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SubjectRe: Disk write cache (Was: Hyper-Threading Vulnerability)
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.

>Mikulas
>-
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--
Cheers, Gene
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
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Copyright 2005 by Maurice Eugene Heskett, all rights reserved.
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