proxmox-rrd: use a journal to reduce amount of bytes written

Append pending changes in a simple text based format that allows for
lockless appends as long as we stay below 4 KiB data per write.

Apply the journal every 30 minutes and on daemon startup.

Note that we do not ensure that the journal is synced, this is a
perfomance optimization we can make as the kernel defaults to
writeback in-flight data every 30s (sysctl vm/dirty_expire_centisecs)
anyway, so we lose at max half a minute of data on a crash, here one
should have in mind that we normally expose 1 minute as finest
granularity anyway, so not really much lost.

Signed-off-by: Dietmar Maurer <dietmar@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

proxmox-rrd/src/lib.rs

@@ -13,9 +13,11 @@ mod cache;
 pub use cache::*;
 
 /// RRD data source tyoe
+#[repr(u8)]
+#[derive(Copy, Clone)]
 pub enum DST {
     /// Gauge values are stored unmodified.
-    Gauge,
+    Gauge = 0,
     /// Stores the difference to the previous value.
-    Derive,
+    Derive = 1,
 }