iSCSI fragmentation

[xyzzy]

I've just started to experiment with iSCSI and am looking at creating one or more iSCSI-only pools, each with a single ZVOL device extent.

After reading these forums, I have some admittedly noob-ish questions:

1) With regards to the "50% rule" is it okay to create a single ZVOL that uses the entire pool but keep an eye on the usage so it doesn't exceed 50%?

2) Or is it better to create a ZVOL that's 50% of the total pool size?

3) If I do the #2 approach, does the ZVOL only use the first 50% of the pool's blocks (somewhat like a drive partition)? Or, to avoid fragmentation, can it use any blocks anywhere in the pool (as long as it doesn't occupy more than 50%)?

4) Does the "50% rule" also apply to SSDs? Or can I go higher?

5) Does NFS have similar fragmentation issues as iSCSI does?

Thanks!

 

[Ericloewe]

3) If I do the #2 approach, does the ZVOL only use the first 50% of the pool's blocks (somewhat like a drive partition)?

That's not even possible, since ZFS is copy-on-write.

Does NFS have similar fragmentation issues as iSCSI does?

Yes, obviously. A block device is a block device, no matter how it's being shared.

Does the "50% rule" also apply to SSDs? Or can I go higher?

SSDs are interesting. Nobody has actually thoroughly benchmarked the effect of free space on block device performance and shared the data.

Intuitively, I expect the performance drop to be more towards the full side of the spectrum (with consistent enterprise drives like the Intels). Since SSDs also have much better IOPS in the first place, you can probably sustain a larger percentual drop while maintaining acceptable performance, too.

 

[xyzzy]

Thanks Ericloewe!

With regards to the different options for avoiding fragmentation (whether the target is 50% or say 80%), is there any disadvantage to giving the ZVOL the full pool space and then keeping an eye on the usage? Or does doing that make it harder for ZFS to allocate blocks efficiently?

 

[Ericloewe]

It doesn't make things harder, assuming that the other side issues trims and/or doesn't spread the writes around unnecessarily.

 

[rs225]

If you make the ZVOL that large, there won't be space for snapshots, unless you know that TRIM/UNMAP works and keep a close eye on usage in the ZVOL.