I know what you and Wols are talking about and I think it's actually two separate things. Wol's is referring to traditional read caching where it only benefits if you are reading the same thing over and over again, cache hits. For streaming it won't help as you'll never hit the cache. What you are talking about is a write cache, something I have seen implemented before. Basically the idea is for writes to hit the SSD's first, the SSD acting as a cache or buffer between the filesystem and the slower RAID array. To the end process they're just writing to a disk, they don't see the SSD buffer/cache. QNAP implements this in their NAS chassis, just not sure what the exact implementation is in their case. On Fri, Aug 28, 2020 at 9:14 PM R. Ramesh <rramesh@xxxxxxxxxxx> wrote: > > On 8/28/20 7:01 PM, Roger Heflin wrote: > > Something I would suggest, I have found improves my mythtv experience > > is: Get a big enough SSD to hold 12-18 hours of the recording or > > whatever you do daily, and setup the recordings to go to the SSD. i > > defined use the disk with the highest percentage free to be used > > first, and since my raid6 is always 90% plus the SSD always gets used. > > Then nightly I move the files from the ssd recordings directory onto > > the raid6 recordings directory. This also helps when your disks start > > going bad and getting badblocks, the badblocks *WILL* cause mythtv to > > stop recording shows at random because of some prior choices the > > developers made (sync often, and if you get more than a few seconds > > behind stop recording, attempting to save some recordings). > > > > I also put daily security camera data on the ssd and copy it over to > > the raid6 device nightly. > > > > Using the ssd for recording much reduces the load on the slower raid6 > > spinning disks. > > > > You would have to have a large number of people watching at the same > > time as the watching is relatively easy load, compared to the writes. > > > > On Fri, Aug 28, 2020 at 5:42 PM Ram Ramesh <rramesh2400@xxxxxxxxx> wrote: > >> 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 > >> > I was hoping SSD caching would do what you are suggesting without daily > copying. Based on Wol's comments, it does not. May be I misunderstood > how SSD caching works. I will try it any way and see what happens. If > it does not do what I want, I will remove caching and go straight to disks. > > Ramesh -- Drew "Nothing in life is to be feared. It is only to be understood." --Marie Curie "This started out as a hobby and spun horribly out of control." -Unknown
