Re: bcache and LVM

On Feb 08 2016, "Jens-U. Mozdzen" [off-list ref] wrote:

Hi Nikolaus,

Zitat von Nikolaus Rath [off-list ref]:

quoted

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.

quoted

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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