Mutlitple technical questions

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tyraelmetanamede
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Mutlitple technical questions

Post by tyraelmetanamede »

Hi There,

i am still within the trial phase of PRimoCache and still have not figured it out completely.
Usually i am a linux user so i have used bcache for many years which is quite similar to PrimoCache so i will compare it to that.

The most important question i still have:
If i only use L2-Cache and activate Defer-Write with let's say "infinite" delay, will my data be corrupted when the PC crashes/looses power etc.?
Bcache on Linux is pretty much safe against that as the kernel documents state, i could not find a clear answer on that on the forums as many replies (even from @support) seem to contradict.

On my Test-Device i currently have a 1TB spinning disk and a 250 GB 850 Evo SSD completly reserved for caching (25% Cache-Capacity). I am quite fond of how PRimoCache works (usually i have a hit rate above 95%). I also dedicate around 10 GB of write-cache on the SSD.

But the hard-disk never seems to spin down, is there a config which will prevent writing to the disk and instead always use the SSD? even while the SSD write-cache is only 5% full, there seem to be urgent writes to the disk...

Another question is about the "Free Cache (L2)": The Read Cache is currently 195 GB free, the Write Cache stays at 1,9 GB free - even if i flush the defered data manually. Why is that? Why is the write cache not freeing up some more?

Best regards
Kadrim
tyraelmetanamede
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Re: Mutlitple technical questions

Post by tyraelmetanamede »

also: if L2 Cache is not failure resistant (i.e. has to be flushed before boot) i really don't understand why the free L2 write cache statistics survice a reboot.

Clarification on this matter would be really helpful!
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Re: Mutlitple technical questions

Post by Support »

tyraelmetanamede wrote: Tue Feb 12, 2019 12:12 pm If i only use L2-Cache and activate Defer-Write with let's say "infinite" delay, will my data be corrupted when the PC crashes/looses power etc.?
So far, there still is the risk as L1 even you only use L2 with defer-write. We're study to make it safer, however, I'm afraid that this will not come out soon.
tyraelmetanamede wrote: Tue Feb 12, 2019 12:12 pm But the hard-disk never seems to spin down, is there a config which will prevent writing to the disk and instead always use the SSD? even while the SSD write-cache is only 5% full, there seem to be urgent writes to the disk...
Could you upload a screenshot of the PrimoCache main dialog which shows cache configuration and statistics?
tyraelmetanamede wrote: Tue Feb 12, 2019 12:12 pm The Read Cache is currently 195 GB free, the Write Cache stays at 1,9 GB free - even if i flush the defered data manually. Why is that? Why is the write cache not freeing up some more?

They just indicates how much data stored in cache spaces. PrimoCache will automatically replace old cached data (not deferred write-data) and cache new data, using all cache space if possible. There's no need to free up old cached data because Windows/App may read these data.
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Re: Mutlitple technical questions

Post by Support »

tyraelmetanamede wrote: Tue Feb 12, 2019 1:55 pm also: if L2 Cache is not failure resistant (i.e. has to be flushed before boot) i really don't understand why the free L2 write cache statistics survice a reboot.
PrimoCache will automatically flush before normal reboot/shutdown. However on unexpected shutdown/crash, PrimoCache has no way to flush in which L2 statistics will not survive.
tyraelmetanamede
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Re: Mutlitple technical questions

Post by tyraelmetanamede »

Thanks for your clarification!
Could you upload a screenshot of the PrimoCache main dialog which shows cache configuration and statistics?
Will do, sir! I just need to reset some things (currently working on another tool which need the cache to be quite full, so i cannot reset it right now).

But in the meantime another question bothers my mind:

In bcache there is an option to configure a sequential_cutoff, meaning, if the system detects lots of sequential reads (i.e. 4 MB) the data will not be marked as cachable and as a fact not thrown into the SSD Cache (things like large video files) - does a similar option exist for PrimoCache or is this already a planned feature?

Best regards,
Kadrim
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Re: Mutlitple technical questions

Post by Support »

So far no, but this feature has been considered and we might support it in future.
Thanks.
tyraelmetanamede
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Re: Mutlitple technical questions

Post by tyraelmetanamede »

support wrote: Wed Feb 13, 2019 4:12 am
tyraelmetanamede wrote: Tue Feb 12, 2019 12:12 pm But the hard-disk never seems to spin down, is there a config which will prevent writing to the disk and instead always use the SSD? even while the SSD write-cache is only 5% full, there seem to be urgent writes to the disk...
Could you upload a screenshot of the PrimoCache main dialog which shows cache configuration and statistics?
urgent-writes.png
urgent-writes.png (65.27 KiB) Viewed 4441 times
I did not reset the cache yet, but htis shows pretty much what i meant: The PC has just been booted and has still enough L2-Write-Cache free - but still, there were urgent writes. It does not matter to what delay i set the "defer-write" function (typically 1800).
Or does this happen because i activated L1 cache and that was full? If the urgent-write would mean, that this data from L1 has been written to the HDD (instead of L2) that would explain the behaviour.
tyraelmetanamede
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Re: Mutlitple technical questions

Post by tyraelmetanamede »

Just a quick follow up with deactivated L1 cache:
writes.png
writes.png (62.74 KiB) Viewed 4438 times
the system is up for about 5 minutes, so far no urgent writes yet, but there is already visible some writes to disk (24 kb).

So the real question is: has aynone ever managed to spin down the HDD (boot drive!) when cached by a large enough SSD?

Best regards
Kadrim
tyraelmetanamede
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Re: Mutlitple technical questions

Post by tyraelmetanamede »

and here is the last one:

uptime is less than 300 seconds ( 5 minutes ) and both caches do have enough free space - still i can see another urgent write.
urgent-write.png
urgent-write.png (63.9 KiB) Viewed 4406 times
Best regards,
Kadrim
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Re: Mutlitple technical questions

Post by Support »

I'm sorry for the late reply!
tyraelmetanamede wrote: Fri Feb 15, 2019 8:21 am Or does this happen because i activated L1 cache and that was full? If the urgent-write would mean, that this data from L1 has been written to the HDD (instead of L2) that would explain the behaviour.
Yes. This is an issue which we will improve.

I think you may need to disable L1 write caching if you want to reduce HDD writes because in current versions when L1 write cache is full, it will directly flush deferred data to HDD.
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