IBM DS3500 - Read Caching

7914Admin_Logical.fm Draft Document for Review March 28, 2011 12:24 pm
248 IBM System Storage DS3500: Introduction and Implementation Guide
The logical drive is highlighted depending on the initial selection and we also have several
check boxes for other settings.
You can select more logical drives in Figure 9-29, using the Ctrl or Shift key during selection
or by using the Select All button. All of the selected drives will have the same modification
priority after the change.
Figure 9-29 Logical Drive cache settings - defaults
These cache settings can have a large impact on the performance of the DS3500 storage
subsystem and on the availability of data. Be aware that performance and availability often
conflict with each other. If maximum performance is required, in most cases availability might
have to be compromised and vice versa.
The default settings are read and write cache for all logical drives, with cache mirroring to the
alternate controller for all write data. The write cache is only used if the cache battery is fully
charged. Dynamic cache read prefetch is also used on new logical drives by default.
Read caching
Read caching allows to keep data in controller cache memory even if they were already sent
to host. If a host initially requests data that are not in the cache, the controller reads the
needed data blocks from the disks and places them in the cache. Until the cache is flushed,
any other requests for this data are fulfilled with the cache data instead of initiating another
read operation to the disks.
Write caching
The write caching enables the storage subsystem to cache write data instead of writing it
directly to the disks. This can improve performance significantly, especially for environments
with random writes, such as databases. For sequential writes, the performance gain varies
with the size of the data written. If the logical drive is only used for read access, it might
improve overall performance to disable the write cache for this logical drive.
Write cache with mirroring
The DS3500 storage subsystem write cache mirroring provides the integrity of cached data if
a RAID controller fails. This is excellent feature from a high availability perspective, but it
decreases performance as cache mirroring occupies twice more cache memory for writing.