What is WofCompressedData? Does WOF mean that Windows is a dog?
A customer doing performance analysis of their program discovered that there were reads from an alternate data stream called WofCompressedData. On the Internet, if you search for “WofCompressedData”, you mostly see people wondering what it is. Some people suspect that it’s malware, and others suspect (or even state confidently) that it’s an artifact of anti-malware software and can be deleted.
What is WofCompressedData?
The documentation for wofapi.h says merely “This header is used by Data Access and Storage.” For more information, it refers you to another web page that contains no additional information.
WOF stands for Windows Overlay Filter, which is a nice name that doesn’t really tell you much about what it does or what it’s for.
First, let’s look at how Windows was installed before the introduction of the Windows Overlay Filter.
The Windows installation begins with a install.wim file that contains basically all of Windows. A WIM file is a container file, similar in spirit to other container files, like ZIP and Cabinet. Traditionally, the WIM file is copied to the recovery partition for use during emergencies, such as push-button reset. The contents of the WIM file are then uncompressed, and corresponding files are created on your boot volume, and it is these uncompressed files that are used when you run Windows. The WIM file sits in your recovery partition, ignored, but waiting for its opportunity to spring into action should the need arise.
This traditional layout means that every Windows system file is present twice: A compressed copy is in the WIM file on the recovery partition, and an uncompressed copy in the live Windows installation.
Windows 8.1 introduced a feature known as Windows Image File Boot (WIMBoot): A system manufacturer can set up a system so that the recovery partition contains the install.wim file as well as a custom.wim file which contains the OEM customizations, such as drivers for any special hardware. But instead of uncompressing the files and putting them into the live Windows installation, WIMBoot creates tiny little stub files in the live Windows installation that say, “Hey, um, I’m just a stub. If you want to see the contents, you want to uncompress those bytes over there.” WIMBoot therefore avoids the duplication by allowing the live Windows installation to share the disk storage with the WIM file on the recovery partition.
Furthermore, since the file contents in the WIM are compressed, this reduces disk I/O, though naturally at a cost of higher CPU usage in order to perform the decompression.
The way this magic works is that the live Windows files are formally sparse NTFS files, so that when you ask for the file size, you get the correct number, even though there is no actual data in them. When you open the file, the Windows Overlay Filter steps in and generates the data by decompressing the data in the WIM file on demand.
Unlike native NTFS file compression, the Windows Overlay Filter supports only read operations. This means that it doesn’t need to sector-align each compressed chunk,¹ so the compressed data can be packed more tightly together. If you open the file for writing,² the Windows Overlay Filter just decompresses the entire file, turning it back into a plain file.³ At the time WIMBoot was released, there was also a guidance document warning you not to run around opening files for writing unnecessarily. Not opening files for writing unnecessarily is good advice in general, but it’s particular important for WIMBoot in order to prevent unnecessary conversion.
The Windows Overlay Filter can take advantage of newer compression algorithms developed over the past 20 years, algorithms which produce better compression ratios, can be run in parallel on multiple cores, and which require less CPU and memory for decompression. It can also use algorithms tailored to the scenario: For example, it can choose algorithms where compression is expensive but decompression is cheap.
Changing the native NTFS file compression would be a disk format breaking change, which is not something taken lightly. Doing it as a filter provides much more flexibility. The downside is that if you mount the volume on a system that doesn’t support the Windows Overlay Filter, all you see is an empty file. Fortunately, WOF is used only for system-installed files, and if you are mounting the volume onto another system, it’s probably for data recovery purposes, so you’re interested in user data, not system files.
It’s called the “Windows Overlay Filter” because it “overlays” virtual files into a directory that also contains normal physical files.
When you read through the above description, you may have realized something: Whenever Windows Update updates a file, that file is converted from a virtual file to a plain uncompressed physical file because the file’s backing data is no longer in the WIM file. This means that over time, the Windows system files occupy more and more disk space as more of them no longer match the copy in the WIM and revert from their compressed form to their uncompressed form.
Windows 10 introduced a feature known as Compact OS, which takes a different approach. With Compact OS, the Windows Overlay Filter gains the ability to recompress files: Based on a hardware performance check, the system may decide to take the updated files, recompress them, store the compressed data in the WofCompressedData alternate data stream, and free the original uncompressed data using the same “sparse file” trick to make the file appear as if it were a normal file.
If you open one of these recompressed files, the file is decompressed on the fly based on data in the WofCompressedData alternate data stream. And as before, if you open one of these files for writing, then the file reverts to its uncompressed form.
Bonus chatter: You can use the WofShouldCompressBinaries function to determine whether the system is using WOF to compress system files. From the command line, you can use the compact.exe program to inspect the compression state of a file, or of the system.
Oh, and going back to the customer’s original question: the system’s choice to use Windows Overlay Filter compression spends a small amount of parallel computation in order to save a small amount of I/O. It’s theoretically possible that you stumbled across a hardware configuration where the system’s automatic evaluation suggested using the Windows Overlay Filter even though it was a net performance loss. I guess that would happen if you had a really fast storage device attached to a low-end CPU, and it somehow managed to trick the the automatic evaluation into thinking that compression was a good idea. In practice, it is rather unusual to have a hardware configuration consisting of fast storage and a slow CPU.
Many thanks to Malcolm Smith for his assistance with this article.
¹ The sector alignment was necessary to permit data to be rewritten into the middle of the file. But since the Windows Overlay Filter doesn’t support writing, it doesn’t need to enforce sector alignments.
² Since these files are Windows system files, opening them for writing requires administrator access. Normal usage therefore would not trigger a full decompression.
³ This “decompress on write” behavior merely describes the current behavior and is not contractual.
This means that or This means [duplicate]
I’m currently reworking my technical report and was wondering which of the following wordings is correct. If both are correct, which one is the better one?
This means that the behavior of the cmd.exe is accessible [. ].
This means the behavior of the cmd.exe is accesible [. ].
2 Answers 2
Both are correct. I’d usually go for the first one because it’s clearer, the brain has less work to do to parse the sentence. It could depend on the rest of the sentence though; if you have fifty thousand «that»s you might want to take a few out to improve the flow, or restructure the sentence entirely.
Both the sentences are correct and it seems the second one is better. The ‘that clause‘ here acts as the object of ‘means’. The meaning of the sentence is clear even without the presence of the conjunction ‘that’ and it has nothing extra to add to the meaning as well.
The inclusion or omission of ‘that’ is a matter of formal or informal style. We can often leave out the conjunction or relative pronoun ‘that’ in the informal style.
‘That‘ cannot be dropped after certain verbs (e.g. reply, telegraph, shout), and it is not dropped after nouns.
She shouted that she was busy.
NOT, She shouted she was busy.*
He disagreed with Copernicus’ view that the earth went round the sun.
NOT, He disagreed with Copernicus’ view the earth went round the sun.
But, we can usually leave out the relative pronoun that when it is the object in a relative clause.
Look! There are the people (that) we met in Brighton.
Do it the way (that) I showed you.
what this means is that .
1 This means that you do not believe that day will ever come. In other words you are being sarcastic.
2 It means that we hope the situation improves.
3 means that the starter uses the full voltage, and only runs the motor in one direction.
4 это означает, что
In this case the reaction is exothermic; that is ( to say that) energy is released.
What this means is (or By this is meant that) classical broad-band excitation of the Sl state results in.
5 это означает, что
6 М-80
7 значить
8 означать
9 практика
10 как сивый мерин
11 что
он не знает, что это значит — he does not know what this means
для чего это употребляется, служит? — what is that used for?
что он, она и т. д. из себя представляет? — what is he, she, etc . , like?
он пришёл поздно, что не было обычно — he came late, which was not usual
(та) книга, что на столе — the book that is on the table
(та) книга, что он дал ей — the book that he gave her; the book he gave her
это всё, что там написано — that is all that is written there
всё, что он знал — all he knew
тот самый. что — the same. that
это та самая книга, что он дал ей — this is the very book that he gave her
дайте ему не это письмо, а то, что она принесла вчера — do not give him this letter, but the one she brought yesterday
то, что — what
он помнит то, что она сказала — he remembers what she said
это не то, что он думал — it is not what he thought
это не то, чего он ожидал — it is not what he expected
что. что ( одно. другое ) — this. that:
что оставил, что взял с собой — this he left, that he took with him
что за, что. за разг. — ( при вопросе: какой ) what; ( какого рода и т. п. ) what kind / sort of; ( при восклицании ) what (+ a , an , если данное слово может употребляться с неопред. артиклем )
что за книги там?, что там за книги? — what are those books over there?
♢ что до — with regard to, concerning
что до него, он согласен — as to / for him, he agrees
что до меня. — as far as I am concerned.
что ему и т. д. до этого — what does he, etc . , care for / about it; what does it matter to him, etc .
что ж, он сделает это см — why, he will do it himself
ну и что ж(е)? — well, what of that?
оставить это здесь, что ли? — perhaps leave it here; leave* it here, eh?
что ни день, погода меняется — the weather changes every day
что он ни скажет, интересно — whatever he says is interesting
что пользы, что толку разг. — what is the use / sense
не что иное как — nothing other than, nothing less than, nothing short of
это ему хоть бы что — that is nothing to him; he thinks nothing of that
чего бы не — what:
чего только не — what. not:
чего только он не видел! — what hasn’t he seen!, the things he has seen!
в чём дело?, что случилось? — what is the matter?
с чего бы это вдруг? — what’s the cause?, now, why?
что и говорить разг. — there is no denying, it cannot be denied
не понимать, что к чему — not know* what is what
знать, что к чему — know* the how and why of things
уйти ни с чем — go* away empty-handed, или having achieved nothing; get* nothing for one’s pains
он сказал, что она придёт — he said (that) she would come
это так просто, что каждый поймёт — it is so simple that anybody can understand it
это такое трудное слово, что он не может его запомнить — it is such a difficult word that he cannot remember it
то, что — (the fact) that
то, что он это сделал, их удивило — (the fact) that he did it surprised them
он узнал о том, что она уехала — he learnt that she had left
они узнали, думали, воображали, предполагали и т. п. , что он умный человек — they knew, thought, imagined, supposed, etc . , him to be a clever man*
они ожидали, что он придёт — they expected him to come
потому. что см. потому I
12 что
что э́то (тако́е)? — what is this?
что зна́чит э́то сло́во? — what does this word mean?
он не зна́ет, что э́то зна́чит — he does not know what this means
что (вы сказа́ли)? — what did you say?
что е́сли он не придёт? — what if he does not come?
что де́лать? — what is to be done?
для чего́ э́то употребля́ется / слу́жит? — what is it (used) for?
что он из себя́ представля́ет? — what kind of person is he?
он пришёл по́здно, что бы́ло необы́чно — he came late, which was unusual
(та) кни́га, что на столе́ — the book that / which is on the table
(та) кни́га, что он дал ей — the book (that) he gave her
э́то всё, что там напи́сано — that is all that is written there
всё, что он знал — all he knew
э́то та са́мая кни́га, что он дал ей — this is the very book he gave her
да́йте ему́ не э́то письмо́, а то, что она́ принесла́ вчера́ — don’t give him this letter, but the one she brought yesterday
е́сли что случи́тся — if anything happens
что. что (одно. другое) — this. that; some. other
что оста́вил, что взял с собо́й — this [some things] he left, that [other things] he took with him
что вы! (нет, не верно) — no!, by no means!, far from it!
что до — as for; with regard to, concerning
что до него́, он согла́сен — as to / for him, he agrees
что до меня́ — as for me; as far as I am concerned
что ему́ до э́того — what does he care for / about it; what does it matter to him
что ж, он сде́лает э́то сам — well [all right], he will do it himself
что ж(е) из э́того?, ну и что ж(е)? — well, what of that?; so what does it mean?
(ну и) что ж(е), что. — what does it matter if.
ну и что ж, что он не умён — what does it matter if he is not too bright
что за, что. за разг. (при вопросе: какой) — what; (какого рода и т.п.) what kind / sort of; ( при восклицании) what (+ a, an, если данное слово может употребляться с неопределённым артиклем)
что за кни́ги там?, что там за кни́ги? — what books are those?
что э́то за де́рево? — what kind of tree is it?
что и говори́ть вводн. сл. разг. — there is no denying; it has to be admitted; let’s face it
что к чему́ — what is what
не понима́ть, что к чему́ — not know what is what
знать, что к чему́ — know the how and why of things; know a thing or two
что ли разг. — perhaps, may be
оста́вить э́то здесь, что ли? — shall I perhaps leave it here?
что ни день, пого́да меня́ется — the weather changes every day
что он ни ска́жет, интере́сно — whatever he says is interesting
что бы ни случи́лось — whatever happens
что по́льзы / про́ку / то́лку разг. — what is the use / sense
что с ва́ми? — what is the matter with you?
что тут тако́го? — what’s wrong with that?
в чём де́ло?, что случи́лось? — what is the matter?
не что ино́е как — nothing other than, nothing less than, nothing short of
ни за что — 1) (тж. ни за что на све́те; ни в коем случае) not for anything in the world 2) ( напрасно) for nothing at all
оста́ться ни при чём — get nothing for one’s pains
с чего́ бы э́то вдруг? — what’s the cause?, now, why?
то, что — what
он по́мнит то, что она́ сказа́ла — he remembers what she said
э́то не то, что он ду́мал — it is not what he thought
э́то не то, чего́ он ожида́л — it is not what he expected
уйти́ ни с чем — go away empty-handed; get nothing for one’s pains
чего́ бы. не — what. wouldn’t
чего́ бы он не дал за э́то! — what wouldn’t he give for that!
чего́ до́брого разг. — may. for all I know
он чего́ до́брого опозда́ет — he may be late for all I know
чего́ там разгова́ривать — what is the use of talking
чего́ то́лько. не — what. not
чего́ то́лько он не ви́дел! — what hasn’t he seen!, the things he has seen!; there’s precious [‘pre-] little he hasn’t seen!
он сказа́л, что она́ придёт — he said (that) she would come
э́то так про́сто, что ка́ждый поймёт — it is so simple that anybody can understand it
э́то тако́е тру́дное сло́во, что он не мо́жет его́ запо́мнить — it is such a difficult word that he cannot remember it
то, что — (the fact) that
то, что он э́то сде́лал, их удиви́ло — (the fact) that he did it surprised them
он узна́л о том, что она́ уе́хала — he learnt that she had left
они́ узна́ли [ду́мали, вообража́ли, предполага́ли], что он у́мный челове́к — they knew [thought, imagined, supposed] him to be a clever man
они́ ожида́ли, что он придёт — they expected him to come
что. что (как. так и) — whether. or
он всегда́ мра́чный — что до́ма, что на рабо́те — he is always gloomy, whether at home or at work
потому́. что — см. потому I
что ты не ложи́шься спать? — why aren’t you going to bed?
что же ты молча́ла? — why didn’t you say anything?
13 что
он не знает, что это значит — he does not know what this means
он пришел поздно, что не было обычно — he came late, which was not usual
та книга, что на столе — the book that is on the table
(та) книга, что он дал ей — the book that he gave her; the book he gave her
это все, что там написано — that is all that is written there
все, что он знал — all he knew
дайте ему не это письмо, а то, что она принесла вчера — do not give him this letter, but the one she brought yesterday
к чему — what for, what’s the use of
не за что — don’t mention it, no problem
не к чему — there is no sense/point in doing
не что иное как — nothing but, nothing else than, nothing more nor less than
ни к чему — разг. it is of no use to smb.; there is no need to do smth.
что к чему — what’s what, what it’s all about
что ли — разг. perhaps, maybe, or something
А мне-то что? — what’s it to me, what do I care?
вот что — now, look here
к чему бы это — разг. what could that mean
ни за что на свете — not for anything, not for all the world
уйти ни с чем — to go away empty-handed, to go away having achieved nothing; to get nothing for one’s pains
что ни слово, то ложь — every word is a lie
то, что — (the fact) that
он сказал, что она придет — he said (that) she would come
это так просто, что каждый поймет — it is so simple that anybody can understand it
это такое трудное слово, что он не может его запомнить — it is such a difficult word that he cannot remember it
то, что он это сделал, их удивило — (the fact) that he did it surprised them
он узнал о том, что она уехала — he learnt that she had left
они думали, что он умный человек — they thought him to be a clever man
они ожидали, что он придет — they expected him to come
14 исчезнувший
This means that there may be 10 3 to 10 4 as many defunct pulsars as live ones.
15 Вы отдаёте себе отчёт, что это значит?
16 Смысл его в том
17 аппликационная кожная проба /у клиентов/ (Эту пробу применяют главным образо
18 лечение под
19 не может быть получено
20 объясните мне, что это значит
См. также в других словарях:
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