[kwlug-disc] BTRFS Was: Transferring large backup files over the network

[Chris Irwin]

On Thu, May 23, 2019 at 10:06:32PM -0400, Remi Gauvin wrote:
> many places, including the official
>BTRFS wiki, recommend disabling Copy-On-Write for database or VM Image
>files.

I use BTRFS for my desktop and laptop (both home and work) and have no 
issues or complaints. Automatic snapshots have saved me several times.

However, BTRFS is a tool. You can use it for the features it was 
intended for (COW and checksumms), or you can use it wrong. Anything 
suggesting disabling COW on BTRFS is simply plain wrong, regardless of 
whether it is some blog or official documentation.

Disabling COW for VMs on BTRFS means:

    * You're not getting data checksums (they require COW)
    * You're not using snapshots (they silently enable COW)

If you're not using either of those: Why are you bothering with BTRFS?

BTRFS with COW disabled is worse than ext4: You're not getting any 
actual feature benefits, and you've lost the more plentiful and robust 
ext4 recovery tools (should you ever need them).

Additionally some features (like snapshots) will silently enable COW,
even on files that COW was "disabled", and defragmenting (including 
autodefrag) will cause data duplication between snapshots (which were 
deduped by virtue of COW).

For Virtual Machines and Databases, I'd really just recommend having an 
ext4 LV for those. I keep VMs and databases on ext4 (at home) or xfs (at 
work, due to RHEL).

(I prefer ext4 use for home, because you can't shrink xfs)

>However, the BTRFS raid implementation is completely dependent
>on CoW to maintain synchronization between mirrors. Any event that
>interrupts writes to a mirrored file with CoW disabled will result in
>discrepancies between the mirrored copies, and they are never
>automatically fixed.. not even if you run a scrub.

Which makes sense. Disabling COW disables data checksums, and scrub 
searches for checksum errors.

-- 
Chris Irwin

email:   chris at chrisirwin.ca
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