Re: [RFC] raid5: add a log device to fix raid5/6 write hole issue

[Jens Axboe <axboe@fb.com>]

On 04/01/2015 01:57 PM, Wols Lists wrote:
> On 01/04/15 19:46, Alireza Haghdoost wrote:
>>> Now, how can be assured, in that case, that the "cache"
>>>> device is safe after the power is restored?
>> You do sync write-ahead logging on the Flash cache. If it return
>> successful, you do fire the writes to the RAID. If system crash/fails
>> during the RAID writes (Write-hole), you just recover data by scanning
>> write-ahead log in the flash cache and replay the logs into the RAID
>> drives.
>>
> Just to throw something nasty into the mix, I'm not sure whether it's
> SSDs or SD-cards, but there certainly *was* a spate of corrupted
> *controllers*.
>
> In other words, a power failure would RELIABLY TRASH the device, if it
> happened at the wrong moment. Hopefully that's been fixed ...

You can't protect against shitty devices. If you care about power fail 
events, then you use hw that is specifically tested and vetted for that. 
And they do exist. They might just not be the cheapest you can find on 
newegg or similar places.

This potential problem isn't specific to what Shaohua is proposing, nor 
is it a show stopper for that.

-- 
Jens Axboe