On 02/12/2018 11:02 AM, Austin S. Hemmelgarn wrote:
I will look into that if using built-in group capacity functionality
proves to be truly untenable. Thanks!
As a general rule, unless you really need to actively prevent a
subvolume from exceeding it's quota, this will generally be more
reliable and have much less performance impact than using qgroups.
Ok ok :). I will plan to go this route, but since I'll want to
benchmark it either way, I'll include qgroups enabled in the benchmark
and will report back.
With qgroups involved, I really can't say for certain, as I've never
done much with them myself, but based on my understanding of how it all
works, I would expect multiple subvolumes with a small number of
snapshots each to not have as many performance issues as a single
subvolume with the same total number of snapshots.
Glad to hear that. That was my expectation as well.
BTRFS in general works fine at that scale, dependent of course on the
level of concurrent access you need to support. Each tree update needs
to lock a bunch of things in the tree itself, and having large numbers
of clients writing to the same set of files concurrently can cause lock
contention issues because of this, especially if all of them are calling
fsync() or fdatasync() regularly. These issues can be mitigated by
segregating workloads into their own subvolumes (each subvolume is a
mostly independent filesystem tree), but it sounds like you're already
doing that, so I don't think that would be an issue for you.
Hmm...I'll think harder about this. There is potential for us to
artificially divide access to files across subvolumes automatically
because of the way we are using BTRFS as a backing store for our
parallel file system. So far even with around 1000 threads across about
10 machines accessing BTRFS via our parallel filesystem over the wire
we've not seen issues, but if we do I have some ways out I've not
explored yet. Thanks!
Now, there are some other odd theoretical cases that may cause issues
when dealing with really big filesystems, but they're either really
specific edge cases (for example, starting with a really small
filesystem and gradually scaling it up in size as it gets full) or
happen at scales far larger than what you're talking about (on the order
of at least double digit petabyte scale).
Yea, our use case will be in the tens of TB to hundreds of TB for the
foreseeable future, so I'm glad to hear this is relatively standard.
That was my read of the situation as well.
Thanks!
ellis
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