LaCie 131018 - StorView Snapshot - Mac, User Manual

Page: 205 / 214

More on Snapshot
197
C.3 Overwrite Data Area Location
The overwrite data areas can be configured in two different ways. On the same disk array as a regular
logical drive or on one or more separate disk arrays dedicated for the use of ODAs.
If the ODA area is located on the same physical disk array as the logical drive being snapshotted, the
performance of both reads and writes to the logical drive will be affected. Instead if the ODA area is
located on its own dedicated disk array, it results in much better performance characteristics.
It is always preferred to dedicate a disk array to be used exclusively by one or more ODAs.
C.4 Selecting the ODA Stripe Size
During ODA initialization, the user is asked to select a stripe size for the ODAs. Five different options are
available: 64 Kbytes, 128 Kbytes, 256 Kbytes, 512 Kbytes, and 1 Mbyte. In an array with multiple ODAs
defined, each ODA can be set up with its own stripe size different from the other ODAs.
The ODA stripe size represents the size of the data area that is always protected during a snapshot copy-
on-write operation. Choosing the best ODA stripe size is not possible without some knowledge about the
intended write data access pattern on the logical drive to be snapshotted. The ODA saves data to be
overwritten in multiples of its stripe size. A large stripe size will offer a smaller number of data areas while
a smaller stripe size will offer more data areas.
If the server application issues a high percentage of small writes in a random address pattern, then it will
be better to select a small ODA stripe size such as 64 Kbytes. That way the ODA has a lot of smaller data
areas to use for a large amount of small random writes. A disadvantage will be that the probability for
follow on hits in an already protected area of the logical drive is smaller compared with using a larger
ODA stripe size.
If the server application issues a high percentage of small random writes clustered together in groups on
the logical drive, then it would be preferred to use a large ODA stripe size. This increases the probability
that other small writes will be addressed to a disk area that is already protected by a larger stripe of data.
This will minimize the amount of copy-on-write sequences needed and therefore minimize the impact on
the disk array’s performance.The disadvantage would be that the total number of ODA stripes would be
reduced if the write access pattern suddenly became very random in its nature.
For applications that perform a large percentage of sequential writes it would be best to select a larger
ODA stripe size to minimize the amount of copy-on-write operations needed. The results will be better
performance.
C.5 Overwrite Data Area Storage
Capacity
The maximum storage capacity of an ODA is dependant on the ODA stripe size.