On 2015-02-11 23:33, Kai Krakow wrote:
Duncan <1i5t5.duncan@xxxxxxx> schrieb:
P. Remek posted on Tue, 10 Feb 2015 18:44:33 +0100 as excerpted:
In the test, I use --direct=1 parameter for fio which basically does
O_DIRECT on target file. The O_DIRECT should guarantee that the
filesystem cache is bypassed and IO is sent directly to the underlaying
storage. Are you saying that btrfs buffers writes despite of O_DIRECT?
I'm out of my (admin, no claims at developer) league on that. I see
someone else replied, and would defer to them on this.
I don't think that O_DIRECT can work efficiently on COW filesystems. It
probably has a negative effect and cannot be faster as normal access. Linus
itself said one time that O_DIRECT is broken and should go away, and instead
cache hinting should be used.
Think of this: For the _unbuffered_ direct-io request to be fulfilled the
file system has to go through its COW logic first which it otherwise had
buffered and done in background. Bypassing the cache is probably only a
side-effect of O_DIRECT, not its purpose.
IIUC, the original purpose of O_DIRECT was to allow the application to
handle caching itself, instead of having the kernel do it. The issue is
that it is (again, IIUC) a hard requirement for AIO, which is a
performance booster for many use cases.
At least I'd try with a nocow-file for the benchmark if you still have to
use O_DIRECT.
I'd definitely suggest using NOCOW for any file you are doing O_DIRECT
with, as you should see _much_ better performance that way, and also
don't run the (theoretical) risk of some of the same types of corruption
that swapfiles on BTRFS can cause.
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