Document ATAPI drive flush command, and mention SSD drives.

diff --git a/doc/src/sgml/wal.sgml b/doc/src/sgml/wal.sgml
index 5f6a6eb2e18d1048f1fc44c38e5fbce498b2331a..9e9489da156c42948b0e50bbf1498dc85064fd42 100644 (file)
--- a/doc/src/sgml/wal.sgml
+++ b/doc/src/sgml/wal.sgml
@@ -1,4 +1,4 @@
-
+
 
 
  Reliability and the Write-Ahead Log
@@ -59,8 +59,11 @@
    same concerns about data loss exist for write-back drive caches as
    exist for disk controller caches.  Consumer-grade IDE and SATA drives are
    particularly likely to have write-back caches that will not survive a
-   power failure.  Many solid-state drives also have volatile write-back
-   caches.  To check write caching on Linux use
+   power failure, though ATAPI-6 introduced a drive cache
+   flush command that some file systems use, e.g. ZFS.
+   Many solid-state drives also have volatile write-back
+   caches, and many do not honor cache flush commands by default.
+   To check write caching on Linux use
    hdparm -I;  it is enabled if there is a * next
    to Write cache; hdparm -W to turn off
    write caching.  On FreeBSD use