L2 Cache 'Instant' Gather Interval isn't gathering blocks fast enough under load.

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csyaru
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L2 Cache 'Instant' Gather Interval isn't gathering blocks fast enough under load.

Post by csyaru »

Hello,

I've stuck with the scenario when I need to fill 100%R L2 on a busy HDD drive (several VMs running) but the cache warm-up speed is too low compared to the idle state.
Usually, the speed is single-digit MB/s under load while on idle drive reads can go up to hundreds of MB/s.

My question is why the L2 read cache isn't filled instantly as data is read from the cached drive? Why idle is required to gather already read data?
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Re: L2 Cache 'Instant' Gather Interval isn't gathering blocks fast enough under load.

Post by Support »

csyaru wrote: Sun Oct 16, 2022 11:32 pm My question is why the L2 read cache isn't filled instantly as data is read from the cached drive? Why idle is required to gather already read data?
If the "Instant" is used in the L2 Gather Interval, PrimoCache will ignore the system busy state and gather read-data as quick as possible. I think the problem here is that the HDD is too busy and fetching data from the HDD to L2 cache is also slow. Besides, Windows may assign too much CPU time to HDD related tasks.
csyaru
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Re: L2 Cache 'Instant' Gather Interval isn't gathering blocks fast enough under load.

Post by csyaru »

I would not agree, I have checked multiple times and if there's some small activity on the cached disk, the cache would not be filled at full speed, it awaits an idle state to crank up cache gathering to maximum speed.

Actually, I was using this behavior to throttle writing speed on caching SSD to leverage SLC cache.
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Re: L2 Cache 'Instant' Gather Interval isn't gathering blocks fast enough under load.

Post by Support »

PrimoCache needs to read data from HDDs and then write to L2 if data is not in the L1 cache. So if the HDD has other activities, reading from the HDD might become slow, especially when the HDD has poor performance on mixed read-write activities.
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