What is the *truth* when it comes to configuring Oracle files on an
Enterprise Virtual Array like an HP EVA3000 or EVA5000 or an EMC
Symmetrix frame.
As I Understand It -
A Virtual Array is a storage appliance with a large number of disks
(say over 50), dual-path controller access and, significantly, a very
large disk cache. The cache is supported by front-end and back-end
(disk-side) processors and a range of algorithms that aim to keep all
read and write disk access via the cache.
And this is not RAID. It's a definite storage evolution. The VA
presents the virtual disks to hosts connected via switched fabric. The
virtual disks have the apparent behaviour of different RAID levels,
e.g. 0, 1 and RAID-5. HP calls these settings Vraid0, VRaid1 and
VRaid5. How they are actually implemented is left to the VA.
In a search for sanity, I have read opinion, history ancient and
modern, experts' comment and practitioners' experiences. This seems to
be a hot topic but it seems that the following need to be verified or
de-bunked:
1) Given the cache-centred architecture of disk arrays, how much
effort should be made separating redo logs from data. I suspect that
using VRaid-1 over VRaid-5 is more about data redundancy than
performance. Where RAID-1 was always better than RAID-5 for log files,
because of the pattern of sequential log writes. Is there any sense in
creating a separate VRaid-1 disk group for logfiles?
2) I/O balancing should be achieved by spreading the virtual disks
between the controllers on creation, so that each controller is
working optimally. The issue seems to shift from hot/cold disks to
hot/cold controllers. Can we influence the disk layout?
3) Is it true that the array handles physical layout totally,
rebalancing and migrating data according to usage patterns? If so, the
best practice may be to choose appropriate stripe size and to choose
an extent size to exploit the many disks in a disk group. The aim
would be to feed the cache efficiently at all times.
4) Do I hang my DBA boots up then and use Vraid-5 throughout? What are
the probable recommendations for rollback/redo? archive logs? control
files?
5) A fear born of experience here with SAN is a delinquent controller.
What can I do to minimise the impact of a controller failure on the
database? A one-disk failure? A two-disk failure?
6) What is the suitability of Virtual Array technology for DW? OLTP?
large operational datastore with heavy IUD activity?
My plan is to review experiences and try to test some options before
my next EVA is built. It looks like the book on storage and Oracle has
been rewritten yet again.
Many thanks,
Peter Hitchmough
"Why are there marks against these errors in the Error Messages
manual?"
"I'm trying to collect the set."
