Cache Drive Recommendations

[Cabuzzi]

Hello all.

I'm currently running (and have been very pleased with) StarWind vSAN Free for my home lab SAN. I have two targets... both with a group of three 2GB 5400rpm drives in RAID5. My XenServer hosts are connected to flat SRs on each via a 10gbe iSCSI connection. The hardware is a Dell R710 with an H700 card and 72GB of RAM. The OS itself is installed on an SSD plugged into an SATA port on the mainboard.

The drives themselves are fairly slow, so I've been trying to gather information on setting up some sort of cache drive for each target. I'm a FreeNAS convert, so I tend to think in terms of L2ARC (read) and ZIL (write) cache drives on SSDs.

Budget is important, and since there the 6-bay 3.5" backplane is full, I've been looking into PCI-Express cards to host M.2 SSDs. Is this a good/bad idea, and why. Also, I'm only really concerned with read cache right now, unless I can cover both targets, read and write, with two SSDs (maybe with partitions?).

Here is the card I am looking at getting. My question is what kind of SSDs should I put on it, and how big should they be sized:

https://www.amazon.com/Best-Compute-PCI ... SD+adapter

[itsecasc]

hi - be aware only one of those slots on the board is PCIe the other one is sata - way lower speeds
i was looking for some reviews before i decided to buy me a new system. one i found:
http://benchmarkreviews.com/32720/silve ... er-review/
and - those pcie m2 strips geht cery hot - i added some additional heat sinks with cool paste and zip ties - looks ugly but works
i had bottle-necks very quick , and im not alone :
http://www.anandtech.com/show/9856/ange ... eatsinks/8

[Michael (staff)]

Hello,
Just a kind reminder that StarWind SSD cache works only in the write-through mode which gives a performance boost only on reads. At the same time, RAID5 has a very slow write performance. I would suggest to set there RAM cache in write-back mode which should improve read and writes.

[Cabuzzi]

Thank you for the insight. I guess I was thinking, if it were anything like L2ARC cache in FreeNAS, even a slow SSD and a dedicated (optimized) cache would work wonders. I was simply astonished what L2ARC did for ZFS. I was not as impressed with the ZIL cache. I know a ton about how those two work, but very little about the StarWind approach to caching. I would definitely like to know more. Other than the 50,000ft view on the StarWind "Service Side Caching" info page, I can't find many details on how StarWind works. For instance, it would be great to know how much cache is recommended per GB of published storage. This info may exist somewhere, I just can't find it.

I'm at the point now where I need to pick an M2 drive. They are expensive, so I can't just go with the "bigger is better" mentality... especially given what I know of caching technologies used by other storage vendors.

[Michael (staff)]

There are two modes for StarWind L1 (RAM) cache: Write-Back and Write-Through. Write-Back caching improves read/write speed and lowers the latency unlike Write-Through caching improves only the read speed. The cache settings are depending on total workload to the storage. The base recommendation is to put 1 GB of L1 cache in Write-Back mode per 1 TB storage capacity. Basically, configuring bigger cache size will give you better performance.
StarWind L2 (SSD) cache works only in Write-Through mode. It is recommended to make it 10% of the total capacity of the corresponding device. Additionally, it is recommended to use it only in high read-intensive environments.

[Cabuzzi]

Thanks for the thorough explanation. I ordered a 128GB M2 PCI-E drive to start. I will use that for the L2 cache to start with. Would this be the best article to follow:

https://knowledgebase.starwindsoftware. ... dance/661/

[Michael (staff)]

Yes, you can use this article to add L2 cache for the existing devices.