Hi! A. James Lewis <james@xxxxxxxxxx> schrieb: > The problem is tho... with a very large backing store, I'm not really > happy with a single point of failure in the cache... is there another > way to mirror the cache device? Well, AFAIR there are plans to add such capabilities into bcache itself - read: make it possible to add more than one caching device to a cache set. It will use some sort of hybrid mirror / striping to get the best combination of speed and safety - at least that's what the idea is about. I just don't remember where I've read about it, neither do I know the status of it. If you want to eliminate the single point of failure, you may want to try mdadm with its write-mostly option instead of using bcache. It's slower for writes obviously but gracefully falls back if the SSD fails. Obviously, you can also not benefit from having a huge storage because it's classic RAID-1 and thus the smallest member will limit your storage size. Bcache also has countermeasures for a failing caching device but I didn't really look into that yet. You should read the documentation about it in Documentation/bcache.txt (Error Handling). The safest mode to use here is writethrough. > On 06/08/15 00:10, Kai Krakow wrote: >> Jens-U. Mozdzen <jmozdzen@xxxxxx> schrieb: >> >>> Zitat von Kai Krakow <hurikhan77@xxxxxxxxx>: >>>> A. James Lewis <james@xxxxxxxxxx> schrieb: >>>> >>>>> I've heard rumours that layering bcache with other block device >>>>> drivers might not be recommended... I wonder what the truth really >>>>> is... perhaps someone can advise. >>>> I think this is not just rumours. Multiple people reported problems >>>> when layering caching or backing devices on top of MD devices. This may >>>> be an implementation problem in MD which is gone in later kernel >>>> versions [...] >>> being rather new to bcache, I did only browse the last few months of >>> mailing list history - are you saying that these problems were fixed >>> (or simply vanished) some point after 3.18.8? Because if so, I'd of >>> course try to upgrade our servers to a more recent kernel :) >> Latest posts imply it is still a problem. It fits with earlier reports: >> Caching on native device, backing on md device... Bcache breaks within >> the caching device (although this is not on md). There seem to still be >> bugs with bcache and md to properly interact. >> >> It was suspected that bcache uses a faulty discard implementation. Some >> reports miss details about this setting. However, my setups are working >> fine with discards fully enabled on SSD - but without using MD. And it >> has been robust to accidental or implied reboots since all time I'm using >> it (even with btrfs as the filesystem on bcache). >> >> So I'd probably remove MD from your plans on using bcache. >> >> BTW: My system uses vanilla gentoo kernel, 4.1.4 currently. >> -- Replies to list only preferred. -- To unsubscribe from this list: send the line "unsubscribe linux-bcache" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
