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SubjectRe: raid is dangerous but that's secret (was Re: [patch] ext2/3: document conditions when reliable operation is possible)
On Thu, 3 Sep 2009, Krzysztof Halasa wrote:

> Ric Wheeler <rwheeler@redhat.com> writes:
>
>>>> Just to add some support to this, all of the external RAID arrays that
>>>> I know of normally run with write cache disabled on the component
>>>> drives.
>>>
>>> Do they use "off the shelf" SATA (or PATA) disks, and if so, which ones?
>>
>> Which drives various vendors ships changes with specific products.
>> Usually, they ship drives that have carefully vetted firmware, etc.
>> but they are close to the same drives you buy on the open market.
>
> But they aren't the same, are they? If they are not, the fact they can
> run well with the write-through cache doesn't mean the off-the-shelf
> ones can do as well.

frequently they are exactly the same drives, with exactly the same
firmware.

you disable the write caches on the drives themselves, but you add a large
write cache (with battery backup) in the raid card/chassis

> Are they SATA (or PATA) at all? SCSI etc. are usually different
> animals, though there are SCSI and SATA models which differ only in
> electronics.

it depends on what raid array you use, some use SATA, some use SAS/SCSI

David Lang


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