Confused: Read length and file fragmentation

NTFSD
1

Hello,

In a win32 application I open a big and badly fragmented file
in non-buffering mode and read a big chunk of data

fh = CreateFile(…, FILE_FLAG_NO_BUFFERING,…);
ReadFile(fh, buf, 1048576, &bytesRead, NULL);

Then in my disk filter driver, the ReadDispatch routine gets an
IRP with

pIrpStack->Parameters.Read.ByteOffset.QuadPart == xxxxxxxx
pIrpStack->Parameters.Read.Length == 1048576

Since the file is fragmented, should the driver get a bunch of IPRs
with disjoint offsets and shorter lengths instead of a big one? what
am I missing here?

TIA,
Chu Bun

2

The file is obviously not fragmented if the filesystem is requesting all
of the data in one continuous chunk. You’re sure you’re filtering the
correct device?

  • Nick Ryan

-----Original Message-----
From: xxxxx@lists.osr.com
[mailto:xxxxx@lists.osr.com] On Behalf Of ChuBun
Sent: Thursday, June 26, 2003 1:16 PM
To: File Systems Developers
Subject: [ntfsd] Confused: Read length and file fragmentation

Hello,

In a win32 application I open a big and badly fragmented file
in non-buffering mode and read a big chunk of data

fh = CreateFile(…, FILE_FLAG_NO_BUFFERING,…);
ReadFile(fh, buf, 1048576, &bytesRead, NULL);

Then in my disk filter driver, the ReadDispatch routine gets
an IRP with

pIrpStack->Parameters.Read.ByteOffset.QuadPart == xxxxxxxx
pIrpStack->Parameters.Read.Length == 1048576

Since the file is fragmented, should the driver get a bunch
of IPRs with disjoint offsets and shorter lengths instead of
a big one? what am I missing here?

TIA,
Chu Bun

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3

And how is the object manager gonna know about it?
Usually you would get paging I/O IRPs, if you used buffered mode, but
since you used non buffering one, all is left to the file system to
handle.

ChuBun wrote:

Hello,

In a win32 application I open a big and badly fragmented file
in non-buffering mode and read a big chunk of data

fh = CreateFile(…, FILE_FLAG_NO_BUFFERING,…);
ReadFile(fh, buf, 1048576, &bytesRead, NULL);

Then in my disk filter driver, the ReadDispatch routine gets an
IRP with

pIrpStack->Parameters.Read.ByteOffset.QuadPart == xxxxxxxx
pIrpStack->Parameters.Read.Length == 1048576

Since the file is fragmented, should the driver get a bunch of IPRs
with disjoint offsets and shorter lengths instead of a big one? what
am I missing here?

TIA,
Chu Bun

You are currently subscribed to ntfsd as: xxxxx@alfasp.com
To unsubscribe send a blank email to xxxxx@lists.osr.com

–
Kind regards, Dejan M. MVP for DDK
http://www.alfasp.com E-mail: xxxxx@alfasp.com
Alfa Transparent File Encryptor - Transparent file encryption services.
Alfa File Protector - File protection and hiding library for Win32
developers.
Alfa File Monitor - File monitoring library for Win32 developers.

4

I shouldn’t reply too early in the morning, I see:-) (I didn’t notice
you were talking about disk and not FS filter. Or are you?)
And are you sure the file is fragmented where you suppose it is
fragmented?

ChuBun wrote:

Hello,

In a win32 application I open a big and badly fragmented file
in non-buffering mode and read a big chunk of data

fh = CreateFile(…, FILE_FLAG_NO_BUFFERING,…);
ReadFile(fh, buf, 1048576, &bytesRead, NULL);

Then in my disk filter driver, the ReadDispatch routine gets an
IRP with

pIrpStack->Parameters.Read.ByteOffset.QuadPart == xxxxxxxx
pIrpStack->Parameters.Read.Length == 1048576

Since the file is fragmented, should the driver get a bunch of IPRs
with disjoint offsets and shorter lengths instead of a big one? what
am I missing here?

–
Kind regards, Dejan M. MVP for DDK
http://www.alfasp.com E-mail: xxxxx@alfasp.com
Alfa Transparent File Encryptor - Transparent file encryption services.
Alfa File Protector - File protection and hiding library for Win32
developers.
Alfa File Monitor - File monitoring library for Win32 developers.

5

What I have is a mirror disk driver. My concern is the driver will get
only a big IRP and when I pass it down to the lower device, I’ll
get back incorrect data.
Maybe my test file is not fragmented. I’ll try an outlook file pst file
later. I
know outlook files are very fragmented.

Regards,

“ChuBun” wrote in message news:xxxxx@ntfsd…
>
> Hello,
>
> In a win32 application I open a big and badly fragmented file
> in non-buffering mode and read a big chunk of data
>
> fh = CreateFile(…, FILE_FLAG_NO_BUFFERING,…);
> ReadFile(fh, buf, 1048576, &bytesRead, NULL);
>
> Then in my disk filter driver, the ReadDispatch routine gets an
> IRP with
>
> pIrpStack->Parameters.Read.ByteOffset.QuadPart == xxxxxxxx
> pIrpStack->Parameters.Read.Length == 1048576
>
> Since the file is fragmented, should the driver get a bunch of IPRs
> with disjoint offsets and shorter lengths instead of a big one? what
> am I missing here?
>
> TIA,
> Chu Bun
>
>
>
>
>
>
>
>

6

Don’t ASSUME - test! There are APIs to tell if a file is fragmented and
exactly how fragmented it is - use them.

ChuBun wrote:

What I have is a mirror disk driver. My concern is the driver will get
only a big IRP and when I pass it down to the lower device, I’ll
get back incorrect data.
Maybe my test file is not fragmented. I’ll try an outlook file pst file
later. I know outlook files are very fragmented.

–
Kind regards, Dejan M. MVP for DDK
http://www.alfasp.com E-mail: xxxxx@alfasp.com
Alfa Transparent File Encryptor - Transparent file encryption services.
Alfa File Protector - File protection and hiding library for Win32
developers.
Alfa File Monitor - File monitoring library for Win32 developers.

7

FSCTL_GET_RETRIEVAL_POINTERS returns the disk cluster numbers
underlying a file.

Max

----- Original Message -----
From: “Dejan Maksimovic”
To: “File Systems Developers”
Sent: Friday, June 27, 2003 8:30 PM
Subject: [ntfsd] Re: Confused: Read length and file fragmentation

>
> Don’t ASSUME - test! There are APIs to tell if a file is
fragmented and
> exactly how fragmented it is - use them.
>
> ChuBun wrote:
>
> > What I have is a mirror disk driver. My concern is the driver
will get
> > only a big IRP and when I pass it down to the lower device, I’ll
> > get back incorrect data.
> > Maybe my test file is not fragmented. I’ll try an outlook file
pst file
> > later. I know outlook files are very fragmented.
>
> –
> Kind regards, Dejan M. MVP for DDK
> http://www.alfasp.com E-mail: xxxxx@alfasp.com
> Alfa Transparent File Encryptor - Transparent file encryption
services.
> Alfa File Protector - File protection and hiding library for Win32
> developers.
> Alfa File Monitor - File monitoring library for Win32 developers.
>
>
>
>
> —
> You are currently subscribed to ntfsd as: xxxxx@storagecraft.com
> To unsubscribe send a blank email to xxxxx@lists.osr.com