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135 lines
4.5 KiB
Markdown
135 lines
4.5 KiB
Markdown
# MDADM
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MDADM is a utility to create and manage raid devices.
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For the rest of this entry `n` is the number of drives inside a raid device.
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## Usage
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### Get Information About a Raid
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To get an info for a running raid (assuming it is `/dev/md0`) run
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`mdadm -D /dev/md0`.
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### Add Disk/Partition to Raid Device
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You can add a disk or partition to a raid device by running the following.
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It is assumed you are adding a partition called `/dev/sdd1` but it could also
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be a whole drive and the mdadm drive is called `/dev/md0`.
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`mdadm --add /dev/md0 /dev/sdd1`
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This will add the disk as spare to the existing raid.
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To make the disk active the raid needs to be grown as described in the
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[regarding article](#growing-raid-device).
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### Growing Raid Device
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Growing a raid device will increase its number of active drives.
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[Check the number of active drives](#get-information-about-a-raid).
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Then change and run the following command accordingly.
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```sh
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mdadm --grow --raid-disks=5 /dev/md0
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```
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`/dev/md0` is the raid device.
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`5` is the number of disks that should be active.
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For adding disks view the [previous section](#add-diskpartition-to-raid-device).
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In the [following section](#changing-reshape-speed) methods are described that
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can be tried to affect the performance of the reshaping of a Raid array.
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### Changing Reshape Speed
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Reshaping can take a long time.
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It can be useful to increase or decrease the speed of it depending on the usage
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of the raid array while resizing.
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The following section is based on a guide by
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[nixCraft](https://www.cyberciti.biz/tips/linux-raid-increase-resync-rebuild-speed.html)
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and addresses the temporary change of speed of the reshape process of a raid
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array.
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There are two values that can be temporarily changed to limit the reshape speed.
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Those are `dev.raid.speed_limit_min` which defaults to `1000` and
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`dev.raid.speed_limit_max` which defaults to `10000`.
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```sh
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sysctl -w dev.raid.speed_limit_min=100000
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sysctl -w dev.raid.speed_limit_max=500000
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```
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Another value to change is the read-ahead.
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The current value can be displayed with the following command.
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```sh
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blockdev --getra /dev/md0
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```
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And it can be set with the following command.
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This commands sets the read-ahead to `32MB`.
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```sh
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blockdev --setra 65536 /dev/md0
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```
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For [Raid 5](#raid-5) the value `stripe_cache_size` can be important too.
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It can be changed with the following command which sets it to `32 MiB`.
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```sh
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echo 32768 > /sys/block/md3/md/stripe_cache_size
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```
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However make sure not to set it too large, as this can result in a
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"out of memory" condition.
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The memory consumed can be calculated with the following formula.
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```txt
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memory_consumed = system_page_size * nr_disks * stripe_cache_size
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```
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### Raid 1
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Raid 1 creates a mirror with even amount of drives.
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For `n=2` [raid 5](#raid-5) and raid 1 are basically the same.
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The space efficiency is `1/n` and the fault tolerance is `n-1` drive failure.
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The read perfromance is `n` and the write performance `1`.
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#### Create Raid 1 Device
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You can create a Raid 1 device with
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`mdadm --create --verbose /dev/md0 --level=1 --raid-devices=2 /dev/sda /dev/sdb`
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where `sda` and `sdb` are the disks you want to include in your raid.
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You can increase the number of raid devices, but they have to be at least 2.
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If it is an uneven amount of disks in the raid, a disk will act as a spare disk.
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#### Convert Raid 1 to Raid 5
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Assuming the raid 1 device is called `/dev/md0`.
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All other drives are part of the `md0` raid device.
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Note that mostly raid 1 devices consisting of 2 drives should be converted to
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[raid 5](#raid-5).
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- Remove all drives but 2 (if there are more drives than that) by running
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`mdadm /dev/md0 --fail /dev/sda1` and `mdadm /dev/md0 --remove /dev/sda1`
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where `sda1` is the drive to remove
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- Make sure your raid 1 array has only 2 active drives by running
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`mdadm --grow /dev/md0 -n 2`
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- Now convert your raid 1 to a raid 5 device with `mdadm --grow /dev/md0 -l5`
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- Add the disks you removed back to the raid with
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`mdadm /dev/md0 --add /dev/sda1`
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- Finally grow the active drive number to your needs (in this example 4)
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`mdadm --grow /dev/md0 -n4`
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- MDADM now reshapes the raid. You can monitor it by running
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`watch cat /proc/mdstat`
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### Raid 5
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Raid 5 creates a raid device with distributed parity and is set to have at least
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3 drives.
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The space efficiency is `1-(1/n)` and the fault tolerance is `1` drive failure.
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The read perfromance is `n` and the write performance `1/4` for single sector
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and `n-1` for full stripe.
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In the special case of 2 drives in a raid 5 it is functionally the same as
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[raid 1](#raid-1).
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