On 01/04/15 21:04, Alireza Haghdoost wrote: > On Wed, Apr 1, 2015 at 2:57 PM, Wols Lists <antlists@xxxxxxxxxxxxxxx> 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 ... >> > > That is certainly true. As Dan mentioned, the cache device it-self > should be safe against power failure. I agree this is not the case for > all SSD cards in the market but might be the case for Facebook. I hate > to say this but It seems these efforts are useful dependent to what > kind of hardware is deployed for cache device. > It would be nice, but probably not possible, to have some form of black-list of "these devices are unsafe/dangerous". Along the lines of "mdadm --probe /dev/sda" or whatever, that gets the device type, checks it, and says "this SSD can be destroyed by a power failure" or "this is a cheap disk with the timeout problem" or something. But even if someone did it, the database would probably bit-rot fairly quickly :-( Cheers, Wol -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
