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This is a native Linux Kernel Driver aiming at providing a simple way for REST-based storage to provide volumes and attach them as native linux block devices, thus taking advantage from Linux's efficient Block Device cache.

A secondary aim is also to help automate management of volumes from the client machine.

A lot of features are planned, as you can see by looking at the current issues. You're welcome to report bugs and propose ideas for new features through github's issue tracker; as well as providing patches through pull requests.


Supported OS

Ubuntu 14.04.4 LTS CentOS 7 system

Minimum Linux kernel version: v3.10

Installing the driver

Installing the driver is relatively simple, as it's essentially done by loading the module into the linux kernel.

Here is how it's done:

# insmod srb.ko

Now, the Rest Block Driver is set for use, and you only need to know how to control the driver to do the management tasks. To learn that, please continue reading.

In order to set the driver's parameters, you can add those to the loading command line of the driver as follows:

# insmod srb.ko thread_pool_size=16

The following parameters are available:

  • debug: log level for the LKM (integer number, 0 to 7: emergency, alert, critical, error, warning, notice, info, debug)
  • req_timeout: timeout for requests
  • nb_req_retries: number of retries before aborting a Request
  • server_conn_timeout: timeout for connecting to a server
  • thread_pool_size: size of the thread pool of each device

Volume Provisioning

Currently, the driver does not yet support failover between multiple servers providing the same repository of volumes, but it is nonetheless a feature that we are aiming for. For this reason, we chose to provide a facility to manage the server urls the driver is associated to, and then the usual operations will operate on one of those.

For this reason we provide you with three /sys files controlling the URLs to the servers:

  • urls: allows listing the server urls currently available/configured
  • add_urls: allows adding one or more server urls to the driver
  • remove_urls: allows removing one or more server urls from the driver

Then, the following management files are available:

  • create: allows creating a volume file on the storage
  • extend: allows extending a volume (increasing size), whether it is attached or not
  • destroy: deletes the volume file from the storage

The way those files work is described in the following sections, each dedicated to one management (/sys) file. Please mind that each one of theses files can be displayed (using cat on them) to show a simple usage text.

Listing the server urls

To list the server urls currently configured within the driver, you can simply display the contents of the urls file:

# cat /sys/class/srb/urls

This file displays the list of server urls separated by a coma, using the same format you would to add or remove one or more server urls.

Adding server urls

To add one (or more) new server urls to the driver, you need to write the list of urls separated by commas into the associated file. The format includes protocol, host (IP only: DNS is not supported yet), optional port, and the path to the volume repository ("path") which does no require an ending '/'. In essence, a volume repository URL would look like this:


Thus, to concatenate the multiple urls, you can add them all at once like in the example:

# export REST_REPO1=
# export REST_REPO2=
# echo "$REST_REPO1,$REST_REPO2" > /sys/class/srb/add_urls

The driver will properly separate all repositories from the string you gave it, and add them one by one. In case of error, only the error-yielding server will not be added to the list. All valid server urls that did not yield any error will be properly added. You might want to check which ones could be added by listing the urls if you cannot add all your server urls.

Be careful, though:

  • Every server url must point to the same volume repository. Doing otherwise is an unsupported use, and behavior is undefined and untested
  • /!\ Currently, the failover not being supported, not all of the servers may actually be used.

Removing server urls

In order to remove one or more server urls, the same format is used as for adding some. Also, since the listing of urls outputs them this way, you could copy and paste part of the urls listing if you wished to. In the end, removing server urls can be done as follows:

# echo "" > /sys/class/srb/remove_urls

Please note that if a device is attached, you will not be able to remove the last server url. You need to detach manually every device attached by the module before manually removing the last server url.

Note also that when unloading the module, the devices are detached automatically before the module can actually be unloaded.

Creating a new volume

To create a volume, just give the name of the file to create to the driver, accompanied by a byte size, such as in the following example:

# echo "filename human\_readable\_size" > /sys/class/srb/create

The volume will be created on the storage with the requested size. But beware:

  • The human readable size formats understood by the LKM are the following:
    • [integer number]: size in bytes
    • [integer number]k: size in Kilobytes
    • [integer number]M: size in Megabytes
    • [integer number]G: size in Gigabytes
  • At least one server url must exist and be valid for the volume to be created
  • Created volume are not automatically attached as a device on the system.

Before you can use the volume, you have to attach it through the attach /sys file (see below).

Extending a volume

Sometimes, a volume might look to be provisionned too small for the actual need. For this reason, you can actually extend it through the extend /sys control file. This command follows the same usage as the create command, thus using it is simple:

# echo "volumename human\_readable\_size" > /sys/class/srb/extend

Be aware that this command can only extend a volume, meaning the size you give must be higher than the current size. Also, this is a supported operation on an attached volume, though any file-system formatted onto the volume should be extended to the new volume's size manually since most of the filesystems don't support flexible partition or volume extension (unless you are using LVM underneath).

Once the operation is complete, the new size will be properly reported to the system without any additional administrative task. For instance, displaying the contents of the file /proc/partitions will show you the updated size of the volume.

Note that the human readable size format of the extend command follows the same rules as that of the create command.

Destruction of an existing volume

Destroying a volume means that it volume will no longer be accessible after a successful operation. To destroy a volume, give the driver the name of the file to remove from the storage as the following example shows:

# echo filename > /sys/class/srb/destroy

The volume is then removed from storage and is no longer accessible. But beware:

  • The destroyed volume must exist beforehand
  • Destroying a volume used by other drivers on other machines srb can lead to errors and unexpected behaviors; this is untested.

Attaching and Detaching devices

For multiple reasons, the driver does not attach automatically the devices when adding a server url or creating a new volume. Those reasons include:

  • automatization does not always gain from having generated device names
  • it's sometimes more difficult to synchronize with a generated name than defining a name yourself

For those reason, you need to attach the volumes manually to the system, using the three following management files are available:

  • volumes: Reading the file lists the volumes available on the server
  • attach: Attaches an already provisioned volume as a device
  • detach: Detaches an attached device from the system (does not delete the volume)

They are described in detail in the following sections.

Listing the volumes

Since you might not know from memory which volumes exist on your servers, you might want a way to list those, to attach them easily. One of the ways provided is to read the content of the volumes /sys file:

 # cat /sys/class/srb/volumes

This way, you can know that you have five volumes available on your servers, and know their names, which will allow you to either attach, extend or destroy them.

Attaching a device

In order to attach an existing Volume file in the system, you simply need to write the name of the Volume to the attach control file, followed by the name of the device you want to appear, as the example states:

# echo VolumeName DeviceName > /sys/class/srb/attach

Then, a device named "DeviceName" is created in /dev. You can now use your device as you wish, be it by writing and reading data directly to it, creating a file system or even using LVM on top of it.

Detaching a device

A device attached may be detached by writing the device's name into the detach control file as the example shows:

# echo DeviceName > /sys/class/srb/detach

The DeviceName is the same Device Name used as the one used for Attach operations.

Using the devices

Partitioning a device

The devices can be partitioned as conventional disks for instance:

# fdisk /dev/DeviceName

Then, you use it as any other device: each partition will appear with the same name as the device itself with a number suffix.

sysfs interface

For each device, an entry is created in /sys/block

  • /sys/block/MySmallDevice for the volume attached as 'MySmallDevice'
  • /sys/block/srba for ithe volume attached as 'srba' And so on...

Log & Debug

Logs are enable in the Linux Kernel Module and default is set to INFO. In order to change the log level of the driver you can do it while loading it as follow:

# insmod srb.ko srb_log=3

The log level can also be changed using sysfs as follow:

# echo 3 > /sys/module/srb/parameters/debug

Each device inherit the Linux Kernel Module log level. The log level of a device can be changed as follow:

# echo 6 > /sys/block/srb?/srb\_debug

The log level can be set from debug(7), info(6) ... to emergency (0).

Get information on the device

The URL associated on CDMI (one server only):

# cat /sys/block/srb?/srb\_urls

disk size:

# cat /sys/block/srb?/srb\_size

volume name:

# cat /sys/block/srb?/srb\_name


Playground Server

In the playground directory, you will find a minimalistic REST server written in python that will allow you to try out the features of the Scality Rest Block Driver. It was written to support Scality's REST protocol's mandatory semantics.

The Playground Server uses the filesystem to store its data; using it extensively might fill your disk up. By default, the server stores the volumes in the 'playground_data' directory, within the directory you started the server from; and listens on the port 80 (meaning that you might have to start it as root). By using the options '--port' and '--datapath', you can change either the port it listens on, or the directory where the volumes are stored.

Please keep in mind that as it is a minimal server script, it is not designed for performance, but for functional testing.

Remaining Tasks :

  • Fault tolerance when more servers are available (reset or timeout).
  • Optimize sector sizes
  • Flag the device as non-rotational
  • Support DKMS
  • Rollback if connection lost