202 lines
3.3 KiB
HTML
202 lines
3.3 KiB
HTML
<HTML
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><HEAD
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><TITLE
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>Direct and Mmap-ed IO</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+
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"><LINK
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REL="HOME"
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TITLE="The Linux SCSI Generic (sg) HOWTO"
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HREF="index.html"><LINK
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REL="PREVIOUS"
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TITLE="SCSI_IOCTL_SEND_COMMAND"
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HREF="scsi_snd_cmd.html"><LINK
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REL="NEXT"
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TITLE="Direct IO"
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HREF="dio.html"></HEAD
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><BODY
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CLASS="CHAPTER"
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BGCOLOR="#FFFFFF"
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TEXT="#000000"
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LINK="#0000FF"
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VLINK="#840084"
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ALINK="#0000FF"
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><DIV
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CLASS="NAVHEADER"
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><TABLE
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SUMMARY="Header navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>The Linux SCSI Generic (sg) HOWTO</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="scsi_snd_cmd.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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></TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="dio.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="CHAPTER"
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><H1
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><A
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NAME="DMMIO">Chapter 9. Direct and Mmap-ed IO</H1
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><DIV
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CLASS="TOC"
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><DL
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><DT
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><B
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>Table of Contents</B
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></DT
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><DT
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>9.1. <A
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HREF="dio.html"
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>Direct IO</A
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></DT
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><DT
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>9.2. <A
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HREF="mmapio.html"
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>Mmap-ed IO</A
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></DT
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></DL
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></DIV
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><P
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>The normal action of the sg driver for a read operation (from a device)
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is to request the lower level (adapter) driver to DMA
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<A
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NAME="AEN693"
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HREF="#FTN.AEN693"
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>[1]</A
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>
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data into kernel buffers that the sg driver manages. The sg driver will
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then copy the contents of its buffers into the user space. [This sequence
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is reversed for a write operation (towards a device)]. While this double
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handling of data is obviously inefficient it does decouple some hardware
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issues from user applications. For these and historical reasons the
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"double-buffered" IO remains the default for the sg driver.</P
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><P
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>Both "direct" and "mmap-ed" IO are techniques that permit the data
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to be DMA-ed directly from the lower level (adapter) driver into
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the user application (vice versa for write operations). Both techniques
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result in faster speed, smaller latencies and lower CPU utilization
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but come at the expense of complexity (as always). For example the Linux
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kernel must not attempt to swap out pages in a user application that a
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SCSI adapter is busy DMA-ing data into.</P
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></DIV
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><H3
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CLASS="FOOTNOTES"
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>Notes</H3
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><TABLE
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BORDER="0"
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CLASS="FOOTNOTES"
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WIDTH="100%"
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="5%"
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><A
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NAME="FTN.AEN693"
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HREF="dmmio.html#AEN693"
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>[1]</A
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="95%"
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><P
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>Older SCSI adapters and some pseudo adapter drivers don't have DMA
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capability in which case the CPU is used to copy the data.</P
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></TD
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></TR
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></TABLE
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
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HREF="scsi_snd_cmd.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="index.html"
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ACCESSKEY="H"
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>Home</A
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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><A
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HREF="dio.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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>SCSI_IOCTL_SEND_COMMAND</TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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> </TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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>Direct IO</TD
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></TR
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></TABLE
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></DIV
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></BODY
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></HTML
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> |