Re: Disk write cache (Was: Hyper-Threading Vulnerability)

Jeff Garzik
2005-05-15T15:30:20Z

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:
> >> > > > > ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-05:09/ht
> >> > > > >t5.patch
> >> > > >
> >> > > > 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
> >> > value.
> >> >
> >> > 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
> >> > cases.
> >>
> >> 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.

There is a large amount of yammering and speculation in this thread.

Most disks do seem to obey SYNC CACHE / FLUSH CACHE.

Jeff