On Thu, Jul 15, 2010 at 10:30 AM, Scott Carey <scott@richrelevance.com> wrote:
>
> On Jul 14, 2010, at 7:50 PM, Ben Chobot wrote:
>
>> On Jul 14, 2010, at 6:57 PM, Scott Carey wrote:
>>
>>> But none of this explains why a 4-disk raid 10 is slower than a 1 disk system. If there is no write-back caching
onthe RAID, it should still be similar to the one disk setup.
>>
>> Many raid controllers are smart enough to always turn off write caching on the drives, and also disable the feature
ontheir own buffer without a BBU. Add a BBU, and the cache on the controller starts getting used, but *not* the cache
onthe drives.
>
> This does not make sense.
Basically, you can have cheap, fast and dangerous (drive with write
cache enabled, which responds positively to fsync even when it hasn't
actually fsynced the data. You can have cheap, slow and safe with a
drive that has a cache but since it'll be fsyncing it all the the time
the write cache won't actually get used, or fast, expensive, and safe,
which is what a BBU RAID card gets by saying the data is fsynced when
it's actually just in cache, but a safe cache that won't get lost on
power down.
I don't find it that complicated.