The ZIL is a write-only log that is only read after a power failure. Several GB 
is large enough for most workloads. 

You can't use the Intel X25-E because it has a 32 or 64 MB volatile cache that 
can't be disabled neither flushed by ZFS. 

Imagine your server has a power failure while writing data to the pool. In 
normal situation, with ZIL on a reliable device, ZFS will read the ZIL and come 
back to a stable state at reboot. You may have lost some data (30 seconds) but 
the zpool works.   With the Intel X25-E as ZIL some log data has been lost with 
the power failure (32/64MB max) which lead to a corrupted log and so ... you 
loose your zpool and all your data !!

For the ZIL you need 2 reliable mirrored SSD devices with a supercapacitor that 
can flush the write cache to NAND when a power failure occurs. 

A hard-disk has a write cache but it can be disabled or flush by the operating 

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