Il 27-07-2017 13:09 Igor Cicimov ha scritto:
And in case of live migration I'm sure the tool you decide to use
will freeze the guest and make sync() call to flush the os cache
*before* stopping and starting the guest on the other node.
Yeah, I was busy architecting a valid example [1] until I realized that
libvirt/KVM surely issues a sync() before live migrating the guest :p
Thank you Igor.
[1] For reference, here you find an example showing as the kernel buffer
can sometime act as a writeback cache...
Consider the following command and output:
[root@localhost ~]# dd if=/dev/zero of=/dev/sdb bs=1M count=64
64+0 records in
64+0 records out
67108864 bytes (67 MB) copied, 0.981192 s, 68.4 MB/s
As you can see, write bandwidth is in line with the underlying disk real
performance. This means that the close() call (issued by dd just before
exiting) flushes the buffers.
Now, let's concurrently open the block device in *read only* mode:
[root@localhost ~]# exec 3</dev/sdb
[root@localhost ~]# dd if=/dev/zero of=/dev/sdb bs=1M count=64
64+0 records in
64+0 records out
67108864 bytes (67 MB) copied, 0.0439497 s, 1.5 GB/s
This time, write bandwidth as reported by dd is about 1.5 GB/s, which is
clearly so much more of the disk's real write speed. This means that the
close() call returned *before* buffer flushing.
I don't fully understand if, and how, this correlate in read-life with a
opened LVM device on top of a DRBD resource in a dual-primary setup.
Maybe it opens a small opportunity window for an application writing to
a raw block device, and not issuing proper sync/fsync calls, to see
stale data in the event of a node migration in a dual-primary setup
(without no powerloss or hardware failure happening).
However, this is a very contrived scenario. Moreover, I am not sure on
how DRBD interact with kernel's buffers. Time to do some more tests, it
seems ... ;)
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