Veranstaltungen/2009-12-04 ZFS-Workshop/Cache: L2ARC

ZFS cacht nicht nur im RAM der Maschine, sondern - sofern vorhanden - auch auf Cache-Platten. Der Cache sollte immer auf redundant ausgelegten Platten liegen, am besten natürlich auf SSDs. Der ARC ist hierbei der Lese-Cache im RAM. L2ARC beschreibt den Level-2-ARC, also genannte Festplatten. Und komplizierter kann ich das nicht formulieren, sorry.

Aus der man-Page:

  Cache Devices
     Devices can be added to a storage pool as  "cache  devices."
     These devices provide an additional layer of caching between
     main memory and disk. For read-heavy  workloads,  where  the
     working  set  size is much larger than what can be cached in
     main memory, using cache devices allow  much  more  of  this
     working set to be served from low latency media. Using cache
     devices provides the greatest  performance  improvement  for
     random read-workloads of mostly static content.

     To create a pool with cache devices, specify a "cache"  vdev
     with any number of devices. For example:

       # zpool create pool c0d0 c1d0 cache c2d0 c3d0

     Cache devices cannot be mirrored or part of a  raidz  confi-
     guration.  If a read error is encountered on a cache device,
     that read I/O is reissued to the original storage pool  dev-
     ice,  which  might be part of a mirrored or raidz configura-
     tion.

     The content of the cache devices is considered volatile,  as
     is the case with other system caches.

Und warum ich das jetzt nicht zeigen kann:

shl@lightbulb:~$ pfexec zpool create testpool raidz1 ~/HDD0 ~/HDD1 ~/HDD2 cache ~/HDD3
cannot create 'testpool': cache device must be a disk or disk slice

Da hilft nichtmal -f.

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Quellen:

• http://blogs.sun.com/brendan/entry/test
• man-Pages

ZIL und L2ARC | Agenda | Cache: ZIL

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