>but fast cache does favour Random IOs; isn't that right?
FAST Cache does not algorithmecly favor any I/O profile. However, the FAST Caches most efficient usage is with 'read heavy', random I/O workloads. 'Read heavy' means I/O with a high percentage of reads. Its with read heavy random I/O and FAST Cache that you get the lowest host response time.
>IO’s over 128KB in size bypass FASTCache actually.
Its 'equal to and greater' 128 KB are excluded from analysis. However, there are some 'corner cases' where large I/Os, that would normally be excluded, can become fragmented and enter the FAST Cache.
>i don't understand how sequential workload gets promoted to Fast Cache.
Its the locality that does the algorithm in. Three LBA hits in proximity to each other promote a chunk. The granularity of the promoted 'chunk' is 64KB, which is the proximity the algorithm 'watches' for. In the sequential workload the three consecutive LBAs cause their 64 KB 'chunk' to be promoted. In the same way that a random workload where three LBAs, each separated from each other by 8KB, but not in consecutive order would promote their chunk.
Oh...backup workload with 64k (large) sequential I/O seems to be very unsuitable for FAST Cache then. It would be better to disable fast cache for this kind of I/O profile as it would saturate FAST Cache space quickly.
In the simple answer; a small chunk of system memory is used to store data for FAST cache to be able to operate. The system cache is brought offline to allow it to be re configured just a wee bit smaller than it was before. Don't worry, the rest of the system cache come back.
2 - 3. I estimate you'll have 1709 MB or there abouts. It may be lower depending on what else you have installed. The read/write cache ratio shall be maintained through the addition. However, you may want to consider a 1368 MB write and 341 MB read cache, if that's not what you see.
I am not sure how JSP came up with that number, but in general I use an average of around 1GB of DRAM is allcoated to FAST Memory Map per 1TB of FAST Cache used. I am sure that is off by a chunk or two, but is has been fairly accurate in my arrays.
1GB RAM per 1TB FAST Cache ? So if I have 200TB of FAST Cache, the array should have 200GB of RAM ? I'm pretty sure I need a V-MAX or DMX for that. Clariions or VNX's don't have that amount of RAM !
Message was edited by: RRR Of course I meant 200GB of FAST Cache, not 200TB, which would be great.... My mistake 😉
Ahem. FAST Cache only suports 2TB -- that on the CX4/NX-960. You confusing FAST VP vs FAST Cache?
Hope that helps!