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From: Quartz <quartz@sneakertech.com>
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Subject: Re: Options for zfs inside a VM backed by zfs on the host
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> I am right now exploring the question: are SSD ZILs necessary in an all SSD
> pool?

Something mentioned in another recent thread on this list (or maybe it 
was -questions?) was that yes, you really should consider a separate ZIL 
if you're using primarily SSDs. Without a separate disk, log writes have 
to steal blocks from the pool itself which then have to be deleted 
afterwards to let go of the space. Besides causing excess file 
fragmentation, the write-delete cycle doesn't play well with SSDs and 
trim and can seriously hamper performance. With a dedicated disk, it 
writes and then just leaves it there, only overwriting later if necessary.