RE: Raid5 Vs Raid0+1 -- Raw Vs Solaris 9 Concurrent Direct IO UFS

?

A single-block read from a RAID level 5 array will visit only one block =
on
the array (unless something weird's going on like a block split across
devices, or there's a partial outage going on).

A single-block read from a RAID level 1 array will also visit only one =
block
(unless there's a block split issue), but the advantage of RAID level 1 =
is
that a good controller can fetch the block from the less busy of two =
disks
storing equally valid copies of the block.

There is no "read advantage" of level 5 over level 1. In fact, it's =
quite
the contrary. First, because of what I said above. Second, because a =
RAID
level 5 array has inherently greater load to manage than a RAID level 1
array. If an application generates R 1-block reads/sec, and W 1-block
writes/sec, then the two architectures would compare this way:

- Level 1: would have to process (R +  W) I/O requests per second
- Level 5: would have to process (R + 4W) I/O requests per second

So, for example, if write calls comprise 50% of your I/O call workload =
(that
is, W=3DR), then this is your situation:

- Level 1: load is 2R I/O requests per second
- Level 5: load is 5R I/O requests per second

That is, the RAID level 5 system will have to process 2.5x more I/Os per
second than the RAID level 1 system. How could the RAID-5 system keep =
up?
Either with a /lot/ of cache ($$$, and Tim's right; any amount of cache =
can
be overwhelmed by a high enough sustained I/O rate), or by buying a =
/lot/
more disks.

...By the time you buy all that stuff, your whole economic motivation =
for
buying RAID level 5 ("it's cheaper, because you don't have to buy as =
many
disks...") is out the window.

RAID level 5 is /not/ cheaper, because you /do/ have to buy as many =
disks.
And cache. And controller software. ...BAARF.


Cary Millsap
Hotsos Enterprises, Ltd.
http://www.hotsos.com
* Nullius in verba *

Upcoming events:
- Performance Diagnosis 101: 9/14 San Francisco, 10/5 Charlotte, 10/26
Toronto
- SQL Optimization 101: 8/16 Minneapolis, 9/20 Hartford, 10/18 New =
Orleans
- Hotsos Symposium 2005: March 6-10 Dallas
- Visit www.hotsos.com for schedule details...


-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx =
[mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of Nelson, Allan
Sent: Wednesday, August 25, 2004 3:14 PM
To: oracle-l@xxxxxxxxxxxxx
Subject: RE: Raid5 Vs Raid0+1 -- Raw Vs Solaris 9 Concurrent Direct IO =
UFS

Raid 5 does get good read performance because for reads more disks get
in on the action.  3 in his example vs essentially 2 for the raid 0+1
stuff.

Allan



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