Re: layering question.

Kai Krakow
2015-08-05T23:10:57Z

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.

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