All of us are get used to SATA disk drives connected to our workstations and we call it storage. Some organizations has RAID arrays. RAID is one level of logical abstraction which combine several hard drives to form logical drive with greater size/reliability/speed. What would you say if I’d tell you that NetApp has following terms in its storage architecture paradigm: disk, RAID group, plex, aggregate, volume, qtree, LUN, directory, file. Lets try to understand how all this work together.
RAID in NetApp terminology is called
RAID group. Unlike ordinary storage systems NetApp works mostly with RAID 4 and RAID-DP. Where RAID 4 has one separate disk for parity and RAID-DP has two. Don’t think that it leads to performance degradation. NetApp has very efficient implementation of these RAID levels.
Plex is collection of RAID groups and is used for RAID level mirroring. For instance if you have two disk shelves and SyncMirror license then you can create plex0 from first shelf drives and plex1 from second shelf. This will protect you from one disk shelf failure.
Aggregate is simply a highest level of hardware abstraction in NetApp and is used to manage plexes, raid groups, etc.
Volume is a logical file system. It’s a well-known term in Windows/Linux/Unix realms and serves for the same goal. Volume may contain files, directories, qtrees and LUNs. It’s the highest level of abstraction from the logical point of view. Data in volume can be accessed by any of protocols NetApp supports: NFS, CIFS, iSCSI, FCP, WebDav, HTTP.
Qtree can contain files and directories or even LUNs and is used to put security and quota rules on contained objects with user/group granularity.
LUN is necessary to access data via block-level protocols like FCP and iSCSI. Files and directories are used with file-level protocols NFS/CIFS/WebDav/HTTP.