Re: Raid 10 chunksize

On 4/1/09 9:54 AM, "Scott Marlowe" <scott(dot)marlowe(at)gmail(dot)com> wrote:

> On Wed, Apr 1, 2009 at 10:48 AM, Stef Telford <stef(at)ummon(dot)com> wrote:
>> Scott Marlowe wrote:
>>> On Wed, Apr 1, 2009 at 10:15 AM, Stef Telford <stef(at)ummon(dot)com> wrote:
>>>
>>>>     I do agree that the benefit is probably from write-caching, but I
>>>> think that this is a 'win' as long as you have a UPS or BBU adaptor,
>>>> and really, in a prod environment, not having a UPS is .. well. Crazy ?
>>>>
>>>
>>> You do know that UPSes can fail, right?  En masse sometimes even.
>>>
>> Hello Scott,
>>    Well, the only time the UPS has failed in my memory, was during the
>> great Eastern Seaboard power outage of 2003. Lots of fond memories
>> running around Toronto with a gas can looking for oil for generator
>> power. This said though, anything could happen, the co-lo could be taken
>> out by a meteor and then sync on or off makes no difference.
>
> Meteor strike is far less likely than a power surge taking out a UPS.
> I saw a whole data center go black when a power conditioner blew out,
> taking out the other three power conditioners, both industrial UPSes
> and the switch for the diesel generator. And I have friends who have
> seen the same type of thing before as well. The data is the most
> expensive part of any server.
>
Yeah, well I¹ve had a RAID card die, which broke its Battery backed cache.
They¹re all unsafe, technically.

In fact, not only are battery backed caches unsafe, but hard drives. They
can return bad data. So if you want to be really safe:

1: don't use Linux -- you have to use something with full data and metadata
checksums like ZFS or very expensive proprietary file systems.
2: combine it with mirrored SSD's that don't use write cache (so you can
have fsync perf about as good as a battery backed raid card without that
risk).
4: keep a live redundant system with a PITR backup at another site that can
recover in a short period of time.
3: Run in a datacenter well underground with a plutonium nuclear power
supply. Meteor strikes and Nuclear holocaust, beware!