Logical Volumes Howto
New disk format ("format41") allows to create volumes with scaling-out abilities. That is, you are able to easily add a brick (i.e. formatted block device) to your volume, thus, increasing its capacity. Bricks don't need to be of equal size: data will be distributed fairly among different sized bricks. Also, at any time you are able to remove such brick. Moreover, at any time you can change an abstract throughput of any brick, thus, controlling a portion of IO requests being issued against each brick in your logical volume. More information can be found here.
Finally, you can add a proxy-brick created over a high-performance block device (e.g. SSD or NVRAM) to your logical volume. It allows to increase performance of file operations in the case when your volume is composed of slow media.
To get started with logical volumes you will need Reiser4 kernel module of SFRN (Software Framework Release Number) 5.1.3, or higher. The patches against stock kernels can be found here. When building the kernel, make sure that "Plan-A key allocation scheme" is disabled in the configuration menu (or CONFIG_REISER4_OLD is not set, which is the same). The important note is that volumes based on the old disk format ("format40") are not scalable (you are not able to add a brick to such volume). Moreover, it is impossible to mount volumes managed by new and old disk format in the same system: you need to choose (by enabling, or disabling the mentioned configuration option). This is for performance reasons.
To create a scalable volume, just build the first brick of your volume by formatting some block-device with mkfs.reiser4 utility and mount it. Then you will be able to grow it by adding new bricks by volume.reiser4 utility. See logical volumes administration guide for details. For managing proxy-bricks see proxy-device administration guide.