On 8/28/20 5:12 PM, antlists wrote:
On 28/08/2020 18:25, Ram Ramesh wrote:
I am mainly looking for IOP improvement as I want to use this RAID in
mythtv environment. So multiple threads will be active and I expect
cache to help with random access IOPs.
???
Caching will only help in a read-after-write scenario, or a
read-several-times scenario.
I'm guessing mythtv means it's a film server? Can ALL your films (or
at least your favourite "watch again and again" ones) fit in the
cache? If you watch a lot of films, chances are you'll read it from
disk (no advantage from the cache), and by the time you watch it again
it will have been evicted so you'll have to read it again.
The other time cache may be useful, is if you're recording one thing
and watching another. That way, the writes can stall in cache as you
prioritise reading.
Think about what is actually happening at the i/o level, and will
cache help?
Cheers,
Wol
Mythtv is a sever client DVR system. I have a client next to each of my
TVs and one backend with large disk (this will have RAID with cache). At
any time many clients will be accessing different programs and any
scheduled recording will also be going on in parallel. So you will see a
lot of seeks, but still all will be based on limited threads (I only
have 3 TVs and may be one other PC acting as a client) So lots of IOs,
mostly sequential, across small number of threads. I think most cache
algorithms should be able to benefit from random access to blocks in SSD.
Do you see any flaws in my argument?
Regards
Ramesh
