Level 3 cache?

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Level 3 cache?

Post by points »

As we might plan to add SATA SSD to a machine, it is considered to use them as cache as well. However there is at least theoretical performance difference between SATA SSD and PCI SSD. Is there a way to add something like a a level 3 cache? So that first of all L1 cache gets filled. After that the slower L2 cache on a PCI SSD and when this is full then to fill up the L3 cache on the SSD SATA drive which would be the slowest of them?
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Re: Level 3 cache?

Post by points »

While this thread viewtopic.php?f=33&t=4797#p13621 seems to answer the question if a Level 3 cache is already implemented, I wonder what would happen when setting up PrimoCache the following way:

1. L1 cache from RAM
2. L2 cache as virtual disk on dedicated logical volume on NVMe
3. L2 cache on SATA SSD set to cache the logical volume of No. 2

If the L2 cache on SATA SSD is much larger than the cache on NVMe, over time it should hold more and more previously cached data available.
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Re: Level 3 cache?

Post by Support »

I'm sorry that we don't have the plan to support level-3 cache.
points wrote: Sat Sep 28, 2019 9:33 am 1. L1 cache from RAM
2. L2 cache as virtual disk on dedicated logical volume on NVMe
3. L2 cache on SATA SSD set to cache the logical volume of No. 2
I don't understand this setting. L2 cache can only use one logical volume. And L2 cache shall not use a slow drive to cache a fast drive, otherwise the fast drive will get slow down and throttled by the slow L2 cache.
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Re: Level 3 cache?

Post by points »

support wrote: Sun Sep 29, 2019 3:13 am I'm sorry that we don't have the plan to support level-3 cache.
points wrote: Sat Sep 28, 2019 9:33 am 1. L1 cache from RAM
2. L2 cache as virtual disk on dedicated logical volume on NVMe
3. L2 cache on SATA SSD set to cache the logical volume of No. 2
I don't understand this setting. L2 cache can only use one logical volume. And L2 cache shall not use a slow drive to cache a fast drive, otherwise the fast drive will get slow down and throttled by the slow L2 cache.
I explain. Maybe I am wrong as we don't have it set up that way and it is all theorey. But I believe it should be possible:

First you set regulard L1 cache from RAM. Then create a virtual disk on a dedicated volume of a NVMe. Mount virtual disk and set this mounted volume as L2. But now still the underlying partition has a drive letter and is a regular disk, so it appears a cachable in PC. So it is possible to cache the entire volume including the virtual disk that serves as L2 cache.
I have not tested but it should easily be possible to create another volume or virtual disk on the SATA SSD, convert it as L2 and use it as cache for the volume where the other L2 cache is on.
I don't know if it will return the desired results but the setup shoud be possible.
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Re: Level 3 cache?

Post by Support »

Thank you for the explanation, I got it now. Yes, such setting is possible, however the performance of both cache tasks will be throttled by slow SATA SSD.
BTW, the performance of L2 which uses virtual disk is much that using native drive partition. So it is not recommended that using virtual disks as level-2 cache.
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