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Log und L2Arc lassen sich aus einem Pool entfernen. Ein plötzlicher Verlust ist unkritisch. Beim L2Arc gibt es dann halt nur noch den RAM Lesecache Arc. Beim Slog wird lediglich auf das langsamere onpool Logging gewechselt.
Bei special vdev ist das anders. Die kann man zwar auch nachtröglich aus dem Pool entfernen, es gibt aber zwei Besonderheiten:
Das special vdev benötigt den gleichen ashift wie die Pool vdevs, sonst klappt Entfernen nicht
Geht das special vdev plötzlich verloren, ist der Pool verloren - es sei denn man kann das Device wiederbeleben. Die Daten auf dem special Device werden ja nicht zusätzlich darauf gespeichert wie es beim L2Arc der Fall ist, sondern ausschließlich- man will ja Top Performance.
Ich gehe weiterhin davon aus, dass das bei OpenZFS (noch/mittlerweile) anders gehandhabt wird, bzw. "special" nicht "meta" aus Solaris entspricht, denn man zpool sagt:
log
A separate-intent log device. If more than one log device is speci-
fied, then writes are load-balanced between devices. Log devices
can be mirrored. However, raidz vdev types are not supported for
the intent log. For more information, see the "Intent Log" section.
meta
A device used to optimize reads of certain types of ZFS metadata,
in particular, deduplication entries. If more than one meta device
is specified, operations will be load balanced between them. Meta
devices can be mirrored. However, raidz vdev types are not sup-
ported. For more information, see the "Meta Devices" section.
cache
A device used to cache storage pool data. A cache device cannot be
configured as a mirror or raidz group. For more information, see
the "Cache Devices" section.
Intent Log
The ZFS Intent Log (ZIL) satisfies POSIX requirements for synchronous
transactions. For instance, databases often require their transactions
to be on stable storage devices when returning from a system call. NFS
and other applications can also use fsync to ensure data stability. By
default, the intent log is allocated from blocks within the main pool.
However, it might be possible to get better performance using separate
intent log devices such as NVRAM or a dedicated disk. For example:
# zpool create pool c0d0 c1d0 log c2d0
Multiple log devices can also be specified, and they can be mirrored.
See the EXAMPLES section for an example of mirroring multiple log
devices.
Log devices can be added, replaced, attached, detached, and imported,
and exported as part of the larger pool. Mirrored log devices can be
removed by specifying the top-level mirror for the log.
Meta Devices
Devices can be added to a storage pool as meta devices. These devices
store copies of critical metadata which needs to be accessed in a non-
sequential manner. This functionality is especially useful for dedupli-
cation entries. Since copies of the metadata are also written to the
main storage pool, I/O errors to this device can be recovered and this
device does not have to be mirrored.
To create a pool with meta devices, specify a meta vdev with any number
of devices. For example:
# zpool create pool c0d0 c1d0 meta c2d0 c3d0
Multiple meta devices can be specified, and they can be mirrored, but
they cannot be part of a raidz configuration.
Meta devices can be added, replaced, attached, detached, imported, and
exported as part of the larger pool.
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 configuration. If a
read error is encountered on a cache device, that read I/O is reissued
to the original storage pool device, which might be part of a mirrored
or raidz configuration.
The content of the cache devices is considered volatile, as is the case
with other system caches