Disk Partitioning and Formatting
================================

n2disk writes files sequentially to disk, using Direct IO (O_DIRECT). The disk can be a single
disk, a RAID volume, or multiple disks (e.g. in case of NVMEs). This section describes what are
the best practices for formatting the volume depending on disk type (HDD vs SSD).

HDDs
----

In our experience the XFS filesystem is the best option you can select when writing to HDDs, 
although other filesystems such as EXT4 can also be used. Supposing that you have created the 
/dev/sda1 disk partition, you can format it as follows:

.. code-block:: console

   mkfs.xfs -f -d sunit=128,swidth=1024,agcount=6000 -l size=64m /dev/sda1

Once you have formatted the disk you can mount it as follows:

.. code-block:: console

   mount -o noatime,nodiratime,attr2,nobarrier,logbufs=8,logbsize=256k,osyncisdsync /dev/sda1 /storage/ 

Note that for small partition sizes, you might need to adapt some of the above parameters.

SSDs
----

If you are using SSDs, EXT4 usually performs better, you can format your partition as follows (assuming you have a RAID 0 array with 16 SSDs):

.. code-block:: console

   mkfs.ext4 -F -b 4096 -E discard,stride=16,stripe-width=256 /dev/sda1

Where:

- -b is the block size (max 4K)
- stride = chunk size (64K) / block size (4K) = 16
- stripe-width = number of disks x stride = 256

And mount it with:

.. code-block:: console

   mount -o rw,user,auto,discard /dev/sda1 /storage/

It is also recommended to set the "noop" scheduler with SSDs:

.. code-block:: console

   echo noop > /sys/block/sda/queue/scheduler

Additional Notes
----------------

You can test the disk write performance as explained in chapter 4.

Note that depending on the disk types and controller, your write performance can be influenced by the chunk size (-C option) that specifies the minimal 
unit of data written to disk. Values in the range between 64 and 512 KBytes should guaranteed adequate performance, 
although some combination of disks/controllers operate better with 1024 KBytes or more. You can check that with "-e 1".