L2 Cache Not Used When Needed

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mell111
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Re: L2 Cache Not Used When Needed

Post by mell111 » Mon Jan 07, 2019 6:10 pm

The Urgent writes definitely increased substantially, but I can't say that it was steady (as opposed to bursts) since I wasn't monitoring the whole time.

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support
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Re: L2 Cache Not Used When Needed

Post by support » Tue Jan 08, 2019 7:20 am

Not sure if this is the cause, but if possible, you may try a smaller latency like 10s, 30s or 60s to check if the problem still happens or not.
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mell111
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Joined: Fri Oct 05, 2018 11:16 am

Re: L2 Cache Not Used When Needed

Post by mell111 » Tue Jan 08, 2019 6:25 pm

I tried that - it didn't help. The longer latency appears to have reduced the incidence of this (though it could be a coincidence), but once the thrashing begins, the only thing that works is to stop app activity to the affected disk and allow PrimoCache to catch up. I find it puzzling that even with this stopped activity, PrimoCache still takes a few minutes to write about 2GB, but I presume its because of internal activity of the drive.

Can you tell me what to look for if I encounter this again? Also, as it stands now, should PrimoCache reduce its writing activity when the underlying disk is very slow and it has cache space available? If not, what do you think about that? Thanks.

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support
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Re: L2 Cache Not Used When Needed

Post by support » Wed Jan 09, 2019 5:51 am

We'll introduce a new writing option to coordinate the work of L1 and L2 write cache. It might be helpful to this problem.
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mell111
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Joined: Fri Oct 05, 2018 11:16 am

Re: L2 Cache Not Used When Needed

Post by mell111 » Wed Jan 16, 2019 2:04 am

I have an additional data point that illustrates the problem with more clarity. The screenshot below shows no writing to L2 cache and plenty of available L1 cache and hardly any urgent writes while the underlying drive was thrashing and the writing app was slowed blocking on writes (indicating, I think, that PrimoCache is passing along the writes directly to the drive, rather than caching). When the deferred blocks reached around 20,000 I suspended the app and PrimoCache slowly recovered as before. This whole episode lasted around an hour with the thrashing probably lasting around 10 minutes. Is this expected behavior under the circumstances? Can you say a few words about the new writing option and how it might help? Thanks.
PrimoCache-20190115.png
PrimoCache-20190115.png (16.92 KiB) Viewed 86 times

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Jaga
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Re: L2 Cache Not Used When Needed

Post by Jaga » Wed Jan 16, 2019 5:24 am

Just an observation based on that screenshot - your Urgent Writes are a little over 100MB, while total writes are over 86GB. Do you ever get into a scenario where urgent writes are a significant portion (i.e. Primocache truly "spills over" and hammers the drive directly with urgents)?

mell111
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Joined: Fri Oct 05, 2018 11:16 am

Re: L2 Cache Not Used When Needed

Post by mell111 » Wed Jan 16, 2019 6:37 am

Typically in my usage urgent writes are less than 3% or %4. It was a coincidence that the scenario I described in this thread came up shortly after I started a session, so the data in the screenshot is not mixed with a lot of "normal" data (although there was no thrashing for approximately 30 minutes from the start). The odd thing to me is that PrimoCache is not caching incoming data as it struggles with slow writes to a thrashing drive (under normal circumstances it does continue to cache as it write to the disk at normal speed.) If it did, this should allow the drive to "catch up" and recover, but being hammered by both PrimoCache and the app doesn't allow this. Also, on two occasions my system crashed almost certainly in this scenario (I can't be 100% sure since it didn't happen while I was observing, but my system has been rock solid for a couple of years now.)

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