From - Tue Sep 16 08:02:35 1997
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From: Andy Neville <nobody@europa.com>
Newsgroups: comp.os.ms-windows.programmer.nt.kernel-mode
Subject: NT I/O Throttling
Date: Mon, 15 Sep 1997 17:09:35 -0700
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I've got a device driver that redirects file I/O intended for a virtual
drive (e.g., Z:) to a file (e.g., C:\Test.dat).  Originally when using
ZwWriteFile to write data to the file, if I copied a huge block of data
NT would write a little bit then hang.  In a previous posting, Jonathan
Goldick described this situation as NT I/O throttling - when the write
cache gets full, NT prevents "normal" writes from occurring so that the
memory manager can write out its cache - the problem being that it
blocks my writes, therefore deadlocking the file system.

Instead of using ZwWriteFile, I tried creating my own IRP and setting
the IRP_NOCACHE flag, but that didn't work any differently than
ZwWriteFile.  I noticed that the IRPs my driver gets also have the
IRP_PAGING_IO and IRP_SYNCHRONOUS_PAGING_IO flags set, and when I set
all three in the IRP I send, everything appears to work, but the data
never really gets written out.

Can anyone give me a hint about what I'm doing wrong?  I'm using
ObReferenceObjectByHandle() after ZwCreateFile to get the FileObject,
IoGetRelatedDeviceObject() to get the FSD DeviceObject, and setting
pIrp->UserBuffer equal to my 64k NonPagedPool buffer.  I even copy the
flags and control members from my stack to the new one.  The rest is:

pIrp->MdlAddress=NULL;
pIrp->AssociatedIrp.SystemBuffer=NULL;
pIrp->UserEvent=&event;
pIrp->UserIosb=&IoStatusBlock;
pIrp->Tail.Overlay.Thread=PsGetCurrentThread();
pIrp->Tail.Overlay.OriginalFileObject=pfoDeviceFile;
pIrp->RequestorMode=KernelMode;
pIrp->Flags=IRP_NOCACHE | IRP_PAGING_IO | IRP_SYNCHRONOUS_PAGING_IO;
pioStackLocation=IoGetNextIrpStackLocation(pWriteIrp);
pioStackLocation->MajorFunction=IRP_MJ_WRITE;
pioStackLocation->MinorFunction=0;
pioStackLocation->Flags=pIrpSp->Flags;
pioStackLocation->Control=pIrpSp->Control;
pioStackLocation->DeviceObject=pFsdDevice;
pioStackLocation->FileObject=pfoDeviceFile;
ioStackLocation->Parameters.Write.Length=pIrpSp->Parameters.Write.Length;

ioStackLocation->Parameters.Write.ByteOffset=pIrpSp->Parameters.Write.ByteOffset;

IoSetCompletionRoutine(pIrp, DoCompletion, 0, TRUE, TRUE, TRUE);

Thanks for any help you might be able to spare!

Andy Neville
andyn@europacom.  (please move the dot before the "com" when replying)