How does 100% Read Cache work...?

FAQ, getting help, user experience about PrimoCache
mell111
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Re: How does 100% Read Cache work...?

Post by mell111 »

Got it about the reads. Thanks.

If you don't mind, would you kindly clarify further on the writes? It might help me better understand the occasional performance problems I'm observing and figure out what to do about them. So: with 64kb block size in underlying hard drive, 512kb PrimoCache block size and Windows/App writing various sizes, typically from 1kb to 4kb. With Defer Write enabled and, for example, with Native mode and a 90sec interval and assume PrimoCache accumulated about 1GB of deferred write data. Also assume little to no fragmentation on the hard drive. How would PrimoCache try to "organize" the data it writes at the 90 second interval: will it try to write 512kb at a time? Something else?
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Re: How does 100% Read Cache work...?

Post by Support »

PrimoCache tries to write all deferred write-data when the 90sec interval arrives. It ignores cache block size and only write dirty data. That is, if Windows/app only wrote 4KB data before, then only 4KB data, not the block size 512KB data, will be written to disk by PrimoCache.
PrimoCache writes data at a maximum possible speed, combining write requests for continuous address.

Regarding the occasional performance problems you mentioned, I wonder this is caused by long latency. If the latency is big and cache space is not enough, cache space might be full of deferred write-data. In such case, PrimoCache will start a process called "urgent write", which will write some amount of deferred data to disk by force and make cache room for new coming data. This process will cause performance issue. You may check the statistics and see if the "Urgent Write" is 0.
mell111
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Re: How does 100% Read Cache work...?

Post by mell111 »

Can you just clarify "combining write requests for continuous address"? Thanks.
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Re: How does 100% Read Cache work...?

Post by Support »

For eg. Windows/App wrote first 4KB data on the address 0 to 4KB, then later wrote another 4KB data to the address 4KB to 8KB. PrimoCache will write these continuous 8KB data to underlying disk at a time when doing the flush.
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