On Jun 30, 2005, at 3:34 PM, Dan Nelson wrote:
In the last episode (Jun 30), Mark Bucciarelli said:
On Thu, Jun 30, 2005 at 04:48:18PM -0400, Simon wrote:
Just because there is no monitoring tool available due to lack of
support, doesn't mean the card itself is bad. I much prefer
hardware implementation than software. True hardware RAID frees up
a lot of CPU time if you have heavy IO and software just can't keep
up if you utilize CPU intensive apps.
Why do you say hardware raid frees up "a lot of CPU time?" Have you
measured this?
Do you have any servers that are cpu-bound instead of io-bound?
I am having this exact discussion with my business partner at the
moment--he is also a proponent of hardware raid. I don't see the big
win in hardware raid.
The three big plusses for hardware raid are: if you get one with
battery-backed cache (strongly recommended), then the array can cache
raid-5 writes until it gets full stripes, and can hold off doing
mirror
writes if there are pending read requests. Also, if your power goes
out or the system spontaneously reboots, you won't have to rebuild
parity or resync the mirrors (assuming battery-backed cache). And
finally, hardware raid cards will automatically rebuild onto a hot
spare if available and you can swap out the dead drive and swap a new
spare in without having to run a single command.
I am not an expert at all, but I believe the following to be true and
advantages of true HW raid cards. To add to the above from Dan Nelson.
-- HW raid cards reduce the traffic on your PCI bus. One read or
write request is issued and one set of data goes over the PCI bus.
The card itself worries about talking to the drives and reading or
writing the data from the appropriate drives
-- even if you are not CPU bound in terms of fully using the complete
CPU, if you are busy, the CPU has a queue of things to do and I like
to keep the CPU queue as small as possible... For example, busy PHP
based sites can queue up lots of processes even if the load does not
peg the CPU due to other considerations, we can avoid extraneous
context switches and extra CPU stuff
-- good HW raid cards will have monitoring SW -- Adaptec, 3ware, and
others do.
-- simpler interface for the OS. The OS treats it as just another
disk and so bugs in the OS (in your disk driver and RAID sw) don't
corrupt your data as easily and in fact make you less OS and HW
versions dependent, not more.
---
Chad Leigh -- Shire.Net LLC
Your Web App and Email hosting provider
[EMAIL PROTECTED]
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