VSAN-Troubleshooting-Reference-Manual

Diagnostics and Troubleshooting Reference Manual – Virtual SAN
2. On the second read I/O, the data is returned from the read cache. This is a
cache hit.
3. The third I/O is a write. This now marks that range in the read cache line as
invalid/dirty. A line in cache is a range of data blocks. When one block in the
range changes, the whole range (or line) is marked as dirty. At this point, the
data has not yet made it from the write buffer to the magnetic disk.
4. The fourth I/O is another read, and it attempts to read a different block on
the same cache line. This will be satisifed by the read cache. If the data block
being read is not dirty, it can still be read from the read cache. This is
considered a Read Cache Partial Hit as Virtual SAN does not know whether
the data block it just read came from flash or magnetic disk.
This will become clearer when we discuss a read of the dirty/invalid block.
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