Re: Backing/caching ration

On Thu, Aug 2, 2012 at 1:01 AM, Piergiorgio Sartor
[off-list ref] wrote:
..

At the moment, I've 1TB RAID-10, of which /usr is
24GiB (10GiB used), /home is 64GiB, and then there

So this is a desktop then?

are two "work area" volumes of 250GiB.

Some other space is spread around /var and similar,
and there is some backup space, for future use.

Any idea on what could be "minimum" SSD size (due to
cost, of course).

What is your objective? To speed up normal Desktop tasks such as
booting and loading applications? I haven't kept to date with bcache,
so my advice may be outdated, but...

Given that you only use 10GB of usr my hunch would be that 16GB would
be plenty; However, at 16GB speed is a concern, particularly as 16GB
SSDs are usually sitting on the other side of a USB2 port. The maximum
write speed of your raid array is going to be capped to the write
speed of the SSD unless you use write-around.

If price is really a concern you could try using an old 4gb usb stick
to writearound cache /usr, and see it that helps somewhat.

In summary, if you want the benefit of writeback caching, get a decent
2.5" internal SSD. Internal SSD are pretty much always at least 32GB,
which should be plenty for normal Linux desktop use.


Hope this helps,


-- 
John C. McCabe-Dansted