Re: BBU still needed with SSD?

On 2011-07-19 09:56, Florian Weimer wrote:
> * Yeb Havinga:
>
>> The biggest drawback of 2 SSD's with supercap in hardware raid 1, is
>> that if they are both new and of the same model/firmware, they'd
>> probably reach the end of their write cycles at the same time, thereby
>> failing simultaneously.
> I thought so too, but I've got two Intel 320s (I suppose, the report
> device model is "SSDSA2CT040G3") in a RAID 1 configuration, and after
> about a month of testing, one is down to 89 on the media wearout
> indicator, and the other is still at 96.  Both devices are
> deteriorating, but one at a significantly faster rate.
That's great news if this turns out to be generally true. Is it on mdadm
software raid?

I searched a bit in the mdadm manual for reasons this can be the case.
It isn't the occasional check (echo check >
/sys/block/md0/md/sync_action) since that seems to do two reads and
compare. Another idea was that the layout of the mirror might not be
different, but the manual says that the --layout configuration directive
is only for RAID 5,6 and 10, but not RAID 1. Then my eye caught
--write-behind, the maximum number of outstanding writes and it has a
non-zero default value, but is only done if a drive is marked write-mostly.

Maybe it is caused by the initial build of the array? But then a 7%
difference seems like an awful lot.

It would be interesting to see if the drives also show total xyz
written, and if that differs a lot too.

regards,
Yeb Havinga