Trying to wrap my head around SLOG.

Drok

Cadet
Joined
Jul 6, 2019
Messages
3
Hi,

I'm new to TrueNas and zfs and I'm trying to wrap my head around SLOG as well as whether I need one. Based on the reading I've done my understanding is that a SLOG is not a write cache but instead holds the ZIL which is normally stored on the array until it can be flushed to disk in a batch. Adding a SLOG device benefits synchronous writes by alleviating the otherwise duplicate work the main disks would have to do. This can increase the performance of the pool when using synchronous writes but provides no benefits to asynchronous writes.

I'm currently using TrueNas Scale virtualized with Proxmox. My pool consists of 6x4TB hdds in 3xmirrors. I plan on using this pool for a mixed workload that includes VMs which I understand should be synchronous to mitigate data loss. With this in mind I decided to test my pool using FIO and found that when I forced sync enabled I was only getting around 7-8MB/s sequential writes which wasn't going to cut it. I added a virtual disk as a temporary SLOG device which brought my sequential synchronous writes up to 50MB/s. Unless my tests are inaccurate for this use or I have a a miss understanding of what a SLOG is or isn't, it seems like purchasing a SLOG device would be of use here. Is this correct?

Thanks
 

jgreco

Resident Grinch
Joined
May 29, 2011
Messages
18,681
Hoping you've seen --


But basically you seem to have your head screwed on straight, congratulations for having done the homework. ;-)
 

Drok

Cadet
Joined
Jul 6, 2019
Messages
3
Can't remember if I've see that exact thread but I'll give it a read. Glad to know I understand this concept correctly and I'm right in thinking I could use a SLOG device here. Is there a good list somewhere of recommended SLOG devices?
 

Patrick M. Hausen

Hall of Famer
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Nov 25, 2013
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The truth is always out there ... :wink:

 

Drok

Cadet
Joined
Jul 6, 2019
Messages
3
The truth is always out there ... :wink:

Thanks. I actually ended up finding that myself before you replied. Not entirely sure how I missed it earlier.
 
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