Options for zfs inside a VM backed by zfs on the host

Chad J. Milios milios at ccsys.com
Thu Aug 27 19:53:46 UTC 2015

> On Aug 27, 2015, at 10:46 AM, Allan Jude <allanjude at freebsd.org> wrote:
> On 2015-08-27 02:10, Marcus Reid wrote:
>> On Wed, Aug 26, 2015 at 05:25:52PM -0400, Vick Khera wrote:
>>> I'm running FreeBSD inside a VM that is providing the virtual disks backed
>>> by several ZFS zvols on the host. I want to run ZFS on the VM itself too
>>> for simplified management and backup purposes.
>>> The question I have is on the VM guest, do I really need to run a raid-z or
>>> mirror or can I just use a single virtual disk (or even a stripe)? Given
>>> that the underlying storage for the virtual disk is a zvol on a raid-z
>>> there should not really be too much worry for data corruption, I would
>>> think. It would be equivalent to using a hardware raid for each component
>>> of my zfs pool.
>>> Opinions? Preferably well-reasoned ones. :)
>> This is a frustrating situation, because none of the options that I can
>> think of look particularly appealing.  Single-vdev pools would be the
>> best option, your redundancy is already taken care of by the host's
>> pool.  The overhead of checksumming, etc. twice is probably not super
>> bad.  However, having the ARC eating up lots of memory twice seems
>> pretty bletcherous.  You can probably do some tuning to reduce that, but
>> I never liked tuning the ARC much.
>> All the nice features ZFS brings to the table is hard to give up once
>> you get used to having them around, so I understand your quandry.
>> Marcus
> You can just:
> zfs set primarycache=metadata poolname
> And it will only cache metadata in the ARC inside the VM, and avoid
> caching data blocks, which will be cached outside the VM. You could even
> turn the primarycache off entirely.
> -- 
> Allan Jude

> On Aug 27, 2015, at 1:20 PM, Paul Vixie <paul at redbarn.org> wrote:
> let me ask a related question: i'm using FFS in the guest, zvol on the
> host. should i be telling my guest kernel to not bother with an FFS
> buffer cache at all, or to use a smaller one, or what?

Whether we are talking ffs, ntfs or zpool atop zvol, unfortunately there are really no simple answers. You must consider your use case, the host and vm hardware/software configuration, perform meaningful benchmarks and, if you care about data integrity, thorough tests of the likely failure modes (all far more easily said than done). I’m curious to hear more about your use case(s) and setups so as to offer better insight on what alternatives may make more/less sense for you. Performance needs? Are you striving for lower individual latency or higher combined throughput? How critical are integrity and availability? How do you prefer your backup routine? Do you handle that in guest or host? Want features like dedup and/or L2ARC up in the mix? (Then everything bears reconsideration, just about triple your research and testing efforts.)

Sorry, I’m really not trying to scare anyone away from ZFS. It is awesome and capable of providing amazing solutions with very reliable and sensible behavior if handled with due respect, fear, monitoring and upkeep. :)

There are cases to be made for caching [meta-]data in the child, in the parent, checksumming in the child/parent/both, compressing in the child/parent. I believe `gstat` along with your custom-made benchmark or test load will greatly help guide you.

ZFS on ZFS seems to be a hardly studied, seldom reported, never documented, tedious exercise. Prepare for accelerated greying and balding of your hair. The parent's volblocksize, child's ashift, alignment, interactions involving raidz stripes (if used) can lead to problems from slightly decreased performance and storage efficiency to pathological write amplification within ZFS, performance and responsiveness crashing and sinking to the bottom of the ocean. Some datasets can become veritable black holes to vfs system calls. You may see ZFS reporting elusive errors, deadlocking or panicing in the child or parent altogether. With diligence though, stable and performant setups can be discovered for many production situations.

For example, for a zpool (whether used by a VM or not, locally, thru iscsi, ggate[cd], or whatever) atop zvol which sits on parent zpool with no redundancy, I would set primarycache=metadata checksum=off compression=off for the zvol(s) on the host(s) and for the most part just use the same zpool settings and sysctl tunings in the VM (or child zpool, whatever role it may conduct) that i would otherwise use on bare cpu and bare drives (defaults + compression=lz4 atime=off). However, that simple case is likely not yours.

With ufs/ffs/ntfs/ext4 and most other filesystems atop a zvol i use checksums on the parent zvol, and compression too if the child doesn’t support it (as ntfs can), but still caching only metadata on the host and letting the child vm/fs cache real data.

My use case involves charging customers for their memory use so admittedly that is one motivating factor, LOL. Plus, i certainly don’t want one rude VM marching through host ARC unfairly evacuating and starving the other polite neighbors.

VM’s swap space becomes another consideration and I treat it like any other ‘dumb’ filesystem with compression and checksumming done by the parent but recent versions of many operating systems may be paging out only already compressed data, so investigate your guest OS. I’ve found lz4’s claims of an almost-no-penalty early-abort to be vastly overstated when dealing with zvols, small block sizes and high throughput so if you can be certain you’ll be dealing with only compressed data then turn it off. For the virtual memory pagers in most current-day OS’s though set compression on the swap’s backing zvol to lz4.

Another factor is the ZIL. One VM can hoard your synchronous write performance. Solutions are beyond the scope of this already-too-long email :) but I’d be happy to elaborate if queried.

And then there’s always netbooting guests from NFS mounts served by the host and giving the guest no virtual disks, don’t forget to consider that option.

Hope this provokes some fruitful ideas for you. Glad to philosophize about ZFS setups with ya’ll :)


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