On Mon, Feb 20, 2017 at 05:50:09PM +0100, Christian Samsel wrote:

Hello raid team,
First of all, thanks for your work.
So i recently read about Linux raid5/raid6 write-cache and journaling and thought about
giving it a try. I'm mainly interested in the additional safety provided by the journal but might
want be future proof to use the write cache as well.
I read how to create an array using a journal but i havent found the slightest indication of how
large the respective device/partition should be. I went through the lwn article [1], the slides [2] of the
respective engineers at facebook and a few commit messages.

So my question is, let's assume i have a 6TB raid5 array (3x3TB), what would a good journal device size be?
I'd probably went with 4GB, as this is kinda the upper bound of what hardware raid controller offer. 

Thanks for trying! Depending on write-through or write-back mode. For
write-through mode, the size could be just several hundreds megabytes. For
write-back mode, the size should be a little bigger, several gigabytes, but 4
GB should be enough.

Also I added doc about raid5-cache in
kernel_source/Documentation/md/raid5-cache.txt recently.

Thanks,
Shaohua