From mboxrd@z Thu Jan  1 00:00:00 1970
From: Wols Lists <antlists@youngman.org.uk>
Subject: Re: [RFC] raid5: add a log device to fix raid5/6 write hole issue
Date: Wed, 01 Apr 2015 20:57:30 +0100
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To: Alireza Haghdoost <alireza@cs.umn.edu>, Piergiorgio Sartor <piergiorgio.sartor@nexgo.de>
Cc: Dan Williams <dan.j.williams@intel.com>, Shaohua Li <shli@fb.com>, Neil Brown <neilb@suse.de>, linux-raid <linux-raid@vger.kernel.org>, Song Liu <songliubraving@fb.com>, Kernel-team@fb.com
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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 ...

Cheers.
Wol