IBM TotalStorage NAS 300, User Reference

Page: 78 / 200

data from being overwritten. Very high traffic systems can devote as much as
40% of a production volume to PSM cache, although 15% (the default) or 20%
will meet the needs of most users. The cache-full persistent image deletion
threshold must also be tuned to automatically delete persistent images in time
to free cache space before the cache fills up. Management of the cache must
be tuned carefully to avoid filling the cache completely, as any missed and
uncached old data renders all the persistent images for a volume inconsistent,
and PSM will automatically delete them.
Persistent Images
This panel lists all of the persistent images that exist on all volumes. On this panel
you can:
v Create a new persistent image immediately (without scheduling it through the
Schedules panel). When you create the persistent image, you can specify
properties for the persistent image, including:
Volume(s) The persistent image can contain a single
volume or multiple volumes. To select multiple
volumes, hold down the
Ctrl key while clicking
the volumes. For multi-volume persistent images,
a virtual directory containing data for a volume
appears under the persistent image directory in
the top level of each volume in the persistent
image (the name of the persistent image
directory is configured in the Global Settings
panel).
Name You can name the persistent image. This
becomes the name of the virtual directory
containing the persistent image, underneath the
persistent image directory in the top level of the
volume (the name of the persistent image
directory is configured in the Global Settings
panel).
Read-only or read-write A persistent image is read-only by default, so no
modifications can be made to it. However, you
can set the persistent image to read-write, which
permits you to modify it. When a persistent
image is written, the modifications made are also
persistent (they survive a reboot of the system).
Changing a persistent image from read-write to
read-only resets the persistent image to its state
at the time you took the persistent image, as
does selecting
Undo Writes for a read-write
persistent image from the Persistent Images
panel.
Retention value A persistent image can be given a relative
retention value or weight. This is important when
PSM needs to delete some persistent images for
a volume because the capacity of the cache file
for that volume has reached a certain threshold,
as described later in this section. If the volume
cache file completely fills, then all persistent
images for that volume are deleted regardless of
64
NAS Gateway 300 User’s Reference