Re: Which RAID Controllers to pick/avoid?

On Sun, Feb 6, 2011 at 2:39 AM, Royce Ausburn <royce(dot)ml(at)inomial(dot)com> wrote:
>
>> On Wed, Feb 2, 2011 at 7:00 PM, Craig Ringer
>> <craig(at)postnewspapers(dot)com(dot)au> wrote:
>>> Whatever RAID controller you get, make sure you have a battery backup
>>> unit (BBU) installed so you can safely enable write-back caching.
>>> Without that, you might as well use software RAID - it'll generally be
>>> faster (and cheaper) than HW RAID w/o a BBU.
>>
>> Recently we had to pull our RAID controllers and go to plain SAS
>> cards.  While random access dropped a bit, sequential throughput
>> skyrocketed, saturating the 4 lane cable we use.    4x300Gb/s =
>> 1200Gb/s or right around 1G of data a second off the array.  VERY
>> impressive.
>
>
> This is really surprising.  Software raid generally outperform hardware raid without BBU?  Why is that?  My company uses hardware raid quite a bit without BBU and have never thought to compare with software raid =/

For raw throughtput it's not uncommon to beat a RAID card whether it
has a battery backed cache or not. If I'm wiriting a 200G file to the
disks, a BBU cache isn't gonna make that any faster, it'll fill up in
a second and then it's got to write to disk. BBU Cache are for faster
random writes, and will handily beat SW RAID. But for raw large file
read and write SW RAID is the fastest thing I've seen.