vSphere Flash Read Cache and esxcli

As most features these days in vSphere you can configure them using the awesome esxcli command. I’ve already mentioned esxcli in my vSphere Flash Read Cache FAQ blog but I wanted to call it out explicitly here as I found it very useful. You can get some nice details using the esxcli command. So where do we start?

First thing would be:

esxcli storage vflash

This will return that there are 3 namespaces: cache, module and device. Lets start top down with device. The command “esxcli storage vflash device list” will show you a list of all flash devices and whether it has been configured for vFRC or not. The module namespace can provide you some more details around for instance cache blocksizes etc. If you run the command this is what the output looks like:

~ # esxcli storage vflash module get
 Min Supported Module Version:
 Supported Cache Block Size Max: 1048576
 Supported Cache Block Size Min: 4096
 Supported Cache Size Max: 214748364800
 Supported Cache Size Min: 4194304
 Supported Disk Size Max: 17592186044416
 Supported Mode Mask: WriteThru

You can see in this case that the minimum supported block size is 4KB and the max 1024KB and that the cache is supported for write through etc. So what about VM statistics / details, that is more interesting right? Well that is where the cache name space comes in to play! So you start with listing your current enabled cache:

~ # esxcli storage vflash cache list

Now that we know what the name is of our cache we can retrieve some details.

~ # esxcli storage vflash cache get -c vfc-413278667-vfrc-test
World ID: 2299121
Cachename: vfc-413278667-vfrc-test
Vmdkname: vfrc-test-flat.vmdk

And of course we can get some, probably the most interesting part, nice metrics about this specific cache file. Note I just powered-on this virtual machine, hence the extremely low cache hit rate.

~ # esxcli storage vflash cache stats get -c vfc-413278667-vfrc-test
Cache hit rate (as a percentage): 4
Total cache I/Os: 196
Mean cache I/O latency (in microseconds): 24823
Mean disk I/O latency (in microseconds): 45261
Total I/Os: 5883
Mean IOPS: 44
Max observed IOPS: 44
Mean number of KB per I/O: 586
Max observed number of KB per I/O: 6936
Mean I/O latency (in microseconds): 22304
Max observed I/O latency (in microseconds): 23623
Last I/O operation time (in microseconds): 0
Number of I/O blocks in last operation: 0
Mean blocks per I/O operation: 0
Total failed SSD I/Os: 0
Total failed disk I/Os: 0
Mean number of cache blocks in use: 11728

And if you want to reset the statistics you can do so using:

~ # esxcli storage vflash cache stats reset -c vfc-413278667-vfrc-test

Nice to know right?!

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    1. says

      When will a block be evicted from the cache?
      I’m curious about the exact algorithm vFlash uses for block promotion and demotion..

      I can’t find any details on this.