SLOG (is not a write cache) with PLP

[dwchan69]

High endurance and MLC are still easy to find, but given it is best to have Powerloss Protection for your write cache, what vendor / model nowadays has this capability and which form factor (M.2 vs 2.5 SSD)?

 

[jgreco]

Fixed yer title for you. I'll leave fixing the body of your message to you; SLOG is not any sort of write cache.


https://www.truenas.com/community/threads/some-insights-into-slog-zil-with-zfs-on-freenas.13633/

https://www.truenas.com/community/threads/slog-benchmarking-and-finding-the-best-slog.63521/

 

[dwchan69]

Thank you for the feedback, based on what I gather here, i am considering either the Intel Optane DC P4801X or Intel Optane SSD 800P Series as a potential option in term of its low latency and high endurance. Given the TrueNAS I am building does leverage a dual 10GB connectivity, but while the backend servers are still only GB, would it be fair to assume a SLOG capacity of 100GB should be more than sufficent?

 

[jgreco]

Size for two or three transaction groups, five seconds per. So you need to be able to handle 15 seconds of whatever your network can deliver. And that's only if you're doing exclusively sync writes with 100% write.

 

[dwchan69]

What is a transaction group? As in a disk pool? vdev? or requestor like iscsi block to a VM? Also, any negative consqueneces if I go too big?

 

[jgreco]

What is a transaction group?

I already linked to an article that talks about this extensively, so, presumably you didn't bother to read it.

 

[Herr_Merlin]

go with the P48x optanes.. you just need 100GB.. that will give you about 11PB of TBW. looking at our productive systems here.. we only write 1 to 2 PB a year to the SLOGs