Re: Questions about RAID 6
Mike Bird put forth on 4/28/2010 1:48 PM:
> On Wed April 28 2010 01:44:37 Stan Hoeppner wrote:
>> On a sufficiently fast system that is not loaded, the user will likely see
>> no performance degradation, especially given Linux' buffered I/O
>> architecture. However, on a loaded system, such as a transactional
>> database server or busy ftp upload server, such a RAID setup will bring the
>> system to its knees in short order as the CPU overhead for each 'real' disk
>> I/O is now increased 4x and the physical I/O bandwidth is increased 4x.
>
> I've designed commercial database managers and OLTP systems.
Are you saying you've put production OLTP databases on N-way software RAID 1
sets?
> If CPU usage had ever become a factor in anything I had designed
> I would have been fired. If they're not I/O bound they're useless.
That's an odd point to make given that we're discussing N-way RAID 1. By
using N-way RAID 1, you're making the system I/O bound before you even
create the db. Given the way most database engines do locking, you'll get
zero additional seek benefit on reads, and you'll take a 4x hit on writes.
I don't know how you could possibly argue otherwise.
> With a few exceptions such as physical backups, any I/O bound
> application is going to be seek bound, not bandwidth bound.
Downloads via http/ftp/scp and largish file copies via smb/cifs, as well as
any media streaming applications will be more b/w bound that seek bound.
For most day to day mundane stuff such as smtp/imap/web/etc, yes, they're
far more seek bound. But again, using N-way RAID 1 will give no performance
boost to any of these applications, whether seek or b/w bound. It will give
you the same read performance in most cases as a single spindle, on some
occasions a slight boost, but will always yield a 4x decrease in seek and
b/w/ performance for writes.
The Linux RAID 1 code is optimized for redundancy, not performance. If you
need redundancy and performance with Linux software RAID, your best bet is
RAID 10. It costs more per GB than RAID 5 or 6, but doesn't have to
generate parity, yielding lower CPU overhead and thus decreasing I/O
latency. And it sure as heck costs less than N-way RAID 1.
--
Stan
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