Snapshots and Automated Storage Tiering
Snapshots are an essential feature of modern SAN/NAS technologies and are A MUST for primary virtual machine storage because all virtual machine backup applications use Logical Unit (LU) snapshots as the first stage of creating VM backups. Nevertheless, storing all the snapshot history on the Tier 1 may be challenging.
All the conventional storage systems that utilize snapshots have two problems: wasted space and degraded performance.
While data (i.e. VM images) doesn’t consume that much space, snapshot history takes up a lot of space.
When talking about having the need to run snapshots, the disk space on the primary storage can be used in the following ways:
- Usable – space available to user after all the space reductions due to one or another reason (RAID, Synchronous Mirroring, Erasure Coding etc.);
- Space for snapshots – part of usable space reserved for snapshots.
This results in a great waste of expensive disk space that turns critical when going from spindle hard drives to flash storage.
Snapshots are usually implemented by either Copy-on-Write (COW) or Redirect-on-Write (ROW). They are characterized as follows:
- COW snapshots utilize read-modify-write sequence. To put it simple, when new data blocks come, the system moves the original data block to another location, and after, the new block is written instead of the original one. The system has to perform 3 operations (one read and two writes), instead of one write. Obviously that creates additional load on the hardware involved in the snapshot related processes, which results in performance decrease.
- ROW creates fragmentation since all the new data is written in another place on the disk, instead of putting it sequentially. That results in random reads and writes, and performance decrease. Also, ROW stores all the snapshot history, thus consuming the disk space.
- The usable space on primary high performance storage system is increased since all the “cold” data resides on dedicated inexpensive storage;
- Absence of performance degradation achieved by offloading to the disk subsystem;
- Life time of SSDs is prolonged significantly since all major decrease in amount of write operations;
- Redundancy and disaster recovery is improved by storing snapshot history physically separately from primary storage.