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Re: [dm-devel] Re: [PATCH] Implement barrier support for single device DM devices



Jeremy Higdon wrote:
On Tue, Feb 19, 2008 at 09:16:44AM +1100, David Chinner wrote:
On Mon, Feb 18, 2008 at 04:24:27PM +0300, Michael Tokarev wrote:
First, I still don't understand why in God's sake barriers are "working"
while regular cache flushes are not.  Almost no consumer-grade hard drive
supports write barriers, but they all support regular cache flushes, and
the latter should be enough (while not the most speed-optimal) to ensure
data safety.  Why to require write cache disable (like in XFS FAQ) instead
of going the flush-cache-when-appropriate (as opposed to write-barrier-
when-appropriate) way?
Devil's advocate:

Why should we need to support multiple different block layer APIs
to do the same thing? Surely any hardware that doesn't support barrier
operations can emulate them with cache flushes when they receive a
barrier I/O from the filesystem....

Also, given that disabling the write cache still allows CTQ/NCQ to
operate effectively and that in most cases WCD+CTQ is as fast as
WCE+barriers, the simplest thing to do is turn off volatile write
caches and not require any extra software kludges for safe
operation.


I'll put it even more strongly.  My experience is that disabling write
cache plus disabling barriers is often much faster than enabling both
barriers and write cache enabled, when doing metadata intensive
operations, as long as you have a drive that is good at CTQ/NCQ.

The only time write cache + barriers is significantly faster is when
doing single threaded data writes, such as direct I/O, or if CTQ/NCQ
is not enabled, or the drive does a poor job at it.

jeremy


It would be interesting to compare numbers.

In the large, single threaded write case, what I have measured is roughly 2x faster writes with barriers/write cache enabled on S-ATA/ATA class drives. I think that this case alone is a fairly common one.

For very small file sizes, I have seen write cache off beat barriers + write cache enabled as well but barriers start out performing write cache disabled when you get up to moderate sizes (need to rerun tests to get precise numbers/cross over data).

The type of workload is also important. In the test cases that I ran, the application needs to fsync() each file so we beat up on the barrier code pretty heavily.

ric


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