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    SubjectRe: Disk write cache
    Kenichi Okuyama wrote:
    >>>>>>"Jeff" == Jeff Garzik <> writes:
    > Jeff> On Sun, May 15, 2005 at 11:21:36AM -0400, Gene Heskett wrote:
    >>>On Sunday 15 May 2005 11:00, Mikulas Patocka wrote:
    >>>>On Sun, 15 May 2005, Tomasz Torcz wrote:
    >>>>>On Sun, May 15, 2005 at 04:12:07PM +0200, Andi Kleen wrote:
    >>>>>>>>>However they've patched the FreeBSD kernel to
    >>>>>>>>>"workaround?" it:
    >>>>>>>>That's a similar stupid idea as they did with the disk write
    >>>>>>>>cache (lowering the MTBFs of their disks by considerable
    >>>>>>>>factors, which is much worse than the power off data loss
    >>>>>>>>problem) Let's not go down this path please.
    >>>>>>>What wrong did they do with disk write cache?
    >>>>>>They turned it off by default, which according to disk vendors
    >>>>>>lowers the MTBF of your disk to a fraction of the original
    >>>>>>I bet the total amount of valuable data lost for FreeBSD users
    >>>>>>because of broken disks is much much bigger than what they
    >>>>>>gained from not losing in the rather hard to hit power off
    >>>>> Aren't I/O barriers a way to safely use write cache?
    >>>>FreeBSD used these barriers (FLUSH CACHE command) long time ago.
    >>>>There are rumors that some disks ignore FLUSH CACHE command just to
    >>>>get higher benchmarks in Windows. But I haven't heart of any proof.
    >>>>Does anybody know, what companies fake this command?
    >>>>From a story I read elsewhere just a few days ago, this problem is
    >>>virtually universal even in the umpty-bucks 15,000 rpm scsi server
    >>>drives. It appears that this is just another way to crank up the
    >>>numbers and make each drive seem faster than its competition.
    >>>My gut feeling is that if this gets enough ink to get under the drive
    >>>makers skins, we will see the issuance of a utility from the makers
    >>>that will re-program the drives therefore enabling the proper
    >>>handling of the FLUSH CACHE command. This would be an excellent
    >>>chance IMO, to make a bit of noise if the utility comes out, but only
    >>>runs on windows. In that event, we hold their feet to the fire (the
    >>>prefereable method), or a wrapper is written that allows it to run on
    >>>any os with a bash-like shell manager.
    > Jeff> There is a large amount of yammering and speculation in this thread.
    > Jeff> Most disks do seem to obey SYNC CACHE / FLUSH CACHE.
    > Then it must be file system who's not controlling properly. And
    > because this is so widely spread among Linux, there must be at least
    > one bug existing in VFS ( or there was, and everyone copied it ).
    > At least, from:
    > there is project name "diskio" which does black box test about this:
    > And if we assume for Read after Write access semantics of HDD for
    > "SURELY" checking the data image on disk surface ( by HDD, I mean ),
    > on both SCSI and ATA, ALL the file system does not pass the test.
    > And I was wondering who's bad. File system? Device driver of both
    > SCSI and ATA? or criterion? From Jeff's point, it seems like file
    > system or criterion...

    The ability of a filesystem or fsync(2) to cause a [FLUSH|SYNC] CACHE
    command to be generated has only been present in the most recent 2.6.x
    kernels. See the "write barrier" stuff that people have been discussing.

    Furthermore, read-after-write implies nothing at all. The only way to
    you can be assured that your data has "hit the platter" is
    (1) issuing [FLUSH|SYNC] CACHE, or
    (2) using FUA-style disk commands

    It sounds like your test (or reasoning) is invalid.


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