Mailinglist Archive: opensuse (1620 mails)

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Re: [opensuse] configuring ssd cache
On 11/23/2014 01:57 PM, jdd wrote:

* 24 Gb seems to be a good size for root :-)

is it better than a cache?

That's a very good question and a very good observation.


Back in the PDP-11 days the V7 UNIX roll-in-roll out & memory management
meant that it was possible to have more physical memory than was useful
from the POV of user applications. So, at one USENIX, Mike Tilson
announced, and IIR even had buttons printer up with the code, which did
a trick with the driver for /tmp to improve its caching where a lot of
files were being created and deleted, such as in a development environment.

That was followed in subsequent years by VAX UNIX from Berkeley and
others using the increased address space and memory to perform other
kinds of caching; better inode caching, name resolution caching
contributed by SUN, and more.

The overall trend is that caching helps and often covers a wider
spectrum of utility.

Yes, having the rootFS on the SSD will dramatically speed up the loading
of programs and access to libraries, and in the situations where that is
a lot of traffic, what I consider the traditional UNIX mode where the
shell is used to to create and manage lots of transient short programs.

But there is another mode we should consider. I admit its not CICS, but
we do have a lot of single long lived programs running. Thunderbird,
Firefox and Darktable are mine. For these, especially for Darktable
doing an import of a large batch of photographs and apply basic
camera/lens filters and processing, its about data not about loading new
fresh code and libraries. Doing other photo processing, video and sound
processing, all involves a lot of temporary files, scratch-pad, and
more. Having the SSD devoted to /tmp (or whatever directory the
application is used for its scratch-pad) will be a lot more effective.

Ultimately the kernel caches are good and if the SSD could somehow be
plugged in to them specifically ... But no, we can't do that. The best
we could do is have the SSD as swap space on systems that are short of
physical memory. But then again, not all systems are short of physical
memory.


There are going to be many detailed use-cases and the optimal way to use
a SSD will differ amongst them. There is no one solution that will keep
everyone happy.


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