Re: [Qemu-devel] [Bug 595117] Re: qemu-nbd slow and missing "writeback" cache option

[Jamie Lokier]

Serge Hallyn wrote:
> The default of qemu-img (of using O_SYNC) is not very sensible
> because anyway, the client (the kernel) uses caches (write-back),
> (and "qemu-nbd -d" doesn't flush those by the way). So if for
> instance qemu-nbd is killed, regardless of whether qemu-nbd uses
> O_SYNC, O_DIRECT or not, the data in the image will not be
> consistent anyway, unless "syncs" are done by the client (like fsync
> on the nbd device or sync mount option), and with qemu-nbd's O_SYNC
> mode, those "sync"s will be extremely slow.

Do the "client syncs" cause the nbd server to fsync or fdatasync the file?

> It appears it is because by default the disk image it serves is open
> with O_SYNC. The --nocache option, unintuitively, makes matters a
> bit better because it causes the image to be open with O_DIRECT
> instead of O_SYNC.
[...]
> --cache=off is the same as --nocache (that is use O_DIRECT),
> writethrough is using O_SYNC and is still the default so this patch
> doesn't change the functionality. writeback is none of those flags,
> so is the addition of this patch. The patch also does an fsync upon
> "qemu-nbd -d" to make sure data is flushed to the image before
> removing the nbd.

I really wish qemu's options didn't give the false impression
"nocache" does less caching than "writethrough".  O_DIRECT does
caching in the disk controller/hardware, while O_SYNC hopefully does
not, nowadays.

-- Jamie