[Bugs] [Bug 1303895] promotions not happening when space is created on previously full hot tier

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1303895



--- Comment #2 from Nithya Balachandran <nbalacha at redhat.com> ---
Analysis:

The tier volume in question had different nodes running the hot and cold
bricks. No node contains both hot and cold bricks, so the tier process on each
node will either promote or demote, but not both.

The initial create operations caused the hot tier usage to cross the high
watermark configured. This was detected by the tier processes running on the
cold tier nodes and tier_conf->watermark_last was set to TIER_WM_HI. 

The files on the hot tier were then deleted and the disk space freed up. The
promotions are now expected to start again.

However, as the cold tier nodes do not demote files, the tier_check_watermark
is not called and the watermark value is not reset. So the tier_check_promote
will always fail, in turn preventing tier_watermark_check from being called .

The cold tier nodes will now never promote files and the hot tier will
eventually empty out.

Such a configuration is likely to hot this issue frequently as hot tier bricks
are usually small and likely to cross the high watermark frequently.

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