to gmirror or to ZFS

[krad]

But then zfs doesn't access every block on the disk does it, only the
allocated ones


On 20 July 2013 21:07, Daniel Feenberg <feenberg at nber.org> wrote:

>
>
> On Sat, 20 Jul 2013, Steve O'Hara-Smith wrote:
>
>  On Sat, 20 Jul 2013 18:14:20 +0100
>> Frank Leonhardt <frank2 at fjl.co.uk> wrote:
>>
>>  It's worth noting, as a warning for anyone who hasn't been there, that
>>> the number of times a second drive in a RAID system fails during a
>>> rebuild is higher than would be expected. During a rebuild the remaining
>>> drives get thrashed, hot, and if they're on the edge, that's when
>>> they're going to go. And at the most inconvenient time. Okay - obvious
>>> when you think about it, but this tends to be too late.
>>>
>>
>>         Having the cabinet stuffed full of nominally identical drives
>> bought at the same time from the same supplier tends to add to the
>> probability that more than one drive is on the edge when one goes. It's a
>> pity there are now only two manufacturers of spinning rust.
>>
>
> Often this is presummed to be the reason for double failures close in
> time, also common mode failures such as environment, a defective power
> supply or excess voltage can be blamed. I have to think that the most
> common "cause" for a second failure soon after the first is that a failed
> drive often isn't detected until a particular sector is read or written.
> Since the resilvering reads and writes every sector on multiple disks,
> including unused sectors, it can "detect" latent problems that may have
> existed since the drive was new but which haven't been used for data yet,
> or have gone bad since the last write, but haven't been read since.
>
> The ZFS scrub processes only sectors with data, so it provides only
> partial protection against double failures.
>
> Daniel Feenberg
> NBER
>
>
>
>
>> --
>> Steve O'Hara-Smith <steve at sohara.org>
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