Row; Safe; San - American Megatrends StorTrends 1300 User Manual

ROW

Reallocate On Write or Redirect On Write. Redirect-on-Write (ROW) snapshots use a
method of protecting data that needs to be overwritten by new writes after a snapshot has
been taken. ROW snapshots preserve the old data in its old location, and instead, redirect
the new write to a new location.
The physical address of the data is changed for the current data. All subsequent reads and
writes of data for the volume are performed at the new location. Snapshot reads continue
to be performed from the old location.
As with Copy-on-Write snapshots, systems that implement Redirect-On-Write snapshots
also need to maintain tables that contain a map of where old data and new data are stored.
However, this mapping needs to be updated and referred to for volume reads in Redirect-
on-Write, as opposed to merely for snapshot reads in Copy-on-Write. As mentioned
earlier, it is therefore very important to maintain these tables in main memory for
Redirect-on-Write snapshot systems. And in order to keep the amount of memory
manageably small, the chunk size for these systems is usually more than one sector, for
example 64kB.
Because write requests arriving to the disk may be smaller than the chunk size, and since
a chunk's worth of data always must be valid when it is written to the new location,
Redirect-on-Write snapshots sometimes require a read-modify-write cycle to keep data
valid at the chunk granularity. If the chunk size is 64kB, and a new 8kB write arrives,
64kB of old data is read, the 8kB change is applied, and the updated 64kB is written to
the new location. Therefore, the first write after a redirect-on-write snapshot may
sometimes spawn an additional read.

SAFE

Snapshot Agent Framework Environment. SAFE is a component of the StorTrends iTX
Host Attach Kit, which also includes the StorTrends iTX Scheduler service and the
StorTrends iTX Hardware Provider for VSS environments. SAFE automatically verifies
if the environment is VSS or non-VSS, and acts accordingly.

SAN

Storage Area Network. It is a network of shared storage devices. Its architecture works in
a way that makes all storage devices available to all servers on a LAN or WAN. As more
storage devices are added to a SAN, they too will be accessible from any server in the
larger network. In this case, the server merely acts as a pathway between the end user and
the stored data.
332 StorTrends® 1300 User's Guide (StorTrends iTX version 2.7)