with more than one plex, the data will be accessed
multiple times. During this time, userland code could
potentially modify the buffer, thus causing data
corruption. In the case of a multi-plexed volume this
might be cosmetic, but in the case of a RAID-[45] plex it
can cause severe data corruption which only becomes
evident after a drive failure. Avoid this situation by
making a copy of the data buffer before using it.
Note that this solution does not guarantee any particular
content of the buffer, just that it remains unchanged for
the duration of the request.
Suggested by: alfred
2e387f1bd3