VSAN-Troubleshooting-Reference-Manual

Diagnostics and Troubleshooting Reference Manual – Virtual SAN
The block size that Virtual SAN uses to write to the flash cache device is 4KB. The
block size that Virtual SAN uses when de-staging to magnetic disk is 1MB, which is
also the stripe size used on magnetic disks and flash devices when they are used for
capacity in version 6.0.
The Virtual SAN flash is split into a read buffer and a write cache, 70% read and
30% write. One common question that gets asked about the anatomy of a write
operation is whether or not there is a subsequent write to the read cache so we
don't need to re-read the currently written data from the magnetic disk.
In Virtual SAN 5.5, the data in the read cache that may be affected by a new write is
invalidated, and if the VM issues a read for that data block before it has been
destaged to magnetic disk, we will read that data from the write buffer. A
subsequent read after destaging may then incur a read cache miss.
In Virtual SAN 6.0, this behavior has been changed. When the data is to be destaged
to magnetic disk from the write buffer, the corresponding read cache entry (if it
exists) is updated with the most recent data. Cache entries are not invalidated
unless the cache decides that they're no longer useful. This should mean less
magnetic disk I/O in Virtual SAN 6.0 when compared to Virtual SAN 5.5.
V M W A R E S T O R A G E B U D O C U M E N T A T I O N / 2 4 6