VSAN Caching – Hybrid vs All Flash

With the release of VSAN 6.2,  VMware is ready to enter into the enterprise storage market with full functionality. Dedupe, Raid, Checksum features in addition to existing stretch cluster, ROBO, All Flash, RAID 0/1 , Native Snapshot functionalities makes VSAN a true competitor to all existing  storage arrays.

While reading about the new functionalities of VSAN 6.2, it was observed that most of the new features work only on All flash configuration. VMware documentation on VSAN   mentioned that All Flash will not have cache functionality for Read I/O’s .Why …?

Why to  Cache IOs

Caching of I/O ( both Read and Write) helps to avoid the costly lookup’s to the underlying disks. In case of VSAN the SSD disk selected as ‘cache tier’ act as the caching layer which helps to provide the data directly to the VM without allowing the I/O to go to the underlying disk. But , in the case of all Flash disk group, all disks in the disk group are SSD disks. In case of SSD, the latency for I/O is extremely low compared to magnetic disk.    SSD’s  IOPS are measured in terms of thousands and magnetic disks IOPS are measured in hundreds.  Because of this, there wont be any noticeable latency when reads done from SSD’s in ‘capacity tier’. Hence   there is no need to specifically cache the IO’s in the case of All Flash VSAN.  Reads can directly go into the underlying SSD’s in ‘capacity’ tier .

Then why Cache Tier in All Flash..?

If the $/IOPS is too low in All flash, then why we need to configure Cache Tier during VSAN All Flash configuration. Well, that’s for a different purpose. Magnetic disks are described in terms of RPM and protocols ( SAS/SATA ) , SSD’s are described in terms of DWPD , TBW , MLC, SLC etc.  These terminologies determine the endurance of the SSD disks. SSD’s  ability to withstand writes depends on the underlying nature of the NAND flash.  In a nut shell, the SSD disk wears off based on the write I/O happening on the disk. In order to protect the underlying  capacity tier SSD’s  , write cache is enabled, so that majority of the writes are cached in the cache tier and only limited  writes are redirected to the Capacity Tier. Because of this reason, VSAN All Flash deployment will include High endurance SSD disk  as ‘cache tier’ and low endurance SSD disk as ‘capacity tier’.  The ‘elevator algorithm’ of VSAN make sure that cache destaging to capacity disk are done in such as way that writes are more sequential in nature. Sequential I/O allows significantly higher DWPD /TBW which increases the endurance level of the SSD.

In summary, the All Flash VSAN provides 100% write cache whereas the Hybrid VSAN provides 70% read cache and 30% write cache.  With the latest SSD release from Micron and Sandisk, the price of SSD’s are decreasing drastically which should eventually increase the VSAN All Flash adoption .

 

 

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