Re: Best way to add caching to a new raid setup.

[Ram Ramesh <rramesh2400@gmail.com>]

On 8/28/20 12:46 PM, Roman Mamedov wrote:
> On Thu, 27 Aug 2020 21:31:07 -0500
> "R. Ramesh" <rramesh@verizon.net> wrote:
>
>> I have two raid6s running on mythbuntu 14.04. The are built on 6
>> enterprise drives. So, no hd issues as of now. Still, I plan to upgrade
>> as it has been a while and the size of the hard drives have become
>> significantly larger (a indication that my disks may be older) I want to
>> build new raid using the 16/14tb drives. Since I am building new raid, I
>> thought I could explore caching options. I see a mention of LVM cache
>> and few other bcache/xyzcache etc.
> Once you set up bcache, it cannot be removed. The volume will always stay a
> bcache volume, even if you decide to stop using caching. Which feels weird and
> potentially troublesome, going through an extra layer (kernel driver) with its
> complexity and computational overhead (no matter how small).
>
> On the other hand LVM with caching turned off is just normal LVM, that you'd
> likely would have used anyway, for other benefits that it provides.
>
> Also my impression is that LVM has more solid and reliable codebase, but
> bcache might provide a somewhat better the performance boost due to caching.
>
Thanks for the info on bcache. I do not think it will be my favorite. I 
am going to try LVM cache as my first choice. Note that the new disks 
will be spare disks for some time and I will be able to try out a few 
things before deciding to put it into use.

One thing about LVM that I am not clear. Given the choice between 
creating /mirror LV /on a VG over simple PVs and /simple LV/ over raid1 
PVs, which is preferred method? Why?

Ramesh