Re: layering question.

A. James Lewis
2015-08-06T00:54:15Z

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?

Does anyone make an M.2 raid controller... being PCIe, I don't know it
that could even be possible.

James

On 06/08/15 00:10, Kai Krakow wrote:
> Jens-U. Mozdzen <jmozdzen@nde.ag> schrieb:
>
>> Zitat von Kai Krakow <hurikhan77@gmail.com>:
>>> A. James Lewis <james@fsck.co.uk> 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.
>