VSAN-Troubleshooting-Reference-Manual

Diagnostics and Troubleshooting Reference Manual – Virtual SAN
VSAN Disks (deep-dive) – Device-level stats: full graphs
As well as the Host disk-layer aggregate stats, there are device-level stats: full graph
views. Let’s see that this can tell us next.
WriteBuffer
If the device is a flash device used for caching, then this chart related to write buffer
is displayed. Flash devices are partitioned into a WriteBuffer (WB) and ReadCache
(RC) for Virtual SAN hybrid configurations. By default, the split is 30% WB and 70%
RC.
WB Fill Pct – signifies the percentage on the write buffer that is currently
filled.
A note on LLOG and PLOG
The logical LOG (LLOG) and the physical LOG (PLOG) share the write buffer. The
internal behavior of LLOG and PLOG are not necessarily useful for troubleshooting,
but for completeness a description of their behavior and how they interoperate in
include here.
When a data block arrives in the write buffer, a corresponding entry for it is kept in
LLOG, the logical log. This is used by LSOM for log recovery on reboot.
However, once the data block is in the write buffer, Virtual SAN needs to calculate
where to place this block of data on the magnetic disk when destaging in hybrid
configurations. To do this, it consults the filesystem on the magnetic disk. This
placement process could cause the filesystem to generate its own metadata updates
(e.g. logical block address to physical location mapping, etc). This I/O is intercepted
and buffered on the SSD and a record in kept PLOG. Once we get the physical
locations for the data blocks from the filesystem, Virtual SAN stores this in PLOG. At
this point, we no longer need to keep the LLOG entry.
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