Hey guys,

 

First let me say that I love the show and appreciate all the hard work

you guys put in producing it! I learn something new every time I listen!

 

I'm working on new FreeBSD ZFS storage box and I have a question

regarding the use of a mirrored vs. non-mirrored SLOG (separate log /

dedicated ZIL) devices.

 

In the past, it was recommended to always mirror SLOG devices in case of

a HW failure. Now there's conflicting information floating around that

says it's no longer necessary. From what I understand, the purpose of a

SLOG is to speed up SYNC writes to the pool by giving ZFS a fast

(usually SSD), non-volatile, linear block of storage for the TXGs

(transaction groups) before ACK'ing SYNC writes. I believe the TXGs are

still held in RAM and when it comes time to flush the data to the pool,

it's done from RAM - the SLOG is simply a "replay" backup in case of

immediate power failure.

 

If this is the case, would ZFS be able to sustain a SLOG HW failure

since the TXGs are still in RAM? If so, would a pool allocated ZIL (the

default when there's no SLOG) take over and be used for future SYNC

writes until the SLOG was replaced?

 

Obviously this would slow down any future SYNC writes until the SLOG was

replaced, but I think that's acceptable as the primary concern here is

"in-flight" data loss.

 

Thanks in advance!