controller cache + host memory cache

[ofc2logic]

hello...

looking to understand the consequences of cascading software and hardware caches in a virtualized environment.

virtualized IIS, SQL, Exchange, BES, and various workloads with iSCSI (Starwind) connectivity to a Dell server with a PERC 6/i controller that has 256MB cache. The cache could be configured in either "write-back" or "write through" mode.

Would it make sense to utilize a software (Starwind) cache? Would this result in better performance? Does it makes sense to allocate a monsterous amount of memory to the cache (like 1GB or higher)?

Thanks. I love this product.

[anton (staff)]

It's recommended to use StarWind write-back cache because of a set of reasons:

1) It's MUCH faster. StarWind's cache is located in RAM so sits behind wide bandwidth and low latency memory bus using up-to-date memory technology. At the same time RAID controller's cache is sitting behind comparably slow and higher latency PCIe bus and is built using previous generation memory technology.

2) It's MUCH safer. StarWind's cache is distributed (mirrored) between multiple nodes so if one node will go down - no data loss occures. At the same time RAID controller's cache is protected in the best case with a battery (actually big capacitor). In a nutshell: StarWind cache survives node destruction and RAID controller survives power outage only.

3) It's MUCH cheaper. StarWind cache can be controlled by you (size) and returned back to hypervisor if not needed. Also it's built using cheap memory used as a main system memory. At the same time you can do nothing with not used on-board RAID controller RAM and it's modules are 10x times more expensive per gigabyte of storage compared to main system memory.

Keeping in mind modern operating systems can take care of a software RAID in a very good way (storage spaces & ZFS RAID-Zxxx) also utilizing block signatures for reciliency - it's better to use RAID card as a SAS/SATA port number extender ONLY.

[NextCloud]

What is the maximum cache that can be allocated to an image file? I have 288GB of RAM but found that I can't allocate more than 140GB to the first image file, and subsequent image files get even less. Also, I noticed in Windows Task Manager that not always does Starwinds actually allocate the RAM. This is on Starwinds v. 6.0.4768 and fully patched WS2008R2 SP1. The volume is 60TB in RAID60. Ideally I would have just one image file of 60TB and 280GB of RAM allocated for write back cache (leaving 8GB for Windows OS).

Any guidance on how to better utilize the cache would be appreciated.

[Anatoly (staff)]

I can't allocate more than 140GB to the first image file

Could you clarify that? Are you getting some error?

The volume is 60TB in RAID60

I`d like you to know that recommended RAID for implementing an HA are RAID 1, 0 or 10, RAID 5 or 6 are not recommended due to low write performance. RAID 60 may work, but I`d rather started to use 10, but that is just mine opinion.

Any guidance on how to better utilize the cache would be appreciated.

As my colleague said - just assign all you can assign to WB cache. The reasons you can see in his post as well, and I think nobody will disagree with any of those.