Backing/caching ration

Backing/caching ration

[Piergiorgio Sartor]

Hi all,

I'm thinking about buying an SSD, I would like to
know if someone can, more or less, suggest what
could be a reasonable size, in relationship with
the backing device.

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

Thanks,

bye,

-- 

piergiorgio

Re: Backing/caching ration

[Adam Berkan]

In our experiements we've seen around 1% flash to disk ratio gives
reasonable performance.  Of course it depends a lot on workload so you
may need a bit more than that.

Adam

On Wed, Aug 1, 2012 at 10:01 AM, Piergiorgio Sartor
<piergiorgio.sartor-i47jiTeKxPI@public.gmane.org> wrote:
> Hi all,
>
> I'm thinking about buying an SSD, I would like to
> know if someone can, more or less, suggest what
> could be a reasonable size, in relationship with
> the backing device.
>
> At the moment, I've 1TB RAID-10, of which /usr is
> 24GiB (10GiB used), /home is 64GiB, and then there
> 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).
>
> Thanks,
>
> bye,
>
> --
>
> piergiorgio
> --
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Re: Backing/caching ration

[John McCabe-Dansted]

On Thu, Aug 2, 2012 at 1:01 AM, Piergiorgio Sartor
<piergiorgio.sartor-i47jiTeKxPI@public.gmane.org> 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so my advice may be outdated, but...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