304 lines
6.1 KiB
HTML
304 lines
6.1 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML
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><HEAD
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><TITLE
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>Snapshots</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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REL="HOME"
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TITLE="LVM HOWTO"
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HREF="index.html"><LINK
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REL="UP"
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TITLE="Anatomy of LVM"
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HREF="anatomy.html"><LINK
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REL="PREVIOUS"
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TITLE="mapping modes (linear/striped)"
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HREF="mapmode.html"><LINK
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REL="NEXT"
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TITLE="Frequently Asked Questions"
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HREF="faq.html"></HEAD
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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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>LVM 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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>Prev</A
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><TD
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WIDTH="80%"
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>Chapter 3. Anatomy of LVM</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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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="sect1"
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><H1
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CLASS="sect1"
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><A
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NAME="snapshotintro"
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></A
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>3.8. Snapshots</H1
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><P
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> A wonderful facility provided by LVM is 'snapshots'. This
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allows the administrator to create a new block device which
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presents an exact copy of a logical volume, frozen at some
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point in time. Typically this would be used when some batch
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processing, a backup for instance, needs to be performed on
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the logical volume, but you don't want to halt a live system
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that is changing the data. When the snapshot device has been
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finished with the system administrator can just remove the
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device. This facility does require that the snapshot be made
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at a time when the data on the logical volume is in a
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consistent state - the VFS-lock patch for LVM1 makes sure that
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some filesystems do this automatically when a snapshot is
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created, and many of the filesystems in the 2.6 kernel do this
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automatically when a snapshot is created without patching.
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</P
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><DIV
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CLASS="warning"
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><P
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></P
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><TABLE
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CLASS="warning"
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WIDTH="100%"
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BORDER="0"
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><TR
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><TD
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WIDTH="25"
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ALIGN="CENTER"
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VALIGN="TOP"
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><IMG
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SRC="../images/warning.gif"
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HSPACE="5"
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ALT="Warning"></TD
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><TH
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ALIGN="LEFT"
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VALIGN="CENTER"
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><B
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>Full snapshot are automatically disabled</B
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></TH
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></TR
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><TR
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><TD
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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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><P
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> If the snapshot logical volume becomes full it will be dropped
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(become unusable) so it is vitally important to allocate enough space.
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The amount of space necessary is dependent on the usage of the
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snapshot, so there is no set recipe to follow for this. If the
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snapshot size equals the origin size, it will never overflow.
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</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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><P
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> LVM1 has read-only snapshots. Read-only snapshots work by
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creating an <EM
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>exception table</EM
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>, which is
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used to keep track of which blocks have been changed. If a
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block is to be changed on the origin, it is first copied to
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the snapshot, marked as copied in the exception table, and
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then the new data is written to the original volume.
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</P
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><P
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> In LVM2, snapshots are read/write by default. Read/write
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snapshots work like read-only snapshots, with the additional
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feature that if data is written to the snapshot, that block is
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marked in the exception table as used, and never gets copied
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from the original volume. This opens up many new
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possibilities that were not possible with LVM1's read-only
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snapshots. One example is to snapshot a volume, mount the
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snapshot, and try an experimental program that change files on
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that volume. If you don't like what it did, you can unmount
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the snapshot, remove it, and mount the original filesystem in
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its place. It is also useful for creating volumes for use
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with <A
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HREF="http://www.cl.cam.ac.uk/Research/SRG/netos/xen/"
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TARGET="_top"
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>Xen</A
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>.
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You can create a disk image, then snapshot it and modify the
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snapshot for a particular domU instance. You can then create
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another snapshot of the original volume, and modify that one
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for a different domU instance. Since the only storage used by
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a snapshot is blocks that were changed on the origin or the
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snapshot, the majority of the volume is shared by the domU's.
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</P
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><DIV
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CLASS="note"
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><P
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></P
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><TABLE
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CLASS="note"
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WIDTH="100%"
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BORDER="0"
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><TD
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WIDTH="25"
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ALIGN="CENTER"
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VALIGN="TOP"
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><IMG
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SRC="../images/note.gif"
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HSPACE="5"
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ALT="Note"></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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> With the current LVM2/device-mapper code, the origin can be
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grown, but not shrunk. With LVM1, you cannot resize the origin.
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</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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><DIV
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CLASS="warning"
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><P
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></P
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><TABLE
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CLASS="warning"
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WIDTH="100%"
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BORDER="0"
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><TR
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><TD
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WIDTH="25"
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ALIGN="CENTER"
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VALIGN="TOP"
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><IMG
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SRC="../images/warning.gif"
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HSPACE="5"
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ALT="Warning"></TD
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><TH
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ALIGN="LEFT"
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VALIGN="CENTER"
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><B
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>LVM 1 -> LVM 2 Upgrade Info</B
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></TH
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></TR
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><TR
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><TD
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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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><P
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> Make sure to remove snapshot LVs before upgrading from
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LVM 1 to LVM 2. (See <A
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HREF="lvm2faq.html"
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>Section 4.1</A
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>)
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</P
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></TD
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>Prev</A
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WIDTH="33%"
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VALIGN="top"
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>mapping modes (linear/striped)</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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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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>Frequently Asked Questions</TD
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> |