Re: [PATCH 0/4] (RESEND) ext3[34] barrier changes

From: Jamie Lokier
Date: Tue May 20 2008 - 11:13:34 EST

Theodore Tso wrote:
> On Fri, May 16, 2008 at 11:03:15PM +0100, Jamie Lokier wrote:
> > The MacOS X folks decided that speed is most important for fsync().
> > fsync() does not guarantee commit to platter. *But* they added an
> > fcntl() for applications to request a commit to platter, which SQLite
> > at least uses. I don't know if MacOS X uses barriers for filesystem
> > operations.
> Out of curiosity, exactly *what* semantics did MacOS X give fsync(),
> then? Did it simply start the process of staging writes to disk, but
> not wait for the writes to hit the platter before returning? That's
> basically the equivalent of ext3's barrier=0.

I haven't read the code and don't use MacOS myself.

>From its fcntl() man page:

Note that while fsync() will flush all data from the host to the
drive (i.e. the "permanent storage device"), the drive itself may
not physically write the data to the platters for quite some time
and it may be written in an out-of-order sequence.

Specifically, if the drive loses power or the OS crashes, the
application may find that only some or none of their data was
written. The disk drive may also re-order the data so that later
writes may be present while earlier writes are not.

This is not a theoretical edge case. This scenario is easily
reproduced with real world workloads and drive power failures.

For applications that require tighter guarantess about the
integrity of their data, MacOS X provides the F_FULLFSYNC
fcntl. The F_FULLFSYNC fcntl asks the drive to flush all buffered
data to permanent storage. Applications such as databases that
require a strict ordering of writes should use F_FULLFSYNC to
ensure their data is written in the order they expect. Please see
fcntl(2) for more detail.

Some notable things:

1. Para 2 says "if the drive loses power __or the OS crashes__".
Does this mean some drives will abandon cached writes when reset
despite retaining power?

2. Para 3 to be re-read by the skeptical.

3. Para 4 perpetuates the confused idea that write ordering is what
it's all about, for things like databases. In fact, sometimes
ordering barriers are all that's needed and flush is unnecessary
performance baggage. But sometimes an fsync() which only
guarantees ordering is insufficient. An "ideal"
database-friendly block layer would offer both.

I doubt if common unix mail transports use F_FULLSYNC on Darwin
instead of fsync(), before reporting a mail received safely, but they
probably should. I recall SQLite does use it (unless I'm confusing
it with some other database).

-- Jamie
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