Estimated total raw data capacity: 0 B

[kushari]

Hi Everyone,
I'm trying to create a log write cache for my main vdev. I added the two nvme's to the log VDev. But it shows "Estimated total raw data capacity: 0 B" and I cant create the vDev. I assume I don't have to add all devices left (there's a Dell 16GB emmc card that I don't want in there). I know it's supposed to show the full 1TB, but it's showing 0B. This is my first time setting up TrueNAS, so not sure how to trouble shoot this. I searched before I posted this, and found a post about changing block size to 512?.

 

[HoneyBadger]

Hi @kushari

It looks like you're in the "new pool" creation wizard. Go to the Storage page, and select "Manage Devices" beside your existing pool - then click "Add vdev" on that page.

Note that you'll want to change your log vdev to mirror as well, most likely.

As an additional note, a log vdev will only accelerate certain types of writes. What are you using the pool for?

 

[kushari]

Thanks HoneyBadger, sorry for my n00bness. I just want it to be the write cache for my server. I have 10Gbe, so want to make sure writes are as fast as possible.

 

[HoneyBadger]

No worries. Unless you know you are using "synchronous writes" such as from a hypervisor or an NFS client, you probably won't benefit from the log device.

Sustained write speeds, especially at 10GbE, also require a fast pool; multiple SSDs or a whole chassis of HDDs most likely. If you can describe your system in more detail, we can try to make some guesses about potential performance bottlenecks.

 

[kushari]

Oh, is there a normal staged cache, that writes to cache then writes to the spinning disks after?

 

[MrGuvernment]

Oh, is there a normal staged cache, that writes to cache then writes to the spinning disks after?

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[NugentS]

Oh, is there a normal staged cache, that writes to cache then writes to the spinning disks after?

No there is not.
Cache (L2ARC) does not cache writes - it doesn't really cache reads (as most think of a cache)
SLOG only speeds up sync writes such as NFS or iscsi - and only some of them. SLOG is not a cache.

There is nothing that acts like an unraid cache drive

 

[sretalla]

SLOG only speeds up sync writes such as NFS or iscsi - and only some of them. SLOG is not a cache.

Also just to be more precise, a SLOG will make sync write speeds suck less, not actually speed them up when compared to async writes.

 

[HoneyBadger]

To add to the idea that there isn't a traditional "write-back cache" in ZFS - the default behavior is to buffer the pending asynchronous writes (most normal file copies and activity) in RAM before writing it as a single "transaction group" to the disk.

In short, for regular file copying, it will be "as fast as possible" out of the box. If you're using TrueNAS as backing for something like a transactional database, then it will need good synchronous write performance - which is where the log vdev comes into play.