Block Zero or Write Same

ESXi supports three disk formats for provisioning virtual disks:

1. Eagerzeroedthick (thick provision eager zeroed)—the entirety of the virtual disk is completely reserved and pre-zeroed upon creation on the VMFS. This virtual disk allocation mechanism offers the most predictable performance and the highest level of protection against capacity exhaustion.
2. Zeroedthick (thick provision lazy zeroed)—this format reserves the space on the VMFS volume upon creation but does not pre-zero the encompassed blocks until the guest OS writes to them. New writes cause iterations of on-demand zeroing in segments of the block size of the target VMFS (almost invariably 1 MB with VMFS 5). There is a slight performance impact on writes to new blocks due to the on-demand zeroing.
3. Thin (thin provision)—this format neither reserves space on the VMFS volume nor pre-zeros blocks. Space is reserved and zeroed on-demand in segments in accordance to the VMFS block size. Thin virtual disks allow for the highest virtual machine density but provide the lowest protection against possible capacity exhaustion. There is a slight performance impact on writes to new blocks due to the on-demand zeroing.

   

   Prior to WRITE SAME support, the performance differences between these allocation mechanisms were distinct. This was due to the fact that before any unallocated block could be written to, zeros would have to be written first, causing an allocate-on-first-write penalty. Therefore, for every new block that was to be written to, there were two writes, the zeros then the actual data. For thin and zeroedthick virtual disks, this zeroing was on-demand so the effect was observed by the guest OS in the virtual machine that was issuing writes to new blocks. For eagerzeroedthick, zeroing occurred during deployment and therefore large virtual disks took a long time to create but with the benefit of eliminating any zeroing penalty for new writes.

   To reduce this latency, VMware introduced WRITE SAME support. WRITE SAME is a SCSI command that tells a target device (or array) to write a pattern (in this case, zeros) to a target location. ESXi utilizes this command to avoid having to actually send a payload of zeros. Instead, ESXi simply communicates to an array that it needs to write zeros to a certain location on a certain device. This not only reduces traffic on the SAN fabric, but also speeds up the overall process since the zeros do not have to traverse the data path.

   This process is optimized even further on the Everpure FlashArray. Since the array does not store space-wasting patterns like contiguous zeros, the metadata is created or changed to simply denote that these locations are supposed to be all-zero so any subsequent reads will result in the array returning contiguous zeros to the host. This additional array-side optimization further reduces the time and penalty caused by pre-zeroing of newly-allocated blocks.

   The following sections will outline a few examples of WRITE SAME usage to describe expected behavior and performance benefits of using WRITE SAME on the Everpure FlashArray.