test btrfs scrubbing/get system's total capacity

Hello,

1. is there a tool that I can use to test btrfs scrubbing? Namely I need to "damage" a file, i.e. change it without changing its meta information, so I can see how scrubbing performs... Is there something like this?

2. I want to use Ansible to set btrfs quota on a /home subvolume as say 0.8*TotalCapacityOfRoot and then assign to it individual users' subvolumes with certain (smaller) quatos.
2.1 Is it OK to rely on ansible_facts? Does it has correct information about btrfs-based "/" total capacity?
2.2 If not, is there a raw command to get TotalCapacityOfRoot in -TB so I can set it? Is there something like btrfs filesystem usage / -t --device_size? to get only the total Device Size information?

Thank you!
ST

btrfs-corrupt-block.c

It's part of btrfs-progs but many distributions do not include it by default, but is easy to build it stand alone.

No idea how ansible is manipulating quotas on Btrfs.

Chris Murphy

ST posted on Mon, 20 Nov 2017 18:50:26 +0200 as excerpted:

On the narrow subject of btrfs quotas...

Just be sure you're aware of the effect of btrfs quotas on btrfs maintenance command times, and are using a "newish" kernel, as...

* For years quotas were so buggy that they simply weren't sanely reliable (negative numbers, anyone?). In general those bugs are fixed in newer kernels, but there's still minor tweaks going in. I'd for sure want at /least/ 4.9-LTS series if I were relying on kernels (and I'm not /entirely/ sure the appropriate patches have all made it there), and would want to move to 4.14-LTS ASAP. Or just use the current kernel series and upgrade with it.

IOW, don't even consider quotas pre-4.9. They're too buggy to be worth the hassle.

* While the worst quota bugs now appear to be quashed and it's at least usable (the numbers come out correct), quotas continue to have scaling issues and likely will for some time to come.

Generally these scaling issues don't affect generic filesystem (or quota) operation, but they *do* affect various btrfs maintenance commands where they compound already severe scaling interactions with snapshots and reflinks in commands such as btrfs subvol delete, btrfs balance (and thus btrfs device remove due to its implicit balance), and btrfs check, to the point that the commands cannot complete in practical time (days to weeks, even months, instead of hours).

For commands such as btrfs balance, temporarily disabling quotas (renable and rescan if necessary when done) speeds things up dramatically, but that's not likely to be possible in many of the scenarios one would find themselves wanting to run btrfs check in.

If you're already aware of and prepared to work with those caveats, great, but I've seen enough people complaining about slow balance, etc, due to having quotas enabled (and being thrilled with the actually workable speed once they turn them off) when they didn't really need them, that I tend to cringe inside every time I see people saying they plan on using them, without mentioning that they're aware of the issues and find them an acceptable tradeoff for their use-case.

Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman

Btrfs-corrupt-block, which is not compiled by default. And not well documented.

Quota only works on subvolumes.
Although you could do it using multi level qgroups.

Using higher level qgroup, 1/0 for example, as a big pool and then assign real subvolume qgroup to that pool.

However, current qgroup will bring big performance overhead, especially for balance and snapshot deletion.

Although not sure what Ansible is and if it uses btrfs quota.

Not sure about ansible at all.

What is the meaning of TotalCapacityOfRoot? Device size? Fs size?

Thanks,
Qu