[Bug 237807] ZFS: ZVOL writes fast, ZVOL reads abysmal...

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=237807

--- Comment #6 from crest at rlwinm.de ---
There are three caching levels (all, metadata, none). At the default level of
all metadata and data get cached. This offers the best performance in most
cases, but it can become a problem if it leads to double buffering (e.g. on a
VM host with data being cached once by the host and again by the guest OS).
This can cause both caches to fight for resources. Disabling all caching
requires ZFS to walk the B-tree on disk for every access which is very slow.
Limiting caching to metadata is a compromise between the two extremes. Limited
to metadata caching ZFS caches everything but the actual data. This avoids the
need to (re-)read metadata from disk without double caching.

The rule of thumb is to cache data just once per physical system as close to
the user as possible (it's better to cache inside a VM than outside of it).
Also keep in mind that the L2ARC is a pure victim cache. If the primary cache
is limited to metadata it will never contain anything but metadata to be
evicted to the secondary cache.

Can you tell us more about your setup? What are the ZVOLs used for? What else
is running on the system?

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