Turn on iSCSI Sync Write

[eroji]

I found a lot of sources saying to turn iscsi sync=on but nothing saying where or how to set this config. Can someone tell me the steps to set this?

 

[hugovsky]

see this post:

https://forums.freenas.org/index.ph...xi-nfs-so-slow-and-why-is-iscsi-faster.12506/

 

[jgreco]

Yes, you have to set it from the command line. "zfs set sync=always pool/foo/bar"

Expect to see a performance hit.

 

[HoneyBadger]

Expect to see a performance hit.

That's putting it mildly.

If your workload actually requires sync writes (ie: VMs on iSCSI storage) then you should get a good SLOG device and install it first, otherwise your VMs may not appreciate their lightning-fast write speeds getting brought back down to sync-write reality.

 

[jgreco]

That's putting it mildly.

Depends on the workload and whether or not he's already got a SLOG.

 

[GeoffK]

Yes, you have to set it from the command line. "zfs set sync=always pool/foo/bar"

Expect to see a performance hit.

Reiterating this - you need sync=always for vmware iSCSI pools.

And yes, if you don't have a SLOG, its gonna suck - on an array with 7200rpm drives, etc a cap of 700iops or there abouts.

Get a DC S3500/3700 and this will boost to 30-40k - dont' forget to resize it to 8-16GB...

 

[jgreco]

on an array with 7200rpm drives, etc a cap of 700iops or there abouts.

I cannot even begin to imagine how you would come up with that.

A single mirror vdev of two 7200RPM drives might get you 100 IOPS write or 150 IOPS read.

A shelf of 48 7200RPM drives in mirror vdev should get you at least 2000 IOPS write or 3000 IOPS read.

 

[GeoffK]

I cannot even begin to imagine how you would come up with that.

Real world experience.

It was 16 Mirrored vdevs ;)

 

[jgreco]

Real world experience.

It was 16 Mirrored vdevs ;)

Well, the point was that it's really dependent on the size and configuration of the pool, you can't just say "on an array with 7200rpm drives".

But that strikes me as maybe a little on the low side for 16 mirror vdevs. Are you sure you had enough parallelism in the workload while testing?

 

[GeoffK]

But that strikes me as maybe a little on the low side for 16 mirror vdevs. Are you sure you had enough parallelism in the workload while testing?

Eh, it was hardly emperical - it was a single iometer guest on esxi :)

 

[jgreco]

So probably not enough concurrency.