How to decide how big my ZIL and Cache device should be?

Hi all,
I have a multiple LUN connected to my server with different size.
1.
If I want to add ZIL to the pool:
How to calculate the ZIL (size) to fit different Pool size?
it is better to mirror the ZIL? If the ZIL devices is from the SAN storage?
2.
Same as Cache device, how big should I give?
3.
Cache device is recommended to use SSD. However, if my SSD is not the local SSD but it is a SSD in the SAN storage which is the same storage of the pool. Is it useless to give a cache device to the pool? As it limited to the fiber channel throughput.

Hi,
Good questions and to recap these performance features:
Separate log devices (ZIL) are good for improving synchronous write performance
Separate cache devices (L2ARC) are good for improving read performance
In general, we recommend the following:
1. Use SSDs for both ZIL or cache otherwise you won't see the performance boosts when using HDDs.
2. The cache device size should equal your application's warm working set size.
3. General log sizing recommendations are here:
Creating and Destroying ZFS Storage Pools - Oracle Solaris 11.1 Administration: ZFS File Systems
Creating a ZFS Storage Pool With Log Devices
A more specific case is for the Oracle db redo log, where the recommendation is 15 seconds of redo
log activity x 2 or 300 MB.
4. Local attached SSDs as log or cache devices will perform much better than if they are attached
through a SAN array.
5. Mirrored log devices are recommended, but unnecessary for cache devices.
Thanks, Cindy

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How to decide how big my ZIL and Cache device should be?

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