iSCSI ZVOL Block Size Recommendations

[viniciusferrao]

Hello guys,

I've read a lot of topics recommending 128k as the default block size for hosting VMs.

The question is: should this be done? This can be applied only in a particular ZVOL or should be applied in the entire pool? And finally how to this?

Thanks in advance,

 

[dlavigne]

This option is available in Advanced Mode when creating the zvol: http://doc.freenas.org/index.php/Volumes#Creating_a_zvol. It's my understanding that it depends upon the filesystem that will be put on extent.

 

[viniciusferrao]

This option is available in Advanced Mode when creating the zvol: http://doc.freenas.org/index.php/Volumes#Creating_a_zvol. It's my understanding that it depends upon the filesystem that will be put on extent.

I will be using LVM from XenServer.

Thanks for the info!

 

[viniciusferrao]

Looking deeper, I've found this:

Code:

  --- Physical volume ---
  PV Name               /dev/sdb
  VG Name               VG_XenStorage-f3c604cd-325c-2664-7552-a3db49a67882
  PV Size               5.00 TB / not usable 10.00 MB
  Allocatable           yes 
  PE Size (KByte)       4096
  Total PE              1310717
  Free PE               771350
  Allocated PE          539367
  PV UUID               P3AQWb-QPmm-QG4T-n16t-hyz3-rM99-3axr5p

So this is the information for iSCSI LVM on XenServer.

I think PE Size is the block size, but I'm not sure.

In my calculations 4096*1310717 results in 5.0TB, as expected.

I don't know if there any LVM experts here but it appears that the block size is 4MB.