lkml.org 
[lkml]   [2008]   [Feb]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [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



    \
     
     \ /
      Last update: 2008-02-20 14:41    [W:3.476 / U:0.788 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site