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Costa: Designing a Userspace Disk I/O Scheduler for Modern Datastores: the Scylla example (Part 1)

Costa: Designing a Userspace Disk I/O Scheduler for Modern Datastores: the Scylla example (Part 1)

Posted Apr 19, 2016 15:17 UTC (Tue) by Wol (subscriber, #4433)
In reply to: Costa: Designing a Userspace Disk I/O Scheduler for Modern Datastores: the Scylla example (Part 1) by nye
Parent article: Costa: Designing a Userspace Disk I/O Scheduler for Modern Datastores: the Scylla example (Part 1)

> The difference from my perspective is that the volume management layer is bigger, incorporating the moral equivalent of LVM and RAID (and checksumming, and encryption if you're using Oracle's proprietary ZFS), and that the filesystem layer is not intended to be layered on top of anything else, so can be tailored to fit the layer beneath it.

As I understand it (and no way am I an expert in filesystems :-) it comes over that the more the top (user interface) knows about the bottom (the disk interface) the easier it is to make sensible decisions that don't have exponential performance as the load rises ...

> It's not that a layered stack is the problem per se, but more that allowing arbitrary selections of layers chosen from a generous smorgasbord and combined in arbitrary orders dramatically expands the scope of the problem.

:-)

If ZFS is split into two tightly-coupled components, that sounds in line with my understanding. Could you put a ZFS filesystem layer over a linux lvm layer? Or a linux ext4 over the ZFS volume layer? I guess the answer's "no", so ZFS as a whole has control over the whole stack :-) and hence should achieve far better performance.

Cheers,
Wol

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Costa: Designing a Userspace Disk I/O Scheduler for Modern Datastores: the Scylla example (Part 1)

Posted Apr 19, 2016 16:41 UTC (Tue) by zdzichu (subscriber, #17118) [Link] (1 responses)

You can't put ZPL (ZFS Posix Layer, the filesystem) on top of LVM.
You CAN put ext4 on ZVOLs. Or swap. Or export it via iSCSI.

Costa: Designing a Userspace Disk I/O Scheduler for Modern Datastores: the Scylla example (Part 1)

Posted Apr 20, 2016 11:07 UTC (Wed) by nye (subscriber, #51576) [Link]

There are even circumstances where it would actually be reasonably sane to use ZFS (the volume layer) as your volume management layer, and ext4 as your filesystem layer, aside from the obvious like exporting the volume to a VM or an iSCSI client that just does its own thing.

I've seriously considered it for storing my email backups where more than 50% of the space usage is waste due to internal fragmentation, where ZFS (the filesystem layer) suffers fairly badly compared to ext4. Ultimately, losing low double digit gigabytes isn't a pressing concern these days so I've not bothered, but I can well imagine there would be circumstances where ext4 or some other filesystem would be a sensible choice.


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