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diff --git a/en/using-d-i/modules/partman-md.xml b/en/using-d-i/modules/partman-md.xml new file mode 100644 index 000000000..9dffad9f8 --- /dev/null +++ b/en/using-d-i/modules/partman-md.xml @@ -0,0 +1,300 @@ +<!-- retain these comments for translator revision tracking --> +<!-- $Id$ --> + + <sect3 id="partman-md"> + <title>Configuring Multidisk Devices (Software RAID)</title> +<para> + +If you have more than one harddrive<footnote><para> + +To be honest, you can construct an MD device even from partitions +residing on single physical drive, but that won't give any benefits. + +</para></footnote> in your computer, you can use +<command>partman-md</command> to set up your drives for increased +performance and/or better reliability of your data. The result is +called <firstterm>Multidisk Device</firstterm> (or after its most +famous variant <firstterm>software RAID</firstterm>). + +</para><para> + +MD is basically a bunch of partitions located on different disks and +combined together to form a <emphasis>logical</emphasis> device. This +device can then be used like an ordinary partition (i.e. in +<command>partman</command> you can format it, assign a mountpoint, +etc.). + +</para><para> + +What benefits this brings depends on the type of MD device you are +creating. Currently supported are: + +<variablelist> +<varlistentry> + +<term>RAID0</term><listitem><para> + +Is mainly aimed at performance. RAID0 splits all incoming data into +<firstterm>stripes</firstterm> and distributes them equally over each +disk in the array. This can increase the speed of read/write +operations, but when one of the disks fails, you will lose +<emphasis>everything</emphasis> (part of the information is still on +the healthy disk(s), the other part <emphasis>was</emphasis> on the +failed disk). + +</para><para> + +The typical use for RAID0 is a partition for video editing. + +</para></listitem> +</varlistentry> +<varlistentry> + +<term>RAID1</term><listitem><para> + +Is suitable for setups where reliability is the first concern. It +consists of several (usually two) equally-sized partitions where every +partition contains exactly the same data. This essentially means three +things. First, if one of your disks fails, you still have the data +mirrored on the remaining disks. Second, you can use only a fraction +of the available capacity (more precisely, it is the size of the +smallest partition in the RAID). Third, file-reads are load-balanced among +the disks, which can improve performance on a server, such as a file +server, that tends to be loaded with more disk reads than writes. + +</para><para> + +Optionally you can have a spare disk in the array which will take the +place of the failed disk in the case of failure. + +</para></listitem> +</varlistentry> +<varlistentry> + +<term>RAID5</term><listitem><para> + +Is a good compromise between speed, reliability and data redundancy. +RAID5 splits all incoming data into stripes and distributes them +equally on all but one disk (similar to RAID0). Unlike RAID0, RAID5 +also computes <firstterm>parity</firstterm> information, which gets +written on the remaining disk. The parity disk is not static (that +would be called RAID4), but is changing periodically, so the parity +information is distributed equally on all disks. When one of the +disks fails, the missing part of information can be computed from +remaining data and its parity. RAID5 must consist of at least three +active partitions. Optionally you can have a spare disk in the array +which will take the place of the failed disk in the case of failure. + +</para><para> + +As you can see, RAID5 has a similar degree of reliability to RAID1 +while achieving less redundancy. On the other hand, it might be a bit +slower on write operations than RAID0 due to computation of parity +information. + +</para></listitem> +</varlistentry> +<varlistentry> + +<term>RAID6</term><listitem><para> + +Is similar to RAID5 except that it uses two parity devices instead of +one. + +</para><para> + +A RAID6 array can survive up to two disk failures. + +</para></listitem> +</varlistentry> +<varlistentry> + +<term>RAID10</term><listitem><para> + +RAID10 combines striping (as in RAID0) and mirroring (as in RAID1). +It creates <replaceable>n</replaceable> copies of incoming data and +distributes them across the partitions so that none of the copies of +the same data are on the same device. +The default value of <replaceable>n</replaceable> is 2, but it can be +set to something else in expert mode. The number of partitions used +must be at least <replaceable>n</replaceable>. +RAID10 has different layouts for distributing the copies. The default is +near copies. Near copies have all of the copies at about the same offset +on all of the disks. Far copies have the copies at different offsets on +the disks. Offset copies copy the stripe, not the individual copies. + +</para><para> + +RAID10 can be used to achieve reliability and redundancy without the +drawback of having to calculate parity. + +</para></listitem> +</varlistentry> +</variablelist> + +To sum it up: + +<informaltable> +<tgroup cols="5"> +<thead> +<row> + <entry>Type</entry> + <entry>Minimum Devices</entry> + <entry>Spare Device</entry> + <entry>Survives disk failure?</entry> + <entry>Available Space</entry> +</row> +</thead> + +<tbody> +<row> + <entry>RAID0</entry> + <entry>2</entry> + <entry>no</entry> + <entry>no</entry> + <entry>Size of the smallest partition multiplied by number of devices in RAID</entry> +</row> + +<row> + <entry>RAID1</entry> + <entry>2</entry> + <entry>optional</entry> + <entry>yes</entry> + <entry>Size of the smallest partition in RAID</entry> +</row> + +<row> + <entry>RAID5</entry> + <entry>3</entry> + <entry>optional</entry> + <entry>yes</entry> + <entry> + Size of the smallest partition multiplied by (number of devices in + RAID minus one) + </entry> +</row> + +<row> + <entry>RAID6</entry> + <entry>4</entry> + <entry>optional</entry> + <entry>yes</entry> + <entry> + Size of the smallest partition multiplied by (number of devices in + RAID minus two) + </entry> +</row> + +<row> + <entry>RAID10</entry> + <entry>2</entry> + <entry>optional</entry> + <entry>yes</entry> + <entry> + Total of all partitions divided by the number of chunk copies (defaults to two) + </entry> +</row> + +</tbody></tgroup></informaltable> + +</para><para> + +If you want to know more about Software RAID, have a look +at <ulink url="&url-software-raid-howto;">Software RAID HOWTO</ulink>. + +</para><para> + +To create an MD device, you need to have the desired partitions it +should consist of marked for use in a RAID. (This is done in +<command>partman</command> in the <guimenu>Partition +settings</guimenu> menu where you should select <menuchoice> +<guimenu>Use as:</guimenu> <guimenuitem>physical volume for +RAID</guimenuitem> </menuchoice>.) + +</para><note><para> + +Make sure that the system can be booted with the partitioning scheme +you are planning. In general it will be necessary to create a separate +file system for <filename>/boot</filename> when using RAID for the root +(<filename>/</filename>) file system. +Most boot loaders <phrase arch="x86">(including lilo and grub)</phrase> +do support mirrored (not striped!) RAID1, so using for example RAID5 for +<filename>/</filename> and RAID1 for <filename>/boot</filename> can be +an option. + +</para></note><para> + +Next, you should choose <guimenuitem>Configure software +RAID</guimenuitem> from the main <command>partman</command> menu. +(The menu will only appear after you mark at least one partition for +use as <guimenuitem>physical volume for RAID</guimenuitem>.) +On the first screen of <command>partman-md</command> simply select +<guimenuitem>Create MD device</guimenuitem>. You will be presented with +a list of supported types of MD devices, from which you should choose +one (e.g. RAID1). What follows depends on the type of MD you selected. +</para> + +<itemizedlist> +<listitem><para> + +RAID0 is simple — you will be issued with the list of available +RAID partitions and your only task is to select the partitions which +will form the MD. + +</para></listitem> +<listitem><para> + +RAID1 is a bit more tricky. First, you will be asked to enter the +number of active devices and the number of spare devices which will +form the MD. Next, you need to select from the list of available RAID +partitions those that will be active and then those that will be +spare. The count of selected partitions must be equal to the number +provided earlier. Don't worry. If you make a mistake and +select a different number of partitions, &d-i; won't let you +continue until you correct the issue. + +</para></listitem> +<listitem><para> + +RAID5 has a setup procedure similar to RAID1 with the exception that you +need to use at least <emphasis>three</emphasis> active partitions. + +</para></listitem> +<listitem><para> + +RAID6 also has a setup procedure similar to RAID1 except that at least +<emphasis>four</emphasis> active partitions are required. + +</para></listitem> +<listitem><para> + +RAID10 again has a setup procedure similar to RAID1 except in expert +mode. In expert mode, &d-i; will ask you for the layout. +The layout has two parts. The first part is the layout type. It is either +<literal>n</literal> (for near copies), <literal>f</literal> (for far +copies), or <literal>o</literal> (for offset copies). The second part is +the number of copies to make of the data. There must be at least that +many active devices so that all of the copies can be distributed onto +different disks. + +</para></listitem> +</itemizedlist> + +<para> + +It is perfectly possible to have several types of MD at once. For +example, if you have three 200 GB hard drives dedicated to MD, each +containing two 100 GB partitions, you can combine the first partitions on +all three disks into the RAID0 (fast 300 GB video editing partition) +and use the other three partitions (2 active and 1 spare) for RAID1 +(quite reliable 100 GB partition for <filename>/home</filename>). + +</para><para> + +After you set up MD devices to your liking, you can +<guimenuitem>Finish</guimenuitem> <command>partman-md</command> to return +back to the <command>partman</command> to create filesystems on your +new MD devices and assign them the usual attributes like mountpoints. + +</para> + </sect3> |