Re: [PATCH 1/3] nbd: support FLUSH requests

From: Paolo Bonzini
Date: Wed Feb 13 2013 - 08:01:11 EST

Il 13/02/2013 01:03, Alex Bligh ha scritto:
>> > Obviously the changelog was inadequate. Please send along a new one
>> > which fully describes the reasons for this change.
> To be clear I have no complaints about the rest of the patch being
> merged. Supporting FLUSH but not FUA is far better than supporting
> neither. I just didn't understand dropping FUA given the semantics
> of nbd is in essence 'linux bios over tcp'.

Not really bios, since it expects FLUSH requests to include no I/O, but
yes the semantics of NBD (and virtio-blk) are quite close to those of
the Linux block layer.

But as far as I can test with free servers, the FUA bits have no
advantage over flush. Also, I wasn't sure if SEND_FUA without
SEND_FLUSH is valid, and if so how to handle this combination (treat it
as writethrough and add FUA to all requests? warn and do nothing?).

Andrew, here is a better commit message:

------------ 8< ---------------
From: Alex Bligh <alex@xxxxxxxxxxx>
Subject: nbd: support FLUSH requests

Currently, the NBD device does not accept flush requests from the Linux
block layer. If the NBD server opened the target with neither O_SYNC
nor O_DSYNC, however, the device will be effectively backed by a
writeback cache. Without issuing flushes properly, operation of the NBD
device will not be safe against power losses.

The NBD protocol has support for both a cache flush command and a FUA
command flag; the server will also pass a flag to note its support for
these features. This patch adds support for the cache flush command and
flag. In the kernel, we receive the flags via the NBD_SET_FLAGS ioctl,
and map NBD_FLAG_SEND_FLUSH to the argument of blk_queue_flush. When
the flag is active the block layer will send REQ_FLUSH requests, which
we translate to NBD_CMD_FLUSH commands.

FUA support is not included in this patch because all free software
servers implement it with a full fdatasync; thus it has no advantage
over supporting flush only. Because I [Paolo] cannot really benchmark
it in a realistic scenario, I cannot tell if it is a good idea or not.
It is also not clear if it is valid for an NBD server to support FUA but
not flush. The Linux block layer gives a warning for this combination,
the NBD protocol documentation says nothing about it.

The patch also fixes a small problem in the handling of flags:
nbd->flags must be cleared at the end of NBD_DO_IT, but the driver was
not doing that. The bug manifests itself as follows. Suppose you two
different client/server pairs to start the NBD device. Suppose also
that the first client supports NBD_SET_FLAGS, and the first server sends
NBD_FLAG_SEND_FLUSH; the second pair instead does neither of these two
things. Before this patch, the second invocation of NBD_DO_IT will use
a stale value of nbd->flags, and the second server will issue an error
every time it receives an NBD_CMD_FLUSH command.

This bug is pre-existing, but it becomes much more important after this
patch; flush failures make the device pretty much unusable, unlike
discard failures.

Signed-off-by: Alex Bligh <alex@xxxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Paul Clements <Paul.Clements@xxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>

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