[XCSSA] NAS the VIA EPIA way

[X-otic Computer Systems of San Antonio]

On Jun 29, 2009, at 11:11 PM, X-otic Computer Systems of San Antonio  
wrote:

>> I'm not a fan of RAID5 in general, but it has its
>> uses.
>
> That's why I was asking about the battery and cache.. RAID-5 CAN be  
> as fast as
> RAID-0 on a nice high end card.. because you're not writing to  
> disk.. you're
> writing to NV RAM.. as fast as the bus can take input from the CPU.

Hmmm that's a slippery slope :) RAID5 might be faster for a while, but  
you can still fill up the buffer on the RAID controller with a busy DB  
server. We have seen random I/O still be a significant issue on a  
RAID5 versus a RAID10. MySQL specifically handles it poorly. That sort  
of makes sense since indexes tend to generate lots of random I/O. So  
even if you save some on the writes, you still need to get the data  
via reads and if the dataset is larger than the amount of RAM on the  
controller, you still have a problem.

Granted, it's a lot better than it is with software RAID, but we  
actually see RAID1 occasionally beating out a RAID5 (even with a  
hardware controller). Most of these situations are 3-drive RAID5s -  
the random I/O can be mitigated by adding more drives, but you will  
likely never see a RAID5 performance like a RAID0 for random I/O.  
Sequential I/O, however, isn't near as bad on a RAID5.

I should point out that these experiences are specific to ext3.  
Running on another file-system, such as XFS (which Red-Hat was only  
recently toying with supporting in RHEL) mitigates the issues with a  
RAID5 much better than 'ext3'. Among other things, with XFS you can  
describe the underlying drive configuration and make sure it optimizes  
for it. So, in theory, a RAID5 might not be all that bad. But it sure  
does suck for ext3 with MySQL :/

That said, that's why we beat performance issues with our RAID10  
hammer. Multiple failures possible, and it's hella fast on both reads  
and writes. For some customers, simply moving from a 3 drive RAID5 to  
a 4 drive RAID10 was like night and day. Plus there's no parity to  
compute - in some cases, believe it or not, a RAID10 in software has  
been known to beat our a RAID10 hardware (the stats I saw were with a  
ton of drives on some magical Sun box though) and part of that may  
have been due to the lack of needing to XOR everything.
>
>
>> For my application, though, I've been very happy with software RAID.
>> It's not a Porsche and, in fact I paid a premium to get a motherboard
>> which was intentionally slow :) It's more like a Ford Focus Hybrid.
>> There's nothing wrong with a Focus, but a Prosche is definitely going
>> to kick it's ass on a race track (straight or curvy).
>
> Looking forward to seeing it. :)
>
>
>>> Does that card you're selling have a BBCache?
>
> What's the make/model again?
>

LSI MegaRAID SATA 150-6. It's from 2004 I think. I believe it has 64MB  
of onboard cache (non-upgradable).

Tim