Selecting the Right Linux I/O Scheduler for a Host Equipped with Nvme Ssd

Selecting the right Linux I/O scheduler for a host equipped with NVMe SSD?

"none" (aka "noop") is the correct scheduler to use for this device.

I/O schedulers are primarily useful for slower storage devices with limited queueing (e.g, single mechanical hard drives) — the purpose of an I/O scheduler is to reorder I/O requests to get more important ones serviced earlier. For a device with a very large internal queue, and very fast service (like a PCIe SSD!), an I/O scheduler won't do you any good; you're better off just submitting all requests to the device immediately.

Decrease in Random read IOPs on NVME SSD if requests issued over small region

Surely it is to do with the structure of the memory. Internally this drive is built from many memory chips and may have multiple memory buses internally. If you do requests across a small range all the requests will resolve to a single or few chips and will have to be queued. If you access across the whole device then the multiple request are across many internal chips and buses and can be run asynchronously so will provide more throughput.

Best enterprise scheduler that you worked with?

The best one was homegrown.

Second best was Autosys.

Risk of ZIL working on single SSD

You would need to check your zpool version, if it's >= 19 then it supports "Log device removal" or ZIL removal.

$ zpool upgrade -v
This system is currently running ZFS pool version 28.

The following versions are supported:

VER  DESCRIPTION
---  --------------------------------------------------------
 1   Initial ZFS version
 2   Ditto blocks (replicated metadata)
 3   Hot spares and double parity RAID-Z
 4   zpool history
 5   Compression using the gzip algorithm
 6   bootfs pool property
 7   Separate intent log devices
 8   Delegated administration
 9   refquota and refreservation properties
 10  Cache devices
 11  Improved scrub performance
 12  Snapshot properties
 13  snapused property
 14  passthrough-x aclinherit
 15  user/group space accounting
 16  stmf property support
 17  Triple-parity RAID-Z
 18  Snapshot user holds
 19  Log device removal
 20  Compression using zle (zero-length encoding)
 21  Deduplication
 22  Received properties
 23  Slim ZIL
 24  System attributes
 25  Improved scrub stats
 26  Improved snapshot deletion performance
 27  Improved snapshot creation performance
 28  Multiple vdev replacements

This means I have version 28 (>=19), thus, I can remove my ZIL device anytime from my pool. I doubt you'll break your single ZIL drive within a year or two unless you are performing synchronous writes all the time which your ZIL helps even out. There have been a lot of improvements on ZFS and from what I've read when your ZIL device breaks, the pool will simply revert to writing directly to the data pool. Of course, you can still rollback to a 'good' snapshot (do make sure you have snapshots).

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