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mdadm: change reshape speed section

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@ -36,6 +36,57 @@ mdadm --grow --raid-disks=5 /dev/md0
`5` is the number of disks that should be active.
For adding disks view the [previous section](#add-diskpartition-to-raid-device).
In the [following section](#changing-reshape-speed) methods are described that
can be tried to affect the performance of the reshaping of a Raid array.
### Changing Reshape Speed
Reshaping can take a long time.
It can be useful to increase or decrease the speed of it depending on the usage
of the raid array while resizing.
The following section is based on a guide by
[nixCraft](https://www.cyberciti.biz/tips/linux-raid-increase-resync-rebuild-speed.html)
and addresses the temporary change of speed of the reshape process of a raid
array.
There are two values that can be temporarily changed to limit the reshape speed.
Those are `dev.raid.speed_limit_min` which defaults to `1000` and
`dev.raid.speed_limit_max` which defaults to `10000`.
```sh
sysctl -w dev.raid.speed_limit_min=100000
sysctl -w dev.raid.speed_limit_max=500000
```
Another value to change is the read-ahead.
The current value can be displayed with the following command.
```sh
blockdev --getra /dev/md0
```
And it can be set with the following command.
This commands sets the read-ahead to `32MB`.
```sh
blockdev --setra 65536 /dev/md0
```
For [Raid 5](#raid-5) the value `stripe_cache_size` can be important too.
It can be changed with the following command which sets it to `32 MiB`.
```sh
echo 32768 > /sys/block/md3/md/stripe_cache_size
```
However make sure not to set it too large, as this can result in a
"out of memory" condition.
The memory consumed can be calculated with the following formula.
```txt
memory_consumed = system_page_size * nr_disks * stripe_cache_size
```
### Raid 1
Raid 1 creates a mirror with even amount of drives.