast [[multi-core processor]]s will find improvements in application speed by compressing their applications and data as well as a reduction in space used. Even when SSD controllers already compress data, there is still a reduction in I/Os since less data is transferred.<ref>{{cite web |url=http://www.tomshardware.com/reviews/ssd-ntfs-compression,3073-11.html |title=Should You Compress Data On Your SSD? |work=Tom's Hardware |publisher=Bestofmedia Group |first=Manuel |last=Masiero |date=2011-12-01 |access-date= 2013-04-05}}</ref>

According to research by Microsoft's NTFS Development team, 50–60{{nbsp}}[[Gigabyte|GB]] is a reasonable maximum size for a compressed file on an NTFS volume with a 4{{nbsp}}KB (default) cluster (block) size. This reasonable maximum size decreases sharply for volumes with smaller cluster sizes.<ref name="understanding-ntfs-compression" /> 

==== Disadvantages ====
Large compressible files become highly fragmented since every chunk smaller than 64{{nbsp}}KB becomes a fragment.<ref name="understanding-ntfs-compression">{{cite web |url=http://blogs.msdn.com/b/ntdebugging/archive/2008/05/20/understanding-ntfs-compression.aspx |title=Understanding NTFS Compression |work=Ntdebugging Blog |publisher=[[Microsoft]] |access-date=2011-03-16 |first=Dennis |last=Middleton |archive-url=https://web.archive.org/web/20110629043102/http://blogs.msdn.com/b/ntdebugging/archive/2008/05/20/understanding-ntfs-compression.aspx |archive-date=29 June 2011 |url-status=dead}}</ref><ref>{{cite web|url=https://www.forensicfocus.com/articles/shrinking-the-gap-carving-ntfs-compressed-files/|title=Shrinking the gap: carving NTFS-compressed files|access-date=2011-05-29}}</ref> Flash memory, such as [[SSD]] drives do not have the head movement delays and high [[Hard disk drive performance characteristics#Access time|access time]] of mechanical [[hard disk drives]], so fragmentation has only a smaller penalty.

If system files that are needed at boot time (such as drivers, NTLDR, winload.exe, or BOOTMGR) are compressed, the system may fail to boot correctly, because decompression filters are not yet loaded.<ref>{{cite web |url=https://technet.microsoft.com/en-us/library/cc977213.aspx |title=Disk Concepts and Troubleshooting |date=11 September 2008 |publisher=Microsoft |access-date=2012-03-26}}</ref>{{failed verification|date=May 2022}} Later editions of Windows{{which|date=January 2016}} do not allow important system files to be compressed.