Re: bcache and LVM

[Nikolaus Rath <Nikolaus@rath.org>]

On Feb 08 2016, "Jens-U. Mozdzen" <jmozdzen@nde.ag> wrote:
> Hi Nikolaus,
>
> Zitat von Nikolaus Rath <Nikolaus@rath.org>:
>> Hello,
>>
>> The internet claims that using bcache with LVM is not a good idea
>> (eg. on https://wiki.archlinux.org/index.php/Bcache )- but I wasn't able
>> to find any substantial information other than this general
>> recommendation.
>
> looking at that article, it only references possibly ill-handled
> "discard" operations, when running bcache on top of LVM.

Well, it says:

| Warning:
| 
|     it is widely recommended to use Bcache underneath any other block
|    layer.


To me this implies that running it on top of another block layer is
dangerous (why warn about it otherwise?). 

> The article itself does positively mention sub-dividing bcache devices
> using LVM.

Yep, but the talk page then says:

| Initially, LVM did not recognize my /dev/bcache0 when I wanted to
| create a physical volume on it. For anyone else who has that issue,
| this may be relevant:
| http://www.redhat.com/archives/linux-lvm/2012-March/msg00005.html.


>> Is this still (or has ever been) correct? If so, what issues can arise?
>> And does this happen only when using bcache on top of an LVM LV, or also
>> when using a bcache device as an LVM PV?
>
> We're running bcache between MD-RAID (RAID6 for the HDD backing store
> and RAID1 for caching SSDs) and LVM2 (using bcache0 as the only PV for
> the volume group) without any noticeable problem, for moderate to
> significant load (SAN/NAS servers with NFS, Samba, and virtual
> machines' storage via SCST/FC and SCST/iSCSI).

Thanks for the datapoint! What kernel version do you use?


Best,
-Nikolaus

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