Log Device on Truenas 12

[johnbajer]

Hello, I need to know how big Log Device on Truenas 12 should be and up to what extend it speeds up the system. Thanks

 

[c77dk]

I assume you mean SLOG? Then not much for size - 32GB should be more than enough (somebody please correct me if I'm wrong). As for the speedup, that depends on your usage pattern, and the underlying vdev setup - how much sync writes you're doing. No sync = no speedup.

Please explain what you're trying to setup - what hardware and what workload

 

[johnbajer]

I have 2x4tb hdd mirrored on Truenas 12 making a pool. I have an 256gb ssd as cache of this pool. I see that now I can add this kind of device to speed up system somehow. I have copied the name as it is shown on the Truenas system. The workload is that 4 concurrent user maximum.

 

[Patrick M. Hausen]

An SLOG device only speeds up synchronous writes and even that only under specific circumstances. If your workload is file sharing an SLOG device will do nothing for you or even slow down the pool.

 

[johnbajer]

ok. thanks!

 

[hamstercnw]

Adding the SLOG device increased my proxmox speeds drastically because the guest OS is expecting a "write confirmation" for all writes. Raidz will confirm when data is written to all disks but the SLOG SSD confirms, in my case, 10 times faster than the raidz. SLOG device data is written to raidz within the next 5 seconds.

raidz - TrueNAS - 10GB ethernet - Proxmox - Guest OS

 

[sretalla]

Adding the SLOG device increased my proxmox speeds drastically because the guest OS is expecting a "write confirmation" for all writes. Raidz will confirm when data is written to all disks but the SLOG SSD confirms, in my case, 10 times faster than the raidz. SLOG device data is written to raidz within the next 5 seconds.

Although some of what you're saying is at least based on truth, you've put it in a way which could very easily be misinterpreted to mean that you're using a RAIDZ pool for block storage... which is certainly not the way to do that.

The path to success for block storage

It seems like I haven't written a sticky for awhile, but just in the last week I've had to cover this topic several times. ZFS does two different things very well. One is storage of large sequentially-written files, such as archives, logs, or data files, where the file does not have the middle...

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Also, nobody here has mentioned anything about the type of SLOG device (which is very important if you're going to have one).

Some insights into SLOG/ZIL with ZFS on FreeNAS

What is the ZIL? POSIX provides a facility for the system or an application to make sure that data requested to be written is actually committed to stable storage: a synchronous write request. Upon completion of a sync write request, the underlying filesystem is supposed to guarantee that a...

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